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Carburetors on BMW Airhead Motorcycles; how to properly synchronize them
The Whole Story....I HOPE!

Copyright, 2014, R. Fleischer


This is a lengthy article.  It is a REALLY GOOD IDEA for you to read it through
 slowly & carefully several times before you attempt ANYthing.


Do not lubricate the throttle cables.   Originally, 1978+ cables had an inner lining that self-lubricates, wearing very slowly over large mileage.  NOW....ALL throttle and clutch cables from ALL years, that have been replaced, are extremely likely to be the newer lined-types.  Throttle cables should normally be replaced at reasonable intervals.  This used to be about 60-80,000 miles, but I think the mileage should be on-condition.   It is not unusual for properly routed & cared-for cables to go to 100,000 miles.

I replace cables if fraying (typically at the left carb!); or, the throttle feels draggy or catchy...and it isn't due to the gears at the throttle assembly at the handlebars. I have seen a few cables go 150,000 miles with NO problems. 
Some recently shipped BMW cables are lousily made at the ends...although this is mostly clutch cables.

If someone makes a habit of bending the left throttle cable at the carburetor while removing and replacing the dipstick, the throttle cable there may fail, sometimes rather quickly. Once just ONE strand breaks, I have seen the rest go in as little as 200 miles.  A bend at the outer jacket at the carburetor area can cause friction.  Keep in mind that worn cables, friction, etc., means more throttle effort for your right hand, & it means MORE WEAR ON THE THROTTLE GEARS, etc.

Another and BIG! reason for cable problems is failure to lubricate the barrels at the carbs; I do it after EVERY wash job.

>>>Failure to ensure a smoothly rotating cable end barrel & to have a good bushing in the Clutch lever at the handlebars, and in very rare instances the cable at the rear of the transmission, has caused many problems with cables.


If you lubricate the cables, the liner MIGHT swell, and make things much worse.  Frankly, you should not lubricate earlier ones, as they probably by now are replacement types with linings.   Lubrication of the cable innards is generally a last-ditch effort, when you do not have new cables at hand.  It is UNCLEAR if the inner lining will be OK and not swell if you lubricated it with silicone oil.  NEVER use WD40!

>>>Do NOT install carburetor cables with lots of ties, nor sharp curves.
>>>The cables must be free to move about, particularly as you steer the motorcycle, and VERY particularly the clutch cable.

WHY synchronize carburetors?  What's the big deal here?  Why are there so many opinions?  Why are some books like Clymers and Haynes so misleading?...or just plain WRONG!?....OR, how come Snowbum is to be believed, and NOT them? (well, I can't give you any outstanding reasons on the last part, except maybe to listen to your elders who have lots of experience).

Your BMW boxer engine has two pistons going in and out in the cylinders at the SAME time, same direction, same amount.   While the engine would SEEM to be perfectly balanced mechanically, that is not exactly so.   The crankshaft has weights on it, & they are not perfect at  balancing the crankshaft at all rpm, nor at all parts of any piston stroke.   Additionally, parts may not be even near perfectly balanced with regards to weight & reciprocating offset mass.  That will cause some vibration, usually showing up in a narrow rpm band.  Imbalance in the clutch assembly is another area. MANY a clutch assembly is not assembled correctly.
The cylinders are NOT opposite each other, one is BEHIND the other, which produces a situation called 'a rocking couple'.  That is actual terminology, and it, very simply said, is a twisting cyclical vibration.  IN ADDITION, actual internal friction & combustion pressures between cylinders may vary, & there are a MANY other variables, such as small ignition timing differences, differences in valve settings & valve seating areas, camshaft lobes; engine wear....and more.

As the timing chain stretches (actually the wear on the crankshaft sprocket & guides/tensioner are the most common wear items), this affects cam timing & ignition.  While the stock ignition is easily adjusted to compensate, it does NOT do so for both cylinders in a perfect way.  There are jerky things in the timing chain operation, particularly as parts wear, causing IRREGULAR timing between cylinders, & in the same cylinder too.  This is quite easy to see when you check the ignition timing & advance, at the 'flywheel', with a stroboscopic light.

External to the engine core are variations in the carburetors & the cables operating those carburetors. MOST of these can be adjusted to be perfect or near perfect.   

Because of all these things, & other factors, the BMW boxer Airhead engine is rather sensitive to carburetor adjustments.  This is particularly so for the 1980's & later models that were factory tuned to run leaner & have lighter flywheel-clutch-assemblies (the flywheel from 1981 is called a Clutch Carrier, but it IS a flywheel).

An imbalance in synchronization of the cables & carburetors, together with effects & slight differences in what was described above....and maybe even diaphragm differences in the carburetors...I could make a long list of things having effects....will cause engine roughness or vibration, often in a narrow band & sometimes at two different narrow bands of rpm.    Imbalance in the cables & idle mixtures/idle rpm between cylinders will typically result in slight rocking or stumbling as one comes off the idle stops, particularly if doing so gently.   Occasionally things are worse if one of the imbalance points for the various things discussed happens to be at the natural balance frequency of the crankshaft assembly.  That TENDS to be AROUND 3800-4500 rpm, with many centered around 4250 rpm.   

IF the pressure in a combustion chamber/cylinder is different in the left cylinder, compared to the right cylinder; the engine will rock back and forth, sometimes a very little bit, sometimes more.   As rpm is raised, the rocking becomes a faster pulsing.  Further increase in rpm and it becomes a vibration or tingling in the bars.  It is entirely possible AND COMMON for the pressures to be roughly the same at higher rpm and throttle settings and NOT be the same at any other throttle setting INCLUDING idle.

Properly synchronizing the carburetors, assuming all else is even close to reasonably OK, will give smoother throttle operation, particularly noticeable at the just-off-idle area, & will definitely also reduce higher rpm vibration.  Just-off-idle is important, because on quite gentle take-offs, THAT is the point that even slight engine stumbling would be exceptionally annoying.   If the carburetors are NOT in good condition & properly adjusted & synchronized, the engine may well have very poor throttle feel, the engine might stumble, it might even backfire, it might use an excessive amount of fuel, & it might well cause a lot of  'funny vibration'....not to mention an unstable idle, perhaps a large increase in idle rpm after full warm-up; perhaps being hard to start, act very lean, etc.    If the idle mixture is improperly set, it is not unusual to see red exhaust pipes near the heads....& other things.

NOTE:  If you do not have the rest of your engine properly adjusted, such as ignition timing & valve clearances; if you have problems with ignition wiring or coils or points or condenser or spark plugs or spark plug caps; or, if the floats & diaphragms, passageways, jets, and jet needle (in particular) and so on in the carburetors are not in good condition, then you are totally wasting your time playing with carburetor synchronization.  The carburetors are the LAST...LAST!!...item to adjust!   You must NOT adjust your carburetors until the ignition & valves are known to be set properly, the floats are known good, float adjustment proper...& the fuel level in the bowls is known to be correct...&, diaphragms known good.  In addition, the butterfly on the CV carbs & their enrichener parts should be properly assembled.    The CV carb butterflies can be installed backwards, & that will cause you endless problems with idle adjustments. NOTE that some enrichener parts are WRONGLY marked by Bing....YES....that little punch prick on the shaft has been known to be mis-marked.

You should have a relatively clean air cleaner (you do not need a totally clean air cleaner), and no vacuum leaks at the carburetor-to-head adapters.   If your cables are worn enough, or kinked, or routed wrongly, your carburetors will NOT
STAY IN SYNCHRONIZATION!!    If the throttle at the bars is badly worn at the teeth of its barrel and cam, those should be replaced if truly bad, as they cause stiff and irregular throttle action, and eventually will strip.  Once the throttle gear and associated cam gear (that has the chain thing-y) are worn enough, the teeth will won't like that.  Those parts also need to be lubricated and operating smoothly.   BTW...those parts on earlier models have been updated, and you should consult my other articles for the details; or, a good parts person at a BMW dealership or Independent Servicer. 

I see a fair amount of Airheads that do not have sufficient free-play in the throttle cables (sometimes choke cables too).   If the throttle free-play needed is not there, moving the bars may change the idle rpm; and, the engine is likely to drift higher in rpm, usually as it warms up....(however, that is often a symptom of other problems, including idle mixture adjustments and a sticky automatic advance).  Rider's tend to not think about the throttle cables.   It is important that SOME free play be in BOTH throttle cable outer jackets as seen at the carburetors, with the throttle at the bars fully turned off, but your hand NOT on it (that is, NOT rotating the throttle further off than its spring-loaded off point, the springs are located AT the carburetors).  
Conversely, excessive free play in the cables will often result, depending on rider technique, in jerky operation at low rpm....and if there is enough excessive free-play, the outer throttle cable can ride-up onto the carburetor threaded adapter, and cause all sorts of problems.   1/8" is plenty good enough.

 It is beyond the scope of this article...which is lengthy tell you how to check, fix, adjust, & generally be sure everything is correct before you attempt carburetor synchronization.  This article is to tell you HOW to do the synchronization, how to use simple tools, & do the job quickly & accurately.  ANYone can do these things!  You do NOT need to have some sort of vast experience!

What tools do you need?

You will need a single flat blade screwdriver to adjust the idle mixture screws & idle stop screws, and perhaps remove a screw plug from a vacuum port (may not be a screw, could be a push-on hose & you might not even have vacuum ports on very early Airheads).  IF using the Shorting Method (Method #2), then this screwdriver should NOT be quite short in length & it MUST HAVE a plastic electrically insulated handle.  The metal portion of the screwdriver MUST NOT extend to the end of the hand-area, where you could possibly come in contact with it.

You will need a wrench to fit the cable length adjustment locking nuts.  Some folks have wrenches they like better than the BMW one (I am in that category) ...that fits the carburetor cable lock-nuts.  You need a short wrench that fits, avoid rounding the edges of the carburetor cable adjustor lock nuts, which are not overly hardened.    My favorite wrench is a common flat open end type, bent upwards some.

YOU NEED TO DO at least ONE OF THE TWO methods (#1, or #2) JUST BELOW!...or, you certainly CAN use a combination of both methods!  Some folks do method #1, quickly, then take a ride, and then finish up with Method #2.   You can certainly do the entire synchronizing with either, or a combination.  Method #2 gives the BEST synchronization.  Note that if the carburetors were taken apart, for, say, an overhaul, and all the adjustments are now WAY off (you can start with the factory recommended settings though), then any method takes longer.  Once the carburetors ARE synchronized, future adjustments can be done very fast.

Method #1:  This method uses some sort of vacuum sensing gauge or gauge set.  This can be a water manometer, mercury manometer (Sticks or Stix, etc.); or a Walus differential gauge; or an electronic vacuum differential instrument (TwinMax, etc.).  In my opinion, the best carburetor vacuum type synchronizer is the Harmonizer, by Grok.  In MY opinion it is better than the TwinMax, much more useful than the TwinMax, and more useful for most folks than the Walus (which are hard-to-find anyway).

If your carburetors do NOT have vacuum ports, these ports can usually be added.   SOME synch tools don't work too well on the BMW carburetors, or are too slow in use, etc.  The Unisyn comes to mind here.  It IS usable, if you are very careful; I don't like using them on the Airheads.  Note that early CV carburetors that DID have vacuum ports (NOT all did) had the ports sticking out from the carburetor bosses horizontally.  All later models had ports, and the ports stick downwards, from the underside, these later ones usually have soft black rubber hoses that lead to the Pulse Air system in the air filter box.   The carburetor vacuum ports have INternal screw threads, easily seen after the hose is pulled off, and the thread is a uncommon type.  You do NOT have to install a screw (nor washer), unless you are removing the hose permanently for some reason.  

A popular conversion is to remove the rubber hose sections and block the air cleaner area, when removing the Pulse-Air system.  I have mixed feelings on that, and there is NO harm done by leaving the vacuum hoses connected to each other, using the air cleaner area small T-fitting already there, and PLUGGING the rear facing part of that T.   That means NO vacuum port screws to lose.   Disregard folks that tell you that this makes the carburetor action smoother, or that it messes up carburetion.  It does neither.   There is an extensive article on this website that fully describes the Pulse-Air system, how to remove or plug it, and further has information on the tank fumes system and the fuel cutoff system used on the last Airheads:

Method #2:  This method uses spark plug shorting adapter tools (TWO unless dual plugged, then FOUR) or a switching/shorting 'box' (helpful mostly for dual-plugged bikes); and a plastic handled (for electrical insulation) screwdriver, of any reasonable type.   Don't use 3 inch screwdrivers here, they are too short for safety....and do not use wood handled types.....too much chance you will get an electrical shock. Do not use a screwdriver whose metal shank comes completely to the can get an electrical shock.  
While you can imagine-up quite a few types of spark plug shorting tools, there is a photo of some types a bit farther down in this article you are reading.    One version of these tools are available here: It is basically teh same as a tool I have been using for many years.  It is a piece of spoke from a wheel, and a spark plug screw-on barrel fitting.  I have also used threaded spoke fittings.  Wheel spokes have the correct thread, so a spoke fitting can be used, or, the spark plug screw-barrel, same thread.   You may decide on other methods for dual-plugged bikes. 

First, let me describe what these Method #2 shorting adapter tools are for, then you can decide on what to make...or purchase. 

What you are going to do, in part of the synchronization work, if you select my Method #2 (as opposed to a vacuum methods); is to run the engine at idle... and also just off-idle, ....and make adjustments to the carburetors, while you run the engine first with both spark plugs operating, and then with you shorting out first one spark plug (assuming standard two plug engine), then the other, doing only one plug shorting at a time, and listening to the engine... during these shorting's.  You will be doing the adjustments fairly quickly, using a screwdriver to electrically short the spark plug ADAPTER to the engine metal.  The engine must be warmed up a bit, even a garage type 3 minute warmup, for the INITIAL adjustment.   Then, thoroughly warmed-up to do the final adjustments, a 10 mile ride will do.  The final adjustments take only a minute or three at the most, so you don't need a fan.  If you wished, for any garage-type adjustment, you could consider using a big fan, to prevent engine overheating.  This is especially so if you are doing the shorting method for the very first time which will usually take longer. Actually, this fan idea also applies to using method #1, the vacuum method.  While the complete synchronizing can be done with the fan, in your garage, and you will be quite close, the best adjustments are done after a 10 mile+ ride.  

Your EARS and BRAIN will tell you all you need to know.  Your first time at this might take :15 of playing until you get the idea.  Then, you can do it all in 5 minutes OR MUCH LESS (NOT kidding).  You need adapters that absolutely, positively, without question!!!, will allow the existing spark plug caps to connect to these adapter tools, and the adapter tools to then connect to the spark plug top threads.    These adapters must NOT...NOT!! flimsy.  That is, they MUST be such that there is ZERO chance of a spark plug dis-connection.    Whatever adapters you make and use, these adapters must NOT come loose.  If they come loose you could injure the coils (points models) or coils or Hall device or module, in the 1981 and later models. 

  >>>Disregard ANYTHING you read in ANY book or on the internet, or ANYPLACE ELSE... that says to pull the spark plug caps off your engine to 'listen' or otherwise synchronize the carburetors.  That method is SAFE for ONLY the /2 or other early models which use a magneto and have safety spark gaps build into the magneto & such gaps have to be in good condition.   You can ruin or damage coils & electronics by failing to heed my advice here.   Certainly there is a shock hazard too...a serious one especially on 1981 & later bikes. 

DO IT MY WAY...>>EXACTLY my way!

IF you chose a meter or gauge, you don't short the plugs, nor need any adapters.   Some folks do the final adjustment with the shorting adapters after preliminary meter/gauge usage.  In SOME instances that is a NECESSITY...see the Hints section at the end of this article.   YOUR choice.

QUITE frankly, the shorting method IS MORE ACCURATE, as it takes into account ACTUAL differences, not just vacuum differences, between the cylinders, but it is dependent on YOU being careful & methodical.   Of course, this negates that nice feeling some folks seem to get by purchasing pricey test gear (that does not work as well, although SOME think their usage makes things easier).


The shorting adaptor tools:

You need something that fits securely at the spark plug top THREADED POST.  The first thing that comes to mind is likely one of the removable small threaded barrels that many (NOT ALL) spark plugs come with, and that you probably discarded for your Airhead.  Those do have the correct thread, obviously.  Some used to be made of BRASS, and those are preferred, as you can solder to them.  Most are now some sort of UNsolderable, but crimpable, aluminum alloy.  Into the spark plug CAP, you can insert any sort of properly fitting threaded rod as described a bit farther on, and if using one of these barrels of aluminum alloy, perhaps crimp a piece of 14 gauge house wiring solid copper inner conductor to it, the other end of the barrel screwing onto the spark plug..  I made one of these up using, instead of rod or house wire, a few inch piece of old ignition wire, and attached the adapter wire at the spark plug cap end by using the metal part from the top of an old spark plug...simply using a small hammer or vise to crush the old spark plug and remove the top innards.  I don't bother carrying that, it looks unprofessional...although I admit to keeping a couple of especially crude looking ones in my Walus gauge box.    A very nice adaptor can be made with spoke material, see just below.

For the slightly nerdy:  The threads are 4 mm x 0.7 mm pitch; and you can, perhaps, find screws at hardware stores, and nuts, if you wish to make your own version of these shorting adaptors from such items.

Here is a way to make a rather neat adaptor, using an old spoke nipple. The old BMW spokes have the correct thread,  4 x 0.7 mm.  Use that nipple, and the spoke that it fits, and cut the spoke to a couple inches or bit more in length;...and put a teeny notch on the SIDE of the upper end, and then the spoke will be pretty solid in the spark plug cap, which has a cross-wire to 'grip' the spark plug threads.  If you have a dual-plugged bike (two plugs per cylinder), you can make TWO spoke adapters for the top plugs, and an ignition wire type adaptor for the bottom ones, and have the two places to do the shorting (each cylinder) very close to each other...maybe an inch or you can use a single screwdriver in each hand, just like you will do the single plug bike shorting.  Many ways to do this, including insulated parts in an insulated switching box. SOME bikes will allow you to just short the bottom plugs to the cylinder head fins, when doing synchronization.  On some, this reduces the idle speed too much.  You have to THINK when shorting the bottom plugs this way, as you really need the RPM to be in the correct area for doing carburetor adjustments.  That is one of the reasons some folks have made insulated switch boxes with knife switches, to short both plugs on either cylinder a the same time, to engine ground.

Be your own.  Be sure it is secure in its use.

An assortment of spark plug shorting and adapting tools, for your viewing pleasure.  These include the spoke and spoke nipple type. I have used all of these at one time or the other, improvising on the spot at TechDays, etc.   The bungee cord (I actually used two of them) is here as I used the bungee to hold the bottom spark plug wire adaptors close to the upper plug shorting point....see text.  

Here is a source for a simple version of the shorting tools:


Theory behind vacuum-synchronization methods (this is not generally used with the above tools):

A carburetor, as opposed to a fuel injector, works by having a necked-down area through which incoming air passes, and that incoming air is speeded up by the effect of that neck.  Jakob Bernouli's theorem said that there is a DEcrease in pressure as velocity increases The decreased pressure, or 'vacuum', allows fuel to rise up in some opening/port/pipe (sucked up if you want to think of it that way) that has access to the fuel bowl, and turns itself into spray, hopefully well atomized into a gaseous mixture with the incoming air, but is more likely to have superfine droplets.  There are TWO places in the carburetors that fuel is sucked up and utilized.  One is the central jet/needle/etc. assembly, and the other is a passageway to an idle jet, which then feeds one or more very teensy-tiny idle port holes located in the bottom throat area of the carburetor.  (I am avoiding here going into information that the enrichener on the CV carbs works, crudely, in a similar manner).

The piston coming inwards (away from the cylinder head) in our Airheads on the INTAKE stroke (when the exhaust valve is closed, intake valve open); is what allows the 'sucking' a partial vacuum (compared to outside air) is made in the cylinder as the piston moves inwards.    When the butterfly valve (or 'slide' in pure slide carburetors) in a carburetor is closed, or nearly so (it can't be 100.000% closed as the engine needs some air to run), fuel and in some instances fuel/air mixture, is sucked out of the idle port(s) (holes) in the bottom of the carburetor body throat.  As the butterfly or slide is opened more, by you turning the throttle open (on the CV carbs, the butterfly is directly throttle controlled, but the slide is controlled by vacuum passageways inside the carburetor), the idle ports stop working and fuel comes out of the central jet area.   While early stock carburetors did not have vacuum ports, the later ones all did.  The vacuum port is simply a small hole, with an internal passageway into that venturi area, sampling the lowered pressure there, and that hole or enlarged entrance of it, has a small diameter pipe pressed into it, which is blocked off by a plug, screw, or connected to the other carburetor and a system for operating smog device setup called Pulse Air on later models with rectangular air cleaners.    It is possible to sample vacuum after the carburetor, the readings will be of different effect with throttle opening, but it CAN be done, by a probe into the rubber hose, for instance.  I suggest that this method not be done by the average person.  The common method is to sample the vacuum at the carburetor throat, and this is officially called Venturi Vacuum.   For you nerdy folks, if the vacuum was sampled AFTER the carburetor, it is called, confusingly, Throttle Plate Vacuum.

IF the vacuum produced by the engine cylinder piston movement was a perfect vacuum, that is, total vacuum, the maximum vacuum that could be produced, in theory, is approximately 30 inches of mercury vacuum at sea level, where 30 inches of mercury is the standard accepted atmospheric pressure, but in practice it varies a bit with weather conditions.    30" of mercury is roughly 15 pounds per square inch, the pressure on and in our bodies at a constant sea level existence.

As altitude increases, the air pressure around us generally goes down (approximately 1 inch of mercury or half a pound per square inch, per thousand feet).  Some atmospheric effects can make small changes that are up or down with increasing altitude, but the over-all trend is ALWAYS down with increase in altitude.  Thus, at higher altitudes, there is less air pressure, which means more fuel tries to go through the carburetor per weight of air, and the mixture thus gets richer.    The CV ("constant velocity) carburetors tend to compensate for rising altitude but total compensation is generally not possible.

On a practical basis, the cylinder vacuum is not going to be a perfect vacuum, but is going to be a few inches of mercury less. 

As the throttle is opened, especially if suddenly opened and most especially with a directly operated mechanical slide (rather than CV carburetor)....the vacuum on the engine side of the carburetor greatly decreases, until engine rpm rises considerably from where it was when the throttle was suddenly opened.   If the throttle is very suddenly closed from a high rpm, one gets a very high vacuum at the carburetor output area, and the venturi has a fair amount as well, as the passageway is very small due to the closed throttle, air velocity is high.  

For the nerdy, suddenly closing a throttle on a carburetor, especially the slide (only) type, can cause a maximum-possible-fuel-flow through the idle ports. 

As the throttle is opened, slowly to moderately, and rpm rises, and if the throttle is, for example, held for.. let us say 4000 rpm,... the vacuum produced effects are not as distinct as when the rpm is much lower.  This is particularly true in the CV carbs, where the slide is vacuum controlled and the butterfly is directly mechanically coupled to the throttle at the handlebars.  

Due to these types of effects, it is FAR better, FAR more sensitive, to adjust a carburetor's throttle cables not too far above idle, and conversely it is poorer, usually much so, to try to do any cable synchronization at quite high rpm.  Note again:  You do NOT want to adjust the cable synchronization too close to idle rpm nor too high.    No need to get into all this any deeper.   I suggest you DISREGARD any books or literature, that say to set the throttle synchronization at 4000 rpm, for instance.    The best rpm to synchronize the CABLES (after you do the other adjustments FIRST!) is around 1300-1800 rpm, and 1500 is a nice figure.  The BMW carburetors cable synchronization is done by adjusting the cable sheaths, and this is ALWAYS adjusted LAST. During the earlier synchronization process, these throttle cables MUST have some SLACK at throttle-off position!!

The manometers, or any type of gauge or electronic method, either measures the vacuum in each carburetor venturi (separate manometers, or separate mercury sticks or separate gauges), or, reads the difference between the two carburetors.  Either method can work fine, but differential methods are usually more sensitive and thereby more accurate.

NO vacuum method is as good as my shorting method, which takes into account actual engine cylinder differences...and...there is then no need to purchase a vacuum gauge, manometer, or much of anything like those.  Using shorting methods is nothing new, it has been published for many years, not only by me, but by various groups/clubs, etc.....especially in Europe.

Vacuum measuring devices may well incorporate a dampening means, often called a snubber, usually a very tiny restrictive hole, or a porous material or a method that otherwise produces a small hole effect.  Otherwise, the device's indication may pulsate rapidly or be much too sensitive, and thereby be hard to read/average, as carburetor vacuum flow is actually a 'pulse train'.


Preliminary setup:

Before you can do final synchronization, be sure of the following:

Preliminary eyeball synchronization has been done.  If your carburetors have been synchronized before, and you have not messed with anything on the carburetors, you need not do this step, unless you want to be SURE.  I ALWAYS do this eyeball check.   See below for how to do the preliminary eyeball synchronization.

ALWAYS be sure that the barrel fitting at the end of the cables, where they fit into the lever arms at the Bing CV type carburetors, have a bit of lubricant, heavy oil is fine on them, or moly grease, etc. These barrel ends MUST be free to ROTATE.   Failure to have smooth barrels is  a prime cause of frayed and broken cables (at the clutch lever at the bars too!), and lousy synchronization. For the LEFT cable, many will damage the cable by bending it when checking oil level with the dipstick.

Cable condition internally, see below, is important.  If your throttle unit at the bars is rough acting, or the cables are improperly routed (tying them down strongly with extra tie-wraps is not a good idea), are going to be fighting yourself.   There MUST be SOME free play in the outer jacket of both throttle cables, throttle at the bars being off (and hands off the throttle). LIFT the outer cable sheath at the carburetors, about 1/8" is reasonable.

BMW throttle cables (original types, if any still exist, of pre-1978 CAN be lubricated) are NOT to be lubricated, except as a last resort before you can get new ones.   They are LINED with a nylon or teflon-like substance.  Cables DO wear out and should be replaced when they are acting up, getting stiff, etc.   Besides their lining wearing, the steel strands might fray, and one or more break.  This is fairly common at the throttle levers at the CV carburetors, usually due to a lack of lubricant at the barrel fitting.  BUT, it is also a problem from folks bending the throttle cable on the left carburetor, while checking the oil dipstick.    ONE broken strand at the carburetor lever CAN lead to total failure ...I have seen this within a FEW hundred miles. 

BMW has TWO types of throttle cable designs, one design has a single cable at the throttle at the bars; the other design has TWO cables at the bars throttle.  The single type is more stable, and less affected by turning the bars. The single cable type uses a round T junction barrel part located under the fuel tank.  That T-junction has one adjustment for top cable length, for SOME free-play.  It is not adjusted for carburetor cable free-play.  You only need to adjust that top cable adjustor so that there is a tiny bit of free play in the cable AND, that moving the handlebars from side to side has no effect on carburetion.  Of course that is true of the bottom cables too.

When synchronization is completed the upper throttle cable(s), assuming you have properly synchronized the gears and greased them at some time, and have adjusted things properly, will have a small amount of free play. Be sure this is true.  The SINGLE cable model free play can be seen by rotating the throttle or pulling on the cable...., it is better to pull, slightly, on the single throttle cable,  as it enters the throttle assembly.  About 3/32" is about right.   For this single cable model, as noted previously there is an under tank T-junction adjustment.  DO NOT try to rotate the throttle further off for this check.  DO NOT have any friction knob adjusted for any friction for this check.  The free play must exist throughout the turning of the bars, from side to side.    If the free play is way too much, throttle feel will not only be sloppy off-idle, but you MAY not be able to REACH full throttle, no matter how far you twist the grip.  

The free play adjustment is in several places.  The single throttle cable at the bars on bikes having ONE such cable has that upper cable free play set at the T adaptor under the tank.  The bikes having TWO cables to the carburetors, and NO separate single top cable, have the free play adjustment AT THE CARBS via the throttle sheath adjustment.   ALL bikes have the carburetor cables adjustable AT the carburetors.

Note that if the throttle at the bars is improperly assembled (it has synchronizing mechanical marks to line up, INternally, you can see that with the one large screw removed and cover off), you will likely be UNABLE to get full throttle operation, that is, rotation will be limited.  THUS, in working with the cables, lubricating and proper re-assembly of the throttle parts at the bars is to be done FIRST!!!! 

NERDY: the throttle CAM is not the same on 32 mm and 40 mm carburetors.   The BMW throttle design gives a very smooth straight-pull on the cable(s) at the throttle, and also gives a SLOWER carburetor throttle effect at low settings, and as the throttle is opened, the effect increases non-linearly.....a very nice design, due to the CAM shape.

If you have early /5 CV carburetors, the type with the return springs AROUND the throttle cables at the carburetors, and you find the throttle action very stiff and probably has too much return spring force, you may want to modify the spring setup.  Simply reducing the spring force will help only a bit, and if you go too far, the throttles won't return properly.   You will have to fashion your own conversion, and looking at other carburetors will give you ideas.  If you CHANGE to later carburetors/parts, the problem is solved easily, if sometimes expensively.   NOTE that the R75/7 carburetors are GREAT for the /5. Juast what to fashion, or do, has stumped more than a few owners, since the tops of the /5 CV carbs are not all that easy to figure out how to add the later style of spring, which reduces hand force at the bars GREATLY.
HERE's a method you MIGHT LIKE:
An easy conversion for the early /5 CV carbs for much better spring action (and much lighter hand effort needed!), is to add a LARGE washer at each carburetor in the threaded area of the throttle fastening to the carburetor.  This is a bit tricky to do neatly, and one of these days I will do another /5 and post a photo here.   You drill a tiny hole in that washer, and the spring upper mount then becomes the washer, and you use the later type of spring.  Take a look at a later carburetor that has the spring connected to the carburetor body and the levers on the carburetor shaft.

Except for the /5 models in UNmodified condition, lighter CV carburetor springs are available....they are not the same for 32 and 40 mm carburetors. Check with your dealership; or, Ted Porter's Beemershop in California.  There is a type 606 for the 32 mm carburetors that have 3 digit model numbers; a type 908 for the earlier 2 digit 32 mm types, and a type 312 for the 40 mm carbs.  These will give lighter feeling to the throttle, yet are still adequate to return the throttle to idle.  These are NOT for the /5 carbs, unless you modify them.


This could be called a STATIC synchronization; that is, the motor is OFF, and no instruments in use.

If the carburetor adjustments were all changed radically, perhaps you overhauled the carburetors, changed cables; whatever, you SHOULD do a preliminary CABLE synchronization by eyeball.   QUITE frankly, if this is done accurately enough, it is usually good enough to allow you to go riding until you can do it properly with tools or instruments.  

Adjust the throttle cables lengths at the carburetors so the handlebar throttle just begins to lift the carburetors levers (or slide) at the exact same time as you increase the throttle, from idle position, a very tiny amount. #1 eyeball works fine.   Leave the cables with maybe 1/8" of slack in their outer cable sheath at throttle off, but hand off the throttle.  You will need to play with the idea awhile, as it is not the same view to you, left and right.  

Assuming that you are NOT doing JUST an eyeball throttle cable adjustment, proceed as follows.
You may want to do this no matter what you are going to do.

Make sure the two carburetors have close to the same small amount of idle stop adjustment beyond the butterflies being totally closed (also adjust the stops equally for a wee bit of lever off the stop, perhaps 1-1/2 turns).   The slide only carburetors are a bit different, but with the same idea.  If you played with the idle mixture adjustment, set it per whatever book you have, as far as turns outward, from a GENTLE inward stop.  This will be 1/2 to 1-1/2 turns PER THE BOOK, so check for YOUR carburetor, probably the BING book or other literature.  You will also want the choke (enrichener) cables (if you have a CV carburetor) adjusted for full off with their levers on the carburetor stops, and also able to reach approximately or nearly the full-on stops at the same time as you full engage the enrichener lever at the bars, usually marked CHOKE.   There is no official synchronizing of the choke cables beyond this.   If you have T units under the tank, they have adjustments to take up excessive slack for the upper throttle and choke cables, but a wee bit of slack is needed, and turning the bars back and forth should NOT change the slack enough to remove it all.  For those NOT having the T units under the tank, you have TWO throttle cables, which go directly from carburetors to throttle at the bars.  With the throttle OFF, hand NOT rotating the throttle backwards or forwards from idle resting position, there MUST BE approximately the same amount of slack in the outer jacket at the carbs (lift the outer jacket slightly to see this).   Not over 1/8" is about correct.

Assuming that you DO have some throttle outer cable slack at the carburetors and at the bars cable too if you have just one, now you are ready to get your tools, fan, whatever, all laid out, ready for your return.  I said RETURN, because you MUST go for a ride..... that is, if you are doing a complete, rather than eyeball, above, adjustment of the cables.

HINT:   A goodly sized fan is needed, AFTER your warmup ride, if you take more than a FEW minutes to do the adjustments.  Here is a way to get an EXCELLENT fan for basically nothing!  You do not absolutely need this, but it sure is nice!:::

Go to one or more local heating contractors, until you find a cooperative one ("sure, you can have an old squirrel cage motor, for free").   Heating/cooling contractors are always removing old home heaters and replacing them.  MOST of these heaters contain a perfectly good motor, that has shafts running out both ends, and the shafts have mounted to them squirrel-cage type rotary blade fans.  You want the type with a squirrel cage at both ends.   There is a surrounding metal shrouding that you want is all one assembly.   Most of these motors are heavy duty capacitor-start types, with plug tap selection for speeds.   Adapt a power cord and maybe make up some sort of simple wood piece to keep your new FAN from rolling about on your garage floor and maybe to aim the fan slightly have a dual output high volume fan.   It is not a bad idea to put some chicken wire over the intake ends to prevent things in your shop, like rags, or your dog/cat, from flying into the fans.   The lowest speed setting is usually the correct one.   You will find this fan also very nice to use after a ride to cool the engine (and exhaust pipes!) rapidly for other work.  You may even want to use it before covering your bike.  
Go for a 10 mile or longer ride.   1 or 2 miles is NOT enough!  A garage warmup is NOT adequate!   If you are forced to warm the bike up in the garage, the entire engine must be warm, and you MUST use that dual squirrel-cage fan, otherwise the engine will definitely overheat.  Warmup at about 1400 rpm is just fine.  A ride is better....MUCH better!   The fan needs to be in front of the front wheel, not behind the rear wheel.

Immediately upon your return, put the bike on its center stand, do not put it on the side-stand first.  This will avoid any fuel imbalances in the carburetors.   Leave the engine running or restart it when you begin, and turn on the fan, position it in front of the front wheel, blowing over the cylinders.  Have the fan outlets tilted slightly upward for that, if need-be.  Do not overcool the cylinders and carburetors.  If you are experienced, you need no fan.

You are going to do a dynamic synchronization.  NOTE that once the carburetors are fully adjusted, future synchronizations are likely far easier, far less effort, and take very little time.   NOTE that the idle rpm adjustments, and to some reasonable extent the idle mixture adjustments, tend to be rather stable over a long period of time and mileage, and in the future, while you may occasionally have to make an adjustment, usually you will be adjusting only the cable lengths.,,,and even then, only ONE, and much less often will you be adjusting the idle mixtures and idle stops.   With good cables, and the rest of the items previously mentioned all in good condition, 5000 mile intervals are just all that is needed for simple synchronization....  Or, when you feel some imbalance or wrong idle rpm, etc. 

NOTE that it is NORMAL for an Airhead to idle at a different rpm as you ride to a different elevation.  Adjustment to help compensate for a too low or too high idle for altitude changes you think you will do, should be thought of as you do your dynamic synchronization, and compensation IS possible, easily.   Idle rpm will vary a LOT between an engine that is not fully warmed up, and one that is.

DYNAMIC SYNCHRONIZATION:   Because several methods could be used, and because you might have slide carburetors (that means NON-CV, even though CV carburetors DO have slides), I will describe the BASIC and SIMPLE method first, because it is common to all.  I will assume a vacuum operated meter or gauge or manometer, and not the spark plug shorting method.

1.  Adjust the idle stop screws for a balanced gauge/etc. readings.  If the rpm is too high, set each idle stop screw a wee bit less; or a wee bit more if idle is too low, and then go back and adjust one of them for balance.  You want to end up with a balanced indication on your test device, and a rpm of about 900-1000 for bikes before the 1981 changeover to the light clutch carrier, instead of the older heavy flywheel.   For the 1981 and later, and any year with dual-plugging conversion, I recommend a target of 1025 rpm.   If your tach is reasonably accurate, use it.   NOTE that excessively slow idle rpm will result in poor oiling of the timing chain and timing sprockets, and higher sensitivity to mis-adjustments.

NERDY:   You could, you do not have to, select a FINAL idle rpm and idle mixture FINAL setting SOMEWHAT dependent on WHERE you intend to ride, and where the adjustment is being done in the first place.  This is because idle rpm tends to vary with altitude.    Thus, if you ride exclusively at sea level to perhaps about 2500 feet or so, I would target 950 rpm for pre-1981.  If you were doing the synchronization at a higher altitude...say 6000 feet or higher, then I would set the idle for maybe 900 rpm for pre-1981, so the idle rpm will not increase too much going down in altitude. I personally prefer about 1050 when doing idle settings at sea level, or up to a few thousand feet.    Don't set the rpm under 850 for any airhead; too slow means poor chain and sprocket oiling AND too sensitive carb adjustments, and a more abrupt off-idle transition.   ALSO, once you get a bit experienced at this, you will realize that the IDLE MIXTURE is also changing, and you can make a SMALL change to that.  Those adjusting AT high altitudes might want to make the idle mixture screw a WEE bit RICHER, and sea level adjustment folks use a wee bit LEANER.   The Bing CV carbs mixture screws turn INwards for leaner, and the slide carburetors turn OUTwards for leaner.    We are talking about 3/16 turn maximum difference here!   The reason is that if you set the idle mixture screw a wee bit richer at high altitude, then when traveling to low altitude, the mixture will not be too lean, and is a better compromise.

2.  Now that you have the proper idle rpm, and it is balanced on the meter/gauge/whatever, you need to adjust the idle mixture.  You WILL need to blip the throttle now and then to clean the engine of any fuel loading-up, just prior to ANY adjustments, idle, mixture, cables.  After blipping, allow a few seconds for engine stabilization.   Start with ONE carburetor.  The idle mixture screw is adjusted BY EAR, and adjusted very slowly, perhaps initially a 1/4th turn at a time over 5 or 10 seconds, and then as you approach the sweet spot, adjusted 1/10th of a turn at a time, until the engine sounds the smoothest, and the idle speed the highest.  You may have to start with the screw outwards considerably more than you started with, as you may not know what the last person set the screw at, and, the factory initial adjustment recommendation could be too far in to begin with. It is always safe to start 2 full turns outward from gentle seating, thus you won't mistake the proper setting from starting too far in to begin with and trying to move it even further in, and NOT finding the correct adjustment.

Thus, I seldom ever use the recommended initial idle screw adjustment. I just use 1-1/2 or bit more turns outwards to begin with.

THEN do the other carburetor.  If there was any rpm change at the end of any of this, go back and readjust the idle rpm AND its balance.  You may well have to repeat this process of mixture adjustment and idle stop screws SEVERAL times in the beginning.  Take your time.   What you want to end up with is the idle mixture screw in the best position for smoothness and highest idle rpm, and a balanced indication on your meter....and the proper rpm.   The idle stop screw and mixture screws interact with each other!  If things are WAY out of correctness, they interact mightily!    Blip the throttle occasionally, be sure the engine settles down before doing another wee adjustment and reading.    You will find, on the Bing CV carburetors especially, that the idle mixture screw is very sensitive in the INwards direction, and far less sensitive in the OUTward direction. 

>>> I suggest, until you have experience, or need the altitude adjustment, that you leave the mixture screw in the middle of the smallish adjustment range that causes the engine to SLOW or even stumble a wee bit if inwards too much, and slow a bit if outward too much...if anything, leave the screw on the Bing CV a bit outwards from optimum rpm/sound (maybe 1/8th turn). 

You want to end up with the idle mixtures correct, and the idle rpm correct, and the idle balance indication correct.  Do NOT make the mistake of starting this procedure from the idle mixture screw too far inwards, and then rotating it farther inwards.  It is almost always better to start with it too far outwards.  Once done correctly, however, you need make only the tiniest changes in the future (usually under 1/4th a turn each way, to find the proper point).  NOTE!....there MUST BE throttle cable sheath slack!

YOU ARE NOW DONE WITH THE IDLE ADJUSTMENTS!  Future adjustments will be very quick.

3.  Adjusting the cable lengths:

This is BEST done at a modestly low rpm, an rpm above the idle rpm, but not too far above.  The farther above the idle rpm, the less sensitive the adjustments and the harder it is to then get it right for the more critical area, the 'off-idle' position.  Because of this, I recommend 1300-1800 rpm.  You can lock the throttle if you have the factory friction screw at the bars, or, just hold the throttle.

What you will do is rotate the throttle for any specific rpm in the range of 1300-1800 (whatever rpm you want in that range), hold the throttle, and then look at your balancing device.  If not at balance, change the length of ONE of the throttle cables, and try again. You can get an easy feel for what direction, by lifting the outer sheath on one cable, slightly.

You want to end up with a balanced readout, AND, a small amount of cable slack at throttle off (carburetor levers must be on stops at the carburetors)...3/32" is fine.... not critical though.  Do not have so much slack that the outer sheath ferrule could come out of the receptacle in the carburetor fitting, nor so little that turning the handlebars (on TWO handlebar cable models), or any change from temperature, ETC., would cause the idle stop settings to be affected by the cable length.   

Finer point:    raise the rpm a fair amount, and see if the balance still holds relatively well.  If it does not, you might have a diaphragm problem.

NOTE:  when blipping the throttle, or moving the throttle suddenly off from perhaps 4000 rpm...or whatever....if you see largish vacuum changes, SOME of that is due to unequal springs on top of the CV slides (if your model has springs), or differences in diaphragms.....etc.   All will have SOME such vacuum changes....less is better....and unless are advised to leave things alone. 


The shorting-of-spark-plugs method

You simply will be modifying the previous method, and you won't be using gauges. 

Here is the procedure for doing it with the shorting device tools you made:

1.  First, AFTER you put the bike on center-stand following your 10 mile+ warmup ride, you will shut the engine off.   Now you pull the spark plug caps off and install your spark plugs/caps shorting devices.  Be sure it is all secure; it is critical that the spark plug shorting adaptors can NOT fall off, the cap fly off, etc.   Having the system become an open-circuit with the engine running will damage the ignition.  Your two sizable screwdrivers with the plastic insulated handles are ready for you...right?  and the wrench?  Fan?

2.  You can now start the engine.  Use the fan if you need to.  You need to if your adjustment period goes beyond a few minutes.  

3.  The procedure is basically the same, but instead of a meter or other device showing you the balance, you simply will short one spark plug adapter wire to a nearby fin, then remove that screwdriver induced short, and short the other spark plug adapter wire...back and forth.   There are some distinctly different things to know.  First, you will do the IDLE FIRST. You will leave a short in place about 3 actual seconds.   You will make the same adjustment of the idle stops for rpm and balance...balance here being the SAME SOUND/RPM from one to the other cylinder.  You make the same mixture adjustment as previously described, and that MIXTURE adjustment, as in the gauge method, is done with NO shorting of the plugs.    Blip the throttle now and then to clear the engine, then wait a few seconds for the engine to settle down.

You will have the same back and forth, idle stops and mixtures, to do, until it is all OK and no further improvement can be made.

You want to end with proper rpm & proper balance, at idle rpm, throttle off.  Again, repeating; blip the throttle once in awhile during the procedure to be sure the engine is ""cleared"" and then stabilized.  Be sure you have some slack in the throttle cables, throttle off.

4.  NOW you must synchronize the cables.  This is a bit different in a couple of ways.  There are TWO cable synchronizations to watch for.  Both are done at the same 1300-1800 rpm (YOU pick the rpm you like, but I recommend 1500).

a)  Short the plug circuit, as before, left, then right, then left...listening to the engine.  Each time one plug is shorted, listen.  As with the idle adjustments, both plugs are NEVER shorted at the same time (one plug per cylinder meant here, not a dual-plugged bike).  Allow about 4 to 6 seconds of shorting time for EACH plug.  Adjust the cable lengths for balance...the same sound.  Remember to clear the engine if it needs it.

b)  Short the plug circuit, as before, left, then right, then left, listening to the engine, this time allowing only about 1 second of shorting time.  You might need a WEE cable adjustment.

In all the shorting methods, you are NOT looking at the tach, you are LISTENING to the engine.  The only time to look at the tach is to be sure the ending idle rpm, and the rpm during CABLE adjustment, are both at the correct rpm.

Dual plugging conversions:   If you want to use the shorting method, it is sometimes better to short out BOTH plugs on one side, and alternate to the other side, back and forth.  This can be mechanically fun and games unless you make a special switch or device to do this.  I have made an extra long shorting wire to allow it to be done with a single screwdriver in each hand.   This method, on SOME DUAL PLUGGED bikes, may reduce the idle rpm so far that the engine stops. 
    Some have tried shorting out both bottom plugs, leaving them shorted-out during the procedure, which usually works OK, and is my recommended method, as it also eliminates the problem of carboned-up lower plugs.  Some install two lower spark plugs with zero gaps. I do not, but it does work OK. 

Typically I install 4 each spoke adapters, one at each plug.  I make up jumper wires (two are needed), maybe 6 inches long or so; and an alligator clip on the wires ends.  I can then clip a wire onto a lower spoke, and the other end of the wire to a nearby cylinder fin. Then I do my synchronization and balancing work by having only to alternately short the top plugs adaptors.  If that does not work for the particular engine, I short both plugs on one cylinder at a time to the fins, with one screwdriver.

NOTE that the PROPER setup for a dual plugging installation is for one plug of each cylinder fired by ONE of the dual-output coils....and to a lesser importance you can also do it so the top plugs are fired by one coil, the bottom plugs by another coil.  THAT typical setup works FINE. 

Most folks probably do synchronization of the dual-plugged engines with vacuum methods, but, see hints items, below.

That's IT!!

1.  Even with clean idle passages & everything else in good condition, irregular idle & strange symptoms at idle & off-idle, & ESPECIALLY
     irregular idle rpm, are often due to a bad idle adjustment or idle adjustment and/or idle jet rubber O-rings.  This can also happen on CV
     carburetors withOUT the springs above the slides, the purpose of which is to ensure positive seating of the slide at the bottom at idle...and
     to smoothly return the slide downwards.   The earliest R75/5 carburetors had problems & a completely separate article is on this website
     for those.   Springs can be added to dome-type carbs: see the two Bing CV articles on this website.  Be SURE there is throttle cable slack
     at the carbs!!!!
2.  It is CRITICAL that the enrichener parts be not mixed up left to right, & are installed correctly.  It is also very important that the butterfly
     valves on the CV carbs be installed correctly.  Some enricheners were shipped by Bing with WRONG markings.  See my carb articles for
3.  On the /5 models, the intake plastic tubes have slots at the carburetor end, & can drip breather condensate oil on your foot.  A few had the
     slots too deep, and the following does not work, but for most, put the band clamp adjustor at the carburetor INlet at the TOP...which will
     stop the oil dripping.
4.  Once in awhile there will be an Airhead with the CV carbs that will seemingly adjust just fine with the vacuum gauge method....for idle mixture,
     idle rpm & off-idle for the cables... & yet will not idle smoothly.   Sometimes the owner will go crazy trying to find the reason, & check ignition
     points (if present), timing, valve clearance, compression pressure, spark plugs & their caps, cables, coils, ETC.    Nothing is found wrong,
     except the poor idle.   The problem is then thought to be inside the carburetors (which it is!)...but, what?......well, it can be butterfly's are
     improperly installed.  See photos in my carb articles for the butterfly installation.
5.  If you have a R75/5 that is particularly difficult to start & everything else checks out fine, be SURE to check the slides, to be SURE they are
     BOTTOMING fully & not hung up slightly.  This can happen with other airheads, but MUCH less likely.
     SEE the article on these R75/5 carburetors on this website! that article is at:  

04/22/2003:  add .htm title; clarifications
07/31/2003:  add note at top area on lubing cables; minor editing for clarity in the entire body of article,
                    add dual plug info.
09/21/2003:  start a new section..wee hints; go over entire article and clarify details here and there.
09/26/2003:  add picture of spark plug adapter tools
03/08/2004:  add hint 2
10/03/2004:  syntax and grammar here and there; plus updated information on dual plugging &
                    synchronization, and revised the Hints section, adding more information.
03/27/2005:  add 5.
11/03/2006:   Update entire article for emphasis and clarity.
07/06/2011:  clarify idle mixture and rpm compensation methods
11/14/2011:  slight clarifications in dual-plugging comments
11/15/2011:  wee bit more clarification
04/27/2012:  Edit for clarity
08/08/2012:  Expand and update cable maintenance and replacement section
10/14/2012:  Add QR code, add language button, update Google Ad-Sense code
12/13/2012:  Go over article, make minor improvements in clarity
In 2013, remove language button javascript code, it was causing some browsers to have problems.
06/01/2014:  Recheck article.
08/03/2014:  minor changes for clarity, section separation
08/25/2015:  Cleanup
09/14/2015:  Make slight changes to recommendations for synch tools.
10/04/2015:  Clarify vacuum and shorting methods, add link for shorting tools.

Copyright, 2014, R. Fleischer
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