system in your Airhead BMW Motorcycle---miscl. section.
Tank cleaning methods; premium vs. regular; fuel additives; fitting other tanks, seat fit with various tanks; throttle and choke cables, fuel hose, ETC.
© Copyright, 2014, R. Fleischer
>>>>>>>>Refer also to article #1 and article #75 for more
>>>>An article that describes Lead Substitutes, in depth, is: valves.htm which is article 60, sub-section 5
1. An extensive article on Dell'Orto's, covering all their
carburetors, not airhead specific, is now on THIS
It explains in great detail not just how the Dell'Orto works, but is applicable in general to all carburetors....and MIGHT be worthwhile for you to peruse, even if you don't have one of these carburetors, which was used only on the R90S airheads.
2. If you have a high compression ratio airhead (modified later type; or, 1979 and earlier type with stock high compression ratio), and you use high octane fuel, be advised that when fueling up at a service station, it is VASTLY better NOT to fuel up at the type of 'pump' that has only ONE nozzle/hose. Your fuel may well be diluted by a modest quantity of lower octane fuel remaining from the last user......from the internal valve, pump, etc....through the hose, etc....somewhat depending on the type of equipment in use. The dispensing companies don't offer this information easily, and 1/4 gallon or more is VERY common.
There is an old controversy over possible increases in combustion chamber temperatures, ETC., when using premium gasoline's in lower compression BMW airhead engines, where lower octane is supposedly not needed. Gasoline burns at about the same rate under normal, that is not detonating, etc., conditions. The output (BTU) per gallon of premium gasoline is potentially....or even likely...to be a tad LOWER than for regular. THE ENERGY CONTENT OF FUEL VARIES WITH THE SEASONAL CHANGES OF THE FUEL! Therefore, I think it is likely that SOME premium gasoline's WILL give LOWER gas mileage than a regular gasoline will.....assuming here that the engine will run properly on Regular grade gasoline in the first place. Winter gasoline tends to have rather volatile things like butane or propane in them. In the USA in general, so-called oxygenates, such as alcohol, are added to most fuels. These additions GENERALLY cause 6%-10% POORER gas mileage. They are NOT good for your engine, carburetors, hoses, etc. All the oxygenates tend to cause the engine to run leaner. Some Airheads are already running on the lean side of best power mixture...and leaning will cause stumbling, etc.
The fuel content for energy changes with temperature, if you are REALLY anal about knowing stuff.
3. There are a number of
types of aftermarket fuel filters on the market, metal ones,
cleanable element ones, various sizes, colors, etc. I
prefer the impregnated paper type, NAPA
#7-02323. That number may have to be entered at the store or on-line as
702323. With the aging of our fuel tanks, even with
the in-tank or in-petcock screen filters, I feel that a decent aftermarket filter
is a must, in the line just below the petcock. There
is a very much more complete article on this website on the fuel
filters, petcocks, etc: fuelfltrs&petcocks.htm
NOTE that due to the age of our airheads, I am a firm believer in using aftermarket paper filters BELOW the petcocks, and maintaining the petcock mesh screens. Tank debris is a prime cause for a gasoline soaked foot (besides old worn float needles). The 'stuff' gets right by the tank mesh screens....which are designed to trap the bigger particles. I clean the petcocks thoroughly every year, by removing at simple tank cleaning time, blowing them out with compressed air. If the handles are starting to get stiff....usually that means at the every few years point I do the more thorough tank job....I disassemble the petcocks and service them. Almost never do I need any parts, other than a faint smear of silicone grease. Failure to do some sort of regular tank cleaning is fairly likely to result in a rotted tank bottom....AND...a lot of junk getting into your carburetors, or at least fuel screen(s). See the article on petcocks and filters fuelfltrs&petcocks for a fuller description of these things.
will have some interesting information on lead and MTBE for
There are other websites that have considerable technical information on fuels, additives, etc.
There is a fair amount of information scattered throughout this article of mine, that covers leaded fuels: valves.htm
5. Questions often arise as to whether or not the screen, petcock, lines, filter(s), etc., are flowing enough fuel. This is easy to actually calculate, if you figure the worst case, such as 20 mpg at wide-open throttle. But, here is a guideline of mine, that is perfectly adequate, and you don't have to play with mathematics: If you have a small displacement airhead, such as a R65, you need a cupful of fuel flowing from EACH carburetor, separately measured, bowl removed, common household 2 cup measurer underneath carburetor, in one minute or less. For the larger displacement airheads, a cup and a half or more is OK. NOTE: fuel flow problems such as stumbling after a bit of cruising....can be caused by fuel cap venting problems, not just clogged fuel screens and filters, etc. The fuel caps of around 1978 into 1979 were particularly susceptible to vent clogging.
6. Tank cleaning, coating, etc.:
(see my chemicals article and
references article for additional recommendations for
To clean a fuel tank, you must remove it. Remove the petcocks before you remove the tank from the bike (lots easier). I clean the petcocks/screens at the yearly tank cleaning time, and I may take them apart for a more thorough cleaning and silicone grease lubrication of the parts. There are several methods for cleaning the fuel tank. You might want to use the high pressure wand (soap mode, then rinse mode there or at home) at the local car wash. If your local car wash uses non-phosphate soap ('environmentally safe, green, etc.'), then do the entire job at home with a garden hose and common hose nozzle. NOTE!...if the tank cleaning is done because the gasoline, maybe some moisture, whatever, has been in the tank and dried (mostly) from very long term storage, and probably looks like syrup (or??)...then the tank should first be cleaned by adding a fair amount of some strong cleaner like Simple Green. Use a 50-50 mixture, and let it sit overnight. Then go about cleaning the inside with a pressure wand of soapy water, then clean water, from the nearest car wash place. If all the crud does not come out, you can either repeat the process, or start with chemicals.
Note: The information presented in this section is GENERALLY for Airhead steel tanks. If you are reading this and have a fuel-injection bike with the submerged fuel pump, be very cautious about using harsh chemicals.
The rest of this deals with relatively normal yearly or bi-yearly cleaning for Airhead steel tanks::
You can put maybe a couple tablespoons of common dish detergent (works better than "soap", particularly the no phosphates soaps) in the tank, half to each side, then flush strongly with the spigot/hose/nozzle set for a narrow but strong stream. I blast the interior, in other words. Yes, it can loosen and lift the old deteriorating red lining if vigorous enough, I don't find that a problem.
If you do this every year, and then treat the bottom area with phosphoric acid mixture (hardware store etching solution), and then dry the tank.....the tank will likely last forever with no holes or other rusting damage ever occurring and no need for POR15, Caswell stuff; Red-Kote, nor, the lesser Kreem. If you wait too long, the tank will have been rusting enough to cause leaks, or about to. For those of us that quit riding 2 wheels in the Winter and store the bike, I recommend cleaning just before storage, and ensure that the tank is allowed to fully dry. I am so anal about this (fixing and repainting a tank is COSTLY) that I put the washed and cleaned tank over one of my house heater's floor outlets for a week or so, outlets downwards, cap off or open. No, it does not smell if cleaned properly.
For normal riding times, a goodly help, particularly in humid areas where you see dew collecting on the tank overnight, is to refill the tank with fuel before parking it overnight. This can greatly reduce the surface area for water to condense, which then falls as nice large droplets into the bottom...and that nasty to fix seam area....where it helps rusting begin and ...well, you know....
Be sure to have filters below the petcocks and clean the petcock screens yearly at least.
Depending on tank condition, I SOMETIMES remove the petcocks and install cork stoppers (hardware stores stock corks), and swish some 'etching' liquid (1 part to 2 parts water....a rust
preventative/converter material), into the tank, then I drain the
tank after maybe an hour or three or even overnight (better),
flush it once more, and let it sit a day or two or more to fully
dry out. I have tried several of these common
phosphoric acid etching liquids from a hardware store,
they all seem OK. Phosphoric acid converts any remaining
rust, and tends to slightly seal the interior of the
tank, especially the vulnerable bottom seam area. If your tank is a big mess to start with, you
may have to do it twice, and be more thorough about getting the
debris out of the tank in the first place. Once done
properly just once, and it is pretty simple, tanks stop their deterioration to a great extent....and rust
usually is not much of a problem again, with only yearly simple
There are numerous tank coatings one can apply to the tanks. I don't like Kreem, Caswell's is OK, as is POR15. A proper coating job is VERY labor-intensive (preparation), and if not done properly, is a WASTE of all that effort, and can ...fast!....kill your exterior paint. I have had VERY good results using the vastly simpler techniques I describe here. EVEN on tanks that have the red interior lining flaking off. The big problem with tanks is not some red lining peeling off from the tank walls....it is deterioration at the tank bottom. It is water that is the worst offender. NO QUESTION that sometimes a coating is the preferred method. Be SURE to prepare the tank properly...do NOT SKIMP on labor...you will likely need to spend more time on preparation than the manufacturer of the coating says.
Rust-forming water accumulation is GREATLY reduced by refilling the tank before parking the cooling Airhead overnight, where dew could collect INside the tank, if it is not full! The dew collects, runs to the bottom in droplets, and then you have water in the very bottom, and with oxygen that gets in the fuel, etc., rust begins at the slightest microscopic place the tank bottom is not protected.
is possible to use other products and methods to clean or treat the tank insides.
You can also use the reverse-plating method using sodium
carbonate and water to fill the tank (obviously, first close off
the petcock outlets), then fit a big cork (OR?) to the top opening, with
a piece of iron, rebar, etc., going through the cork and into the
solution...but it does NOT touch the metal tank. Use
a DC power source to remove the rust. You can finish by
using a zinc rod (maybe an old carburetor bowl or pieces of one,
and reversing the power leads. You will actually zinc plate the
inside of the tank, which is a very good protectant against rust.
I suggest you look up this type of cleaning and plating via an
Internet search (Google.com is your friend here). Don't use the phosphoric acid method if you plan to do these methods.
SOME folks will purchase a small plastic bottle of gas or fuel-line treatment....the type that is for preventing water from freezing in the fuel system of cars. This stuff is usually a very dry alcohol. Some will clean their tanks with this stuff, using it as a solvent to wash the insides.....and to pick up any water.....and they then drain it from the petcock area. This does work fairly well, for a cleaning....and no other solvents nor detergent/water nor acid treatment is done. If you are a regular tank cleaner type of person this may work fairly well, although I am not so sure that it would not be a bit harsh on the tank liner. But, I have some customers who have been doing it for many years without major flaking. I'm not convince it is at all a universal answer.
The method I use every few years on my own tanks is to remove the petcocks, thoroughly clean what I can out of the insides with a strong detergent solution, preferably one that is NOT phosphate-free, then flush it really well, corks into the outlets, and then put in a quart or so (pint each side of tank) of common aluminum screen door, or galvanized flashing, etc., product, various names, all containing a weak mixture of phosphoric acid. I let it sit from mid-morning to afternoon, or overnight. I then remove the acid mixture and put it back in the plastic container for another use (two or three times seems possible, depending on the amount of rust in the tank), I flush the tank, then dry it, as noted earlier, over the house vent outlet, right side up, with the air going into the petcock holes. Takes a day or three. The phosphoric acid is simply a liquid form of naval jelly. It will convert rusty surfaces to a gray color, generally, and that colored layer, thin as it is, is very protective against further rusting. It works well for this at the tank bottom where the serious rusting begins (and ends with holes, if you let it go). Yearly, clean the tank, but there is usually no need to repeat the phosphoric acid, or not for a long time. This stuff is not a sealant/protectant like POR15, but it works well, and is vastly easier to do the job.
7. Alcohol in fuels
A lot has been printed, a lot said. It is bad for hoses and other parts including plastic types of gas tanks. BMW has had bulletins out about the dangers and problems of using gasohol. We can't get away from it in the USA. Here is a link to an article, for boaters with fiberglass tanks. It explains a lot, if you read it thoroughly. I could cite a LOT of articles about problems from alcohol-laced fuels. Just be aware, that even if you have a steel tank, the lining may be damaged over time, and alcohol-laced fuels are likely to deteriorate MUCH faster and attract condensation of water easier which rots the tank bottoms. In addition, alcohol causes carbureted engines to run leaner, unless you change carburetor adjustments including jetting and needle position, ETC. Common old style rubber fuel lines used on Airheads deteriorate faster. Same for lines on the injected K bikes.
Here is the article about boat tanks: http://www.boatus.com/seaworthy/fueltest.asp
8. Fuels, in
Summer and Winter fuels vary not only for miles per gallon, but for volatility, ease of starting, and the energy content can vary up to about 7%. A GRAM of LEAD will raise the (R+M)/2 value of octane by about 6 numbers. Pure hydrocarbon gasoline, NOT the type with any alcohol in it, can only hold about 0.15 teaspoon of water in each gallon. In other words, water hardly dissolves in pure gasoline. But, if the gasoline has about 10% by volume of Ethanol, a very common additive used for oxygenation purposes, then that fuel can now hold about 4.0 teaspoons per gallon. The problem occurs if just a bit more water is introduced, even by overnight condensation, over time. The gasoline, water, and alcohol can then all SEPARATE, and when that separation occurs, water and alcohol sink to the bottom of the tank....which hardly does any good to steel tanks, and might be hard on the engine if it gets into the engine. The fuel characteristics also change even if that does not happen!
RON specification, in layperson's terminology, is the specification most concerned with LOW and MODERATE power usage; and for potential for engine run-on, also called dieseling. MON specification, in layperson's terminology, is the specification most concerned with HIGH power operation (Knocking).
(R+M)/2 is a compromise specification.
Rule of thumb:
RON is USUALLY 8 to 10 higher than MON.
USA sold 87 octane fuel is approx. 82 MON; and 92 RON
While engines vary, and most very modern engines with fuel injection now have the ignition automatically retarded, and thus the maximum power produced is lessened, if the fuel's octane is lower than the designed-for octane....this is not so for Airheads.
Here is a small chart of the MINIMUM NEEDED RON (remember, RON is perhaps 5 or 6 points HIGHER than USA marked pumps which are (R+M)/2. So, the values below can be REDUCED by about 5 or 6 points, for what YOUR engine is LIKELY to need, for USA marked pumps. Keep in mind that your engine LIKELY has a HIGHER compression ratio than its mechanical calculation...due to carbon buildup, etc. Keep in mind also that RON is NOT done by the most stringent testing; RON is for low and medium throttle/power settings. It is MON that is for high power output. Remember, that in the USA, ROM+MON divided by 2, in other words, an average, is on the USA pumps.
Compression Ratio Needed RON (remember, for USA marked pumps, subtract about 5 or 6 points)
NOTE! The above chart is for a near perfectly machined and clean and smooth combustion chamber, under near ideal conditions, and with single plug per cylinder ignition. For a dual-plugged ignition, might be best to use the same figures and just know that the near perfectly machined and clean and smooth combustion chamber no longer has such strict needs. NOTE that if your Airhead had a stock 8.2 CR, and you milled the head a bit, and have a CR of, say, 8.8; then you need at a minimum, assuming jetting for BEST POWER (this is a tad rich), USA marked pump fuel of about 87-88 octane. This tends to work out quite accurately....if you have dual-plugging. It is marginal, but doable, with single plugging.
I've been asked about how much lead (TEL) was used in the leaded car gasoline's we used to have, years ago. It was up to 2.4 grams per gallon. Aircraft piston engines had, generally, twice (or more).
There is a fair amount of information scattered throughout this article of mine, that covers leaded fuels: valves.htm
There is a good discussion of compression ratios, head milling, cylinder shaving, ETC., here: dualplugging.htm
other tanks to your airhead; SEATS....etc....:::
Many a time someone wants to install a fuel tank or seat or both that is not the original version. There is often confusion on what fits what, and what the PROBLEMS are, if any, when fitting some other tank or seat
There were a number of various /5 tanks; see much further below.
Just before the disc brakes
came out on the /6, BMW modified the underside of the fuel tanks
to allow for the master cylinder that was coming.
The 1973-1/2 (+-) tanks did have the underside cutout for the MC, but the MC did not actually appear until the 1974 models.
(The first disc brakes were the ATE brakes. There were two types, the swinging type, and a slightly later fixed type that somewhat looked like a Brembo, but with ATE's name on them). With the front Brembo brakes came the MC on the handlebars. The reason for the bars Master Cylinder is that in the same era, the pad under the fuel tank where the ATE MC had been mounted was then used for the ignition module. There are some variations.
A /7 tank will fit on a /6. But, best to use a /7 seat. In general...some tanks when substituted, depending on what tank, will require, or might be nice, to have a shorter or longer nose on the seat. The seats themselves may look similar at first glance, but may not be. As example, the /6 tanks have the mounting pins facing each other; the /7 and later are facing the rear. The later tanks have the hinges welded to the steel pan. The early ones had screws, and you might have to unscrew them to get the seat on and off. Because of these variables, frozen screws, etc., many converted the seat to a LIFT OFF style.
Many questions are often asked about tanks. There were a LOT of variables, and although this is not information pertinent to fitting them (note: all /5 tanks fit all /5)...here is some detailed information:
1970-1971: a larger tank was available, and had either 6 or 6.3 gallons, the books are unclear as to official value. In 1972-3 this larger tank was no longer stock, but a special order.
1970-1971 fuel caps were hinged at the FRONT; 1972-3 caps were hinged at the REAR.
1973-1/2 (+-), that underneath master cylinder cutout, as mentioned above, appeared.
Standard tanks had black knee pads.
1972: "Toaster" tanks were STANDARD for U.S. shipped bikes. They had NO pin striping, and this tank WAS available for 1973.
Early 1973 tanks had the chrome panels, but ADDED pin stripes, round the panels. There were rubber pads available for the small tanks after the chrome panels were discontinued.
POLICE (Authorities) tanks look like the /6 tank, even have the rubber pads, but ON TOP there is a LID.
/5 tanks have SCREWED-on Roundels.
GS or R100R tanks WILL fit a R80ST, will add 5 liters.
R100GS tank WILL fit a R80G/S
A PD tank adds 6 liters over stock.
A GS tank, on a ST, needs rubber 16-11-2-307-014, plus fuel splitter and hose...or, block off one petcock outlet. With this arrangement of GS on ST, will be a small seat to tank gap.
The early R90S had a 24 liter tank, with a RAISED filler cap.
YES, you can modify the later tanks with the flapper restrictors, to increase fuel capacity. Remove and plug the crankcase downpipe in the starter motor area. You can remove all the solenoids, etc...too. Do it properly.
With minor work, you can interchange these seats: R100S, RS, RT, from 1977-1984.
AND, R100 and R80, T, RT....from 1981-1984.
Seat dimensions: I have only one I've
measurements for here:
1977 R100/7: 29" front to rear; 12-1/4" widest point; 10" from front lip to seat post.
10. Throttle and choke (enrichener)
cables: This section is not concerned much with what
cable fits what bike with what handlebars. See your
Dealership, and, see Anton Largiader's website on
CABLES. But, here is some information on LENGTHS,
etc., that are not always easy to find. Lengths of BMW
cables MIGHT be printed on the cable sheath. USUALLY the
cable assembly part number IS printed on the cable sheath.
32-73-1-242-135 528 mm long, for 40 mm carbs
32-73-1-454-584 1158 mm long
32-73-1-454-090 1130 mm long, and is the left cable on such as a 1989 G100GS
32-73-1-454-091 1165 mm long, and is the right cable on such as a 1989 G100GS
32-73-2-31--827 1143 mm long, and is the left cable on such as a 91-95 R100PD using 40 mm carbs
32-73-2-311-828 1178 mm long, and is the right cable on such as a 91-95 R100PD using 40 mm carbs
See www.siebenrock.com for more control cable information
11. Fuel hose:
Fuel hose is made in many sizes. For our motorcycles, fuel hose has been used in stock 'nominal 6 mm' size, and some have used 5/16" and 1/4" sizes. The OUTSIDE diameter also varies...and the American, SAE type of hose may not fit easily into the crossover area of the airbox, if you have that crossover hose.
Just because a hose has a nominal inside diameter, does not mean it fits, installs, nor 'pulls off' the
same as other hoses. This includes hoses with the exact same size specification, and it includes 1/4" and nominal 6 mm hose which are almost exactly the same mathematical size. In other words, while size MAY matter, it also may NOT.
Hoses vary in materials, and how constructed. The hose may remain on the spigot/nipple from
friction of the nipple ridges against the mostly unchanged inside diameter. It may remain on from
the nipple ridges & actually making very distinct depressions to match, on the inside of the hose.
A fairly stiff hose material can get some goodly ridges, and need a lot of force to remove the hose
(unless, usually, you pressure the END of the hose). Some fuel hoses are made of two types of rubber in the same hose, with distinctly different
Hoses made of the wrong material to match gasoline may SWELL, and may come off, or eventually
split....but, most fuel hoses seem to harden, over time. Some take many years to harden noticeably.
I have done plenty of hose testing, and I posted an abbreviated summary (verbose as that was).
You CAN get away with most any fuel hose, for awhile. Just how long you want your fuel hoses to
last........or how long before rubber particles start causing problems, or how original you want
them to look........and how easy to fit....particularly if you have a crossover hose at the airbox
area........are questions for you.
Hoses should almost never be 'pulled off'.
The old fabric covered hoses were made in at least two types, and at least one of them would act
like the Chinese finger torture toy, if you pulled on it, it got tighter and tighter.
A lot of more modern hoses are somewhat like that too....the hose tightens while pulling on
it....until your brute force pulls the hose off...or damages the nipple...or?
The PROPER method of removing a hose is to have a tool on the petcock side of the end of the hose,
and push on the tool while gently pulling on the hose. That eliminates any tightening of the hose.
An old hint is to put a large 'fender washer' on the spigot, before pushing on the hose. Then, when
you want to remove the hose, you push on the fender washer while the hose pulling force needed is quite modest.
The original silver colored braided outside fuel hose is still available, from such as Bing Agency in the U.S. The new-style BMW black hose from your BMW dealership is a DIN specification hose and fits better, particularly the cross-over at the aircleaner housing, where American hose is too large in diameter.
Old Volkswagens used a fuel hose that you may find easily: it is a 7 mm ID hose (x 2.5 mm wall). It was used on 60's and 70's, etc., VW's. The VW number is N-203571; sometimes shown as N203571 or N-203 571. It has a list price of maybe $14 per METER, but is available at half that, or tad less than half. There are problems with modern fuels and fuel hoses. When fuel hoses deteriorate, they often release almost microscopic particles of various rubber and other compounds into the fuel, and these can cause problems with the carburetors, INCLUDING overflow of the float bowls due to failure of the float needle and seat to fully shut off the fuel. These problems have WORSENED over the years since ALCOHOLS have been put into fuels (not to mention some of the nastier other stuff.
It is not easy, unless you can find out what the INNARDS of the fuel hose is made of. For K bikes, the fuel pump and some immediate hoses, are all INSIDE the fuel tank, covered by fuel. That is an even worse condition.
In general, fuel hoses used to be made of nitrile rubbers, usually called NBR or acrylonitrile butadiene. These can be softened and will deteriorate in the presence of alcohol. Another type of hose is EPDM, which is ethylene propylene diene monomer. This hose will not stand up to most gasoline's, over time.
The best fuel hose for an airhead USED TO BE, but it certainly does not look original, is a fluorohydrocarbon elastomer, (FKM), such as Dupont's VITON. It USED TO hold up for many dozens of years, wouldn't harden appreciably. It came in various colors. I can no longer recommend this hose due to changes in composition of pump gasoline's.
Fuel hose for Fuel Injected bikes (K bikes) is special...see my K bike section of this website.
The best fuel hose I have ever tested is Tygon type F-4040A. Holds up to everything and does NOT harden much at all.
One that did NOT pass my tests (various solvents, gasoline components, ETC...short term and long term) is Masterflex "FDA Viton" 96412-D
02/06/2004: Add dell.htm
03/14/2004: add #4
10/08/2004: add #5
11/27/2004: add #6
10/28/2005: Expand #2
08/23/2006: Add information on fuels, CR vs octane, and much more.
11/04/2006: more TEL information
04/14/2007: Add #9
04/19/2007: renumber this article from 12 to 12A
06/25/2007: add item 10
11/29/2007: add item 11
01/07/2008: remove bad hyperlink for Heinrich items, was #7
02/18/2010: add more information on fuel hoses
02/16/2011: add link for more information regarding alcohol and fuel tank problems; adding previously blank item 7... and a few
minor other things.
07/21/2012: Remove recommendations on Viton hose; add Tygon and add Masterflex FDS Viton comment
09/28/2012: Add QR code; add language button; update Google code; clean up article a bit and update article with regards to
alcohol, and deleting mention of MTBE which is no longer used in fuels
04/23/2013: Add more hose information
08/01/2013: Edit tank cleaning section
01/21/2014: A bit more editing on tank cleaning methods, and add caution about FI submerged pumps tanks.
© Copyright, 2014, R. Fleischer
Return to Technical Articles LIST Page
Return to HomePage