BMW Airhead Motorcycles:
R 80 series, 1980+ Airbox Modifications; poor fuel mileage
© Copyright, 2011, R. Fleischer
R80-fuel-mileage-problems.htm-9
The information below is for the R80 models, especially GS, ST,
RT.
There is some applicability, not a lot, to other years and models, but
I have seen NO help for R100 series engines.
The FIRST bulletin is #13-003-80 (2021):
Please note that the above bulletin is
typically taken by many to be just for 1980 metal
cover models; although this is not spelled-out.
It also would seem to appear that it applies to subsequent
models; BUT, BMW made other changes!..and NOTE that the SI was
dated in 1980!
NOTE!....the bulletin says "all models".
The ABOVE bulletin is, in my opinion,
WRONG, in that regards.
Background....
The earliest of the flat style airboxes had a
metal cover. The bulletin, unclearly, applies to
those. Later models had a plastic cover with push-together snorkels.,
Some models...like the R80 series (and R65...& maybe R45?) had UNequal
snorkels. I found that the holes affected the mid-range
somewhat on a R100RT I experimented with (40 mm carbs), but there was NO PROBLEM
to cure on that model, the mid-range being fine as it came from the
factory.
The early R80 series metal cover can be replaced by the later cover, or you can
modify your cover.
The only reason to do any of this is if you do not have the drilled
snorkels; AND, IF you have a mid-range flat spot, AND for sure, have eliminated every other possibility!!! AND, we do mean mid-range here,,,,not 'just coming off idle a tad'.
What this modification does is fix a poor throttle response; or,
at least that is supposed to be what it does.
Look at the intake bell-mouth of each snorkel. If, on the curve of the bell-mouth, there are 4 holes, usually it is 2 on top, 2 on
bottom, of each snorkel bell, then go no further, yours are modified
(you might want to read on, see what it is supposed to look like, and to see the
next following bulletin). Your airbox cover is also metal. Or someone has
played with the plastic model.
Assuming you have no holes, and YOU have decided you need them:
Drill 4 holes in each snorkel. The drawing is unclear, but it looks like 4 mm diameter holes. That is the same as 5/32 inch. You may
want to find a copy of the bulletin and make sure ...but I hardly think it is 0.4mm!!
Drill 2 of the holes on the top, 2 on the bottom, of the snorkel INLET BELL, at the curve itself. If you were to follow the bulletin,
the angle of the holes is 30° from a flat plate placed across the bell mouth. The two top holes are each 24 mm apart from each
other, and equally placed about a centerline of the top and bottom of the snorkel. 24 mm is almost the same thing as 1 inch, and 1 inch
can be used. The bottom holes are also 24 mm apart.
Restating this, find the center, horizontally, of the WIDTH of the snorkel, near the bell (forward in the bike) end. From that point,
scribe a mark 1/2 inch to right, 1/2 inch to left. Drill at that distance, in the tight curve of the bell-mouth of the snorkel. Smooth
the edges JUST BARELY (no countersink please) enough to remove the swarf. The 30°
is not critical. An adequate method of determining 30° for this purpose is to hold your drill such
that it is across the bell opening end. That is zero degrees. 90° would be rotating the drill such that it is in line with the
direction of the flow down the snorkel; half that is 45, so 30 is a tad less than that. As I
said, nothing critical here. If you want to use an old school protractor or something, fine, go ahead.
You will have more of a problem trying to hold the drill (center punch it
first), so approximately 30° is just fine.
The SECOND bulletin is dated August 1984 #13-007-84 (2120):
This modification is TWO-fold. ONE, it announces a change
to production bikes;... to help what BMW called the part-load power characteristics, and REDUCE fuel consumption.
It involved
a change of the air-cleaner housing, "to the R65 and R65LS
type with
unequal intake orifices
sizes (snorkels)." The cover part is 13-72-1-337-675.
It also included a change to the carburetor jetting, and
therefore a new carburetor number was assigned.
Don't do this as a modification to your bike unless you have the mid-range and fuel mileage
problems.
NOTE that this bulletin applies only to the R80GS, R80ST, and R80RT.
What this snorkel change does is to help eliminate a slightly overly rich
mixture by the time it reaches the combustion chamber....[..and you thought only jets and needles affected this type of thing!].
The carburetor parts are also changed as follows:
left: Bing V64/32/351 BMW 13-11-1-337-811
right: Bing V64/32/352 BMW 13-11-1-227-812
(the jetting is changed, hence the new part numbers).
main 135
needle 2.66
position 3rd from top
idle 45
slide advance 7.5
It is possible that your R80x already has the modifications. Or, some of them.
For those who already had the
motorcycles and complained: The bulletin says (not a
warranty item) to replace ONE intake
pipe with one having an INTERNAL cross-section at the orifice of 30 x 19 mm.
The part is:
13-72-1-337-501.
The main jet s/b changed and that 135 jet is
13-11-1-256-612.
There seems to be few problems with fuel mileage except on certain R80 models, as listed.
A few have had good over-all results with mileage and power, by keeping the equal snorkels. There seems to be some variances not easily explained.
Revisions:
04/12/2003: incorporate previous revisions, correct spelling errors,
clarify year and changes and emphasis on when not to try the modification,
reverse order of bulletins, add .htm title.
08/07/2005: add jpg of 2021 bulletin
12/01/2009: clean up article somewhat, clarify a few
details.
12/01/2009: 7:36 pm....clarify just exactly what models are
applicable and fix some confusing information.
12/16/2010: Change URL of this page
© Copyright, 2011, R. Fleischer