Please note that the above bulletin is supposedly originally just for 1980 metal cover models. It appears that it applies to subsequent models; BUT, BMW made other changes!
R 80 series, 1980+ Airbox Modifications
©
R80, 1980+, Airboxmods.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.
***THERE ARE TWO BULLETINS!!
The FIRST bulletin is #13-003-80 (2021)
Please note that the above bulletin is supposedly originally just for 1980 metal
cover models. It appears that it applies to subsequent models; BUT, BMW made other
changes!
The earliest of the flat style airboxes had a metal cover. The bulletin, unclearly of course, applies to those. Later models had a plastic cover with push-together snorkles, and some models...like the R80 series (and R65...maybe R45?) had UNequal snorkles. 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 cover can be replaced by the later cover, or you can modify your cover.
The only reason to do this is if you do not have the drilled
snorkels; AND, 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 to eliminate a slightly overly rich mixture over PART of the operating range.
That produces poor mileage, and the throttle response is a bit poor.
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
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 August 1984 #13-007-84 (2120)
This is to help what BMW called the part-load power characteristics, and REDUCE fuel consumption. It involved
dealer changing of the aircleaner housing, to the R65 and R65LS type with unequal intake orifices
sizes (snorkels). The cover part is 13-72-1-337-675.
Obviously, there are no new dealer UNdelivered bikes, so, one would
probably not replace the cover, if one can replace less parts! See a
bit later, herein for the parts numbers.
Don't do this unless you have the mid-range and fuel mileage
problems.
What this modification does is to 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.
Those who have these models with EQUAL snorkels: The bulletin says to replace ONE intake
pipe with one having a 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.
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 explanable.
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