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BMW Airhead Motorcycles
R 80 series, 1980+ airbox modifications; poor fuel mileage.
© Copyright 2020, R. Fleischer
https://bmwmotorcycletech.info/R80-fuel-mileage-problems.htm
9A
Fuel mileage problems, all models: https://bmwmotorcycletech.info/FuelMileage.htm
The information below is for R80 engine'd models. There is some minor/modest applicability to other years and models. Late model R100 engines and R80 engines have different air cleaner intake snorkel tops, and may have unequal snorkel mouth sizes, etc. There seems to be few problems with fuel mileage except on the listed R80-series models.
BE SURE TO READ THIS ENTIRE ARTICLE!
The FIRST bulletin is #13-003-80 (2021). THIS IS A 1980 BULLETIN. It is for 1980 metal cover models, and that is not spelled-out due to the 1980 date of the SI. It may apply to subsequent models. Yes, the earliest of the flat style airboxes had a metal cover. Later models had a plastic cover with push-together snorkels. Some models ...like the R80 series (and R65 ...& maybe R45?) had UNequal snorkels.
Upon receiving some questions from some BMW owners, I experimented on an R100RT, which the bulletin, above, is not for. I found that the holes affected the mid-range somewhat on a 40 mm carbureted R100RT I experimented with, but there was no problem needing fixing 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 drilled snorkels; AND, you have a mid-range flat spot (poor mid-range throttle response), AND, for sure, you have already eliminated every other possibility. I do mean mid-range here ...not 'just coming off idle'. Another indication may be very poor fuel mileage ...if so, also read the second bulletin, below. You should read that bulletin anyway.
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. Note that your airbox cover should be metal. If you have the plastic cover and snorkles, and have holes, modifications, whatever ...someone has played with things.
Some R80 models have excessive fuel consumption, even with the later plastic cover and plastic snorkels. Sometimes playing with the needle position and needle jet does not fix the poor mileage. You can experiment with the snorkel modifications, it may help. There are mixed reports.
Assuming you have no holes, and YOU have decided you need or want to try them: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 OK.
The SECOND bulletin is dated August 1984, #13-007-84 (2120):
This modification has 2 major items. 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 modification to your bike unless you have mid-range and fuel mileage problems .
This bulletin officially applies only to the R80GS, R80ST, and R80RT. I have not experimented with all the R80 models.
What these changes do is to help eliminate a slightly overly rich mixture by the time it reaches the combustion chamber.
The carburetors are 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 carburetor part numbers).
Main jet 135 (13-11-1-256-612_
Needle jet 2.66.
Needle position 3rd from top.
Idle jet 45.
Slide advance 7.5.
It is possible that your R80GS, ST, or RT already has the modifications. Or, some of them. I'd not worry about the slide advance specification; although you could change yours.
For those owners who 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.
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.
NOTE that there was a transition period, May 1983, in which the carbs came with the earlier jetting, notably 150 main jets. This affects 3/4 throttle and higher.
Revisions:
04/12/2003: Incorporate previous revisions, correct spelling errors, clarify
year & changes & emphasis on when to not 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.
10/13/2012: Add QR code, add language button, update Google Ad-Sense code.
Sometime in 2013, removed the language button, due to problems with scripting.
03/18/2016: Updated metacodes, fonts, colors, layout, justify to left.
10/15/2016: Meta's, scripts, excessive html, H.L., border.
04/03/2018: Clean up article. Reduce HTML, fonts, colors. Nicer layout, etc.
© Copyright 2020, R. Fleischer
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