1981 RS Been reading a lot of threads about the gas cap not venting but haven't found one that says what to do about it. I was riding yesterday and a couple timed while cruising at 3000rpm the bike started acting like it was running out of gas. This has been only one of the issues with the bike and I've addressed all the fuel and ignition issues. Complete new ignition, rebuilt Bings etc etc etc. So, I tried unscrewing the gas cap while cruising along and it cured the problem, several times in fact. So how do I fix the gas cap? Replace it? I'm certain this is the issue. I noticed pressure build up when I had stopped for gas at the pump when releasing the cap like on a car. I wonder if this ay be some of the reason I get a lot of popping when down shifting...?
i've had a similar experience on my '83rs.i was running a higher rpm(4500)and the engine stumbled.i backed off a bit and it corrected itself.i thought it was fuel or fire.never touched the gas cap.it did it again a couple of times on that ride.i'll try the gas cap next time.
...incidentally, I'm having a constant problem with the right side carb overflowing. I checked the float several times. Have new float needle valves and all. I figured maybe the tiny safety clip may have been interfering with the needle so left that out. So good so far, but I wonder if the carbs are being pressurized by the tank...? That sound crazy?
Two ideas on the cap: (1) The vent could be plugged (duhh). Is the bike new to you? Or is the cap newly replaced? Does the lock work, or has it been "fixed" by epoxying the lock cylinder in place either by you or a PO? (My ST had the epoxy job, but the PO did punch vent holes). (2) Could the cap have been replaced with one of special ones that go on the special emissions equipped tank systems? (Can't remember the acronym for that system, but have seen them in the Max BMW fiche). Either situation could be remedied with judicious application of a 1/16 or 3/32 drill bit in the appropriate spots to open plugged vent holes or create new ones. I'm saving my good BMW locking cap on a shelf and running a Stant locking cap for late model Ford/Mercury cars. Becasue its from the vapor-tight emission era of today, I had to apply said drill bit up through the bottom and put about 3 holes up through the inner core which hold whatever kind of check valve paraphanalia lives in there. Works great. Some anecdotal info only slightly related to the topic for those interested in using the Stant automotive caps - The threaded portion of the car caps like on my Town Car gas cap are identical to the BMW caps. The gaskets at the top of the threaded portions are identical. For bikes that have the outer trim ring seated in a groove on the BMW caps like my ST, 1/4" wide self adhesive weatherstripping wrapped around the outside of the cap with enough wraps to make a friction fit when slipping the trim rim over the bottom will hold it in place just fine. And if you wrap it just below the top of the cap the trim ring sits correctly and the weatherstrip is unnoticeable. And a side note: While I was playing with the car's OEM gas cap and sorting all this out I discovered that the Lincoln gas cap turns in its little plastic tether which is fitted into a groove identical to the groove on a BMW cap which holds it trim ring. I came really close to going and buying a Genuine Motorcraft cap from a dealer, fitting the BMW trim ring, and seeing how many people would ever notice the logo. But I settled for the Stant so I could have a locking cap.
Noy sure the PO replaved it but it looks new-ish, no wear. The flip up lever feels more plasticky than the metal one on my 1978 /7. I gave this a try, we shall see what haps GAS CAPS:
Just saw your post. I have the same issue on my R90/6, even at the same rpm. Did you find a solution?
The popping could be an issue with the air/fuel mixture screw adjustment..especially after you rebuilt the carbs...and only adjust when the engine is hot from a 10 mile ride. Lot's of posts here on how to adjust the carbs
My gas cap vents through the key barrel. I just make sure it is not plugged by blowing compressed air through it.
Not yet. It’s frustrating. I’ll have to start over, take the carbs down, clean and check every aspect. I did find that the left spark plug was excessively black vs the other side, so it’s running rich on that side. Not sure that has anything to do with it.
I have just fixed venting of my 1987 R80RT petrol cap which now means I do not have open both petrol taps to stop the motor from hesitating when riding up a hill . I have drilled a 1.5 mm hole through one of the top holes in the side of the cap just under the tank seal ,as near as vertical through the plastic into the area that the ratchet appears to work. I can now run full throttle up a hill with out any hesitation from the motor and motor appears to run better now than it has ever run since having the bike all the time.
If you have external filters take them off and run it. There is a procedure to check fuel flow and sometimes the small filters with brass inserts dont pass enough fuel at higher speed. Even the internal petcock filters can get clogged.
Grungy/rusted closed fuel filler caps is a long standing problem . I remove them if metal and drop over night in a cup of white vinegar ~ works a treat .
