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Old 03-06-2011, 12:16 PM   #1
Rapid Dog OP
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Question Airhead gas cap venting...how to fix?

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...?
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Old 03-06-2011, 12:24 PM   #2
princess jamaica
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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.
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Old 03-06-2011, 12:56 PM   #3
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Question

...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?
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Old 03-06-2011, 02:29 PM   #4
mark1305
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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.
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Old 03-06-2011, 02:56 PM   #5
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I gave this a try, we shall see what haps GAS CAPS:
Quote:
rom the /7 onwards, BMW changed to a screwable, removable, fuel cap.

See article 1, on fuel filters, petcocks, and caps, etc. for additional information: CLICK

VENTING:
The late 1977 (or perhaps from 1978) gas caps are changed in design, so that they allow air to pass to the inside of the tank, but fumes are not supposed to pass to the outside of the tank. To accomplish this, their are valves built into the cap. The very first of these caps could have problems. The symptom of a venting problem is when the bike starts running very lean, may buck and seem to run out of fuel, and this typically happens after some time on the highway, the time can be shorter if the tank was rather full to start with. OPENING the cap (unscrewing it a bit) will 'fix' the problem within 15 seconds, and this 'test' is nearly 100% for a bad cap vent. The cap can be modified to fix the problem permanently. Put it upside down on your workbench. Drill AWAY from the center, any direction, about 1/2" from the center. Thus the hole you will drill is off-center. Drill, 3/32", from the bottom of the cap, through the metal shell, and continue drilling carefully....go through the air space, and drill into the softer material that you then feel you are drilling through....it is maybe 1/8" thick. Drill through it, and then stop drilling. Clean out the cap as best you can of drilling swarf. Replace the cap on the tank. That's all there is to this.
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Old 03-06-2011, 02:58 PM   #6
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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:
Quote:
From the /7 onwards, BMW changed to a screwable, removable, fuel cap.

See article 1, on fuel filters, petcocks, and caps, etc. for additional information: CLICK

VENTING:
The late 1977 (or perhaps from 1978) gas caps are changed in design, so that they allow air to pass to the inside of the tank, but fumes are not supposed to pass to the outside of the tank. To accomplish this, their are valves built into the cap. The very first of these caps could have problems. The symptom of a venting problem is when the bike starts running very lean, may buck and seem to run out of fuel, and this typically happens after some time on the highway, the time can be shorter if the tank was rather full to start with. OPENING the cap (unscrewing it a bit) will 'fix' the problem within 15 seconds, and this 'test' is nearly 100% for a bad cap vent. The cap can be modified to fix the problem permanently. Put it upside down on your workbench. Drill AWAY from the center, any direction, about 1/2" from the center. Thus the hole you will drill is off-center. Drill, 3/32", from the bottom of the cap, through the metal shell, and continue drilling carefully....go through the air space, and drill into the softer material that you then feel you are drilling through....it is maybe 1/8" thick. Drill through it, and then stop drilling. Clean out the cap as best you can of drilling swarf. Replace the cap on the tank. That's all there is to this.
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'09 450 EXC Nani Roma Limited/'08 R1200R-Gone to the Hipsters/'02 Dirty lil KTM400EXC-gone to a good home/'10 WR290X/R buhbye!
'81 R100RS Rattlecandy Red/'06 950 ADV Orangeboom/91 R100GSPD Derelict

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