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Fuel caps
For BMW Airhead and Classic K Motorcycles
(K1, K75, K100, K1100 ....)

© Copyright 2018, R. Fleischer

Removing/replacing/rekeying other locks in the later Airhead (and Classic K bike), luggage, etc., will be found in Snowbum's article 75A on his website,

WARNING! ....BMW SCREW CAPS SHOULD NOT BE OPERATED LIKE AUTOMOTIVE CAPS!  AVOID spinning these motorcycle fuel caps to the ratcheting point when tightening them! The ratcheting parts WILL eventually wear out, and WILL eventually cause you problems in trying to remove the cap.   BMW Airhead fuel caps are not constructed like car caps. On most modern car fuel caps, the cap is designed to be rotated to the ratcheting point. That is done because the car caps must fully seal a complicated fuel fumes venting system; if a car cap is not rotated enough, the cap can leak fumes, which will likely be detected by the car's computer monitoring system, and a "Check Engine" light will illuminate on the car's dashboard. On our Airhead BMW's, the cap for the earliest Airheads is simply designed to keep liquid fuel from sloshing out. Later models had cap vents and even later ones had simple venting to direct fumes to the crankcase. These last versions had a more complicated fuel cap, called a SHED cap, but it is best not to rotate them to the ratcheting position; even though that is often done by those locking their caps. If you do tighten to the ratcheting point, try to ratchet only one notch.  Much more on these various caps with the ratcheting mechanisms further down this article!

Fuel caps and venting methods for the BMW fuel tanks vary by year & model. For information on the Pulse-Air system, the fuel tank venting system, & other associated items, please read:

It is common to hear about an Airhead "running out of fuel"....stumbling, loses power, etc. The problem will often be the tank vent ...or the fuel cap. Both problems can be identified by loosening the fuel cap and usually hearing a whoosh of air entering.  Within 15 seconds, the engine will then run OK. Engine stumbling from this venting problem usually happens much more quickly with a more-filled tank and also from high speeds.  While this fuel delivery problem can occur on the models with the fuel solenoid mounted on the underside of the starter motor cover, that is less likely.

There can be other types of problems, such as fuel leaks at the caps, etc.

This comprehensive article, below, deals with fuel cap problems, including the lock.

FYI: Fuel cap cork seals are replaceable. So is the black surround on the later caps.

From the /7 onwards, BMW changed to a screwable, removable, fuel cap. There were changes further on all are not the same, even if they look to be so! My coverage here is general, for USA caps, but most of the information applies to foreign-shipped fuel caps too. The caps with the BMW emblem on top may not be the same internally, which affects how you go about drilling, locking, etc. Late model caps that are used with the crankcase fumes capture system, have SHED stamped into the bottom.

The late 1977-1978 (probably some into 1979) gas caps are changed in design; 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, there are two valves built into the cap. These 2 or 3 years of caps are fairly well known to sometimes have problems. Usually the problem is exclusively that of not allowing outside air into the tank as the fuel level goes down, creating a very slight vacuum in the tank. The symptom of this particular venting problem is that the bike starts running very lean, may buck and give the symptoms of being out of fuel. This typically happens after some time on the highway, the time can be shorter if the tank was rather full to start with & if large amounts of throttle are being used.

This remainder of this article is only available for Airheads Beemer Club members. Please Log In..or..Click Here to join the Airheads Beemer Club, after which you will enjoy access this article and all members only content of the ABC web site. 

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