R100GS Fuel Filler Flap Removal

If you’ve got an R bike from the ’80s or ’90s, chances are good you’ve doused yourself with gasoline more than once while fighting the flapper that lies inside the fuel filler neck. In the photo above, the flap has been removed, leaving an opening wide enough to admit even the crudest filling device (sometimes found at points south of the border and other likely adventure destinations). The procedure isn’t hard, but does require a steady hand and a little nerve. One slip, and its new tank time.

Start by draining the gas from the tank and removing it from the bike. Remove the petcocks to make it easier to get the last bits of gas out. Wash out the tank with plenty of water to remove any traces of fuel. The last thing you want is an explosion from the sparks you’ll be generating as the neck is cut off. Set the tank back on the bike so it doesn’t move around while you work on it.

I used a demolition saw with a 6″ blade to make three cuts in the filler neck that terminated at the circumferential weld about 1/3 up the neck. The cuts were spaced so that the first piece was small enough to come out the flapper hole when the flapper was pushed down. The other cuts were spaced so that each segment was less than half of the neck wide, again to ease removal. I found it helpful to use leather gloves to protect my hands, and to brace the hand holding the front of the saw against the tank. The blade was angled so as to miss the hump in the underside of the tank, and the trigger pulsed to control cutting depth. The saw had a tendency to grab as it broke through the thick initial ring of the neck, so be ready.

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Petcocks, Fuel Filters, Choke, etc.

Each petcock is fastened to the tank by both right hand and left hand threads. As you rotate the large “nut” CW (facing from below), that tightens the nut to the tank while at the same time it brings up the petcock. The petcock large nut has to be FIRST engaged the proper amount onto the petcock BEFORE screwing it upwards to begin to engage the proper amount of tank threads. If you have never done this before you will have to fiddle with this until you get the right amount of petcock and nut threads assembled …to start …and finish…the petcock-to-tank fastening process. Once you learn, it is easy. You want approximately the same number of threads engaged on both the petcock and the tank after fully tightening.

BMW has used numerous petcocks over the Airhead production years. There are five types of petcocks normally seen on our Airhead motorcycles. All except the /5 bullet types are easily re-buildable. Usually this means that you unscrew some knurled or slot “nut” at the handle area (after removing a black trim cup, if it is there on your version), and you can replace a gasket that might be bad, or clean and very-faintly-lightly silicon grease the moving parts. You then have a much smoother operating petcock. On some there are one or two dimpled discs, which can be repaired with a shaped punch, lightly hit, for a better detent action; I usually don’t bother. These discs have a locating tab. They can confuse, so take notice of how they were assembled as you take things apart. You cam figure it out, if you forgot to take notice.

Our petcocks have a Reserve function. Except for the bullet type, photo below, the handle long portion upwards towards the tank is the reserve position. Horizontal (either way) is always off.¬†German “AUF” printed on the body/cap means on, not off. Handle lever downwards means on for the main part of the tank.

Early /5 Bullet Style Everbest Petcocks were different in how the handle indicated fuel flow, & the rotating handle affecting gas flow from was in the opposite way from standard piping flow.

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