Are you sick of only getting 100 miles out of your K100 gas tank before the yellow low fuel light comes on? If so, this may help. I machined an aluminum spacer that fits between the gas tank and the gas cap that allows you to fill the tank up almost all the way. This allows me to go ~135 miles before the yellow 7 liter warning light comes on. This effectively added ~20-30 miles to my range.
I've seen other spacers that perform the same function, but mine re-works the vent system so as to minimize the amount of overflow gas that spills out of the drain hose. Basically, the vent is jacked up higher in the gas tank so that the sloshing gas doesn't reach it as easily.
There are two vents that I re-routed. One is the overflow vent inside the gas tank that spills out gas if the tank is filled above a certain level. I never could understand what the purpose of this feature is. I capped this off underneath the tank with a machine screw and some Teflon tape. I used the vent on the surface of the gas cap recess on the tank for my new gas vent line. The spacer would ordinarily block this, so a new vent passage is drilled through the spacer. On the inner surface of the spacer, the vent should be angled up so that it's a little harder for liquid gasoline to escape. See the included photos for more details.Specifications, aluminum spacer:
I made this spacer using a hole-cutter on a vertical milling machine, and the vent holes were cut on a drill press. The angled vent hole is a bit tricky to cut on a drill press- the tool wants to wander down the angled workpiece. Use a punch to start your hole, and take your time.
You will need to make a trip to the hardware store for four (4) significantly longer gas cap screws. Also, I chucked the stock rubber gasket that originally went between the tanks and gas cap. The rubber stuff was allowing gas to leak between the tank and spacer. I instead used some thin (0.050") gasket cork that I bought in a roll at Pep Boys. I cut my own gaskets to fit between gas cap and spacer, and also spacer and gas tank.
After you're done with the mod, fill up your bike and ride it 10-20 miles. Turn it off and quickly open the gas cap to make sure that the new vent is working correctly and that there's not tank vacuum (indicated by a sucking sound when you open the tank).
After all of this, my gas range is close to the 175 mile mark. Woo hoo! Now if I can just get that Tijuana doctor to perform the bladder enlargement surgery...
Disclaimer: gasoline is dangerous stuff. Don't half ass this mod or you might find yourself looking like a burnt piece of cinnamon toast. Perform this mod at your own risk.
IBMWR, BMWMOA, MGNOC