K100 Gas Cap leakage?

[GJBushman]

I was fortunate to take ownership of a 1985 K100RT a few months ago. My intention when I bought the bike was to use if for winter commuting. My 1150GS with stock windshield just doesn't offer enough protection against the sub freezing temps. Now that the cooler weather is approaching I have started to use the K100 for my commute. I enjoy the bike tremendously. Riding the K bike on a regular basis I find that my tank bag (BMW) now has developed a strong smell of gasoline on the bottom of the bag. I really noticed this when I brought the bag into the office today. I had noticed a slight smell when using the tank bag over the summer. Now the smell is very strong. Looking at the gas cap I do not see any leakage. I have removed the bag and will go without it for a few days to see if I see leakage. The smell only seems to be on the bag. Does this gas cap vent fumes? How would I check for proper tank venting?

[Bobmws]

You can (or could) buy a kit to renew the seals in the cap. I believe the 85 used a different cap though, check with the dealer for availability. It shouldn't be too hard to locate an cap from an 86 or newer model.

[roy]

Does this gas cap vent fumes? How would I check for proper tank venting?

GJ, there should be iether a hose going down from the rear side of the tank or posibly a little plastic funnel attached to the frame member for overflow/vent. I believe you can see these by removing the right side panel & if not remove the bolts that hold the tank on and lift the rear of the tank up slightly. This will allow you to see if the hose is pinched.

I am not sure if the cap vents on this model.

Good luck

[deilenberger]

GJ, there should be iether a hose going down from the rear side of the tank or posibly a little plastic funnel attached to the frame member for overflow/vent. I believe you can see these by removing the right side panel & if not remove the bolts that hold the tank on and lift the rear of the tank up slightly. This will allow you to see if the hose is pinched.

I am not sure if the cap vents on this model.

Good luckThe cap isn't vented by design (it may be vented due to old-age) - the tank vent is a pipe located at the very top of the tank, that runs internally to a small pipe at the right bottom corner of the tank, connected to a hose as Roy explained.

Bigger question is - what cap do you have? Does it open ONLY with a key, or does it have a big pushbutton in the center?

If it's the pushbutton one - plan on replacing it with the later key-only cap. The pushbutton one was known to leak in and out. You can probably find a good used later model cap assembly for cheap at a salvage parts place.

If you already have the key-only cap - replacing all the rubber seals on the cap should help. The topcap - mushroom shaped thing that holds the seals together - DOES come off with some persuasion. Looks like it doesn't but with a bit of force it does. There is also a foam rubber seal under the latch assembly which must be unscrewed from the inside of the cap.

Replace the seals and you should be ready to go.

One other thing - if you don't have a flapper (if you can see right into the tank you don't) - don't overfill. I fill my K75S on the sidestand until the fuel just reaches the inside of the cap. When the bike is upright - that leaves about 1/4-1/2" of air between the fuel and the opening. If you overfill the tank, the vent for it will be submerged, meaning it won't vent correctly and it will tend to have raw fuel going into the vent pipe.

[roy]

GJ, another thing to check for the vent.

With the cap open, look at the mounting ring where the cap assembly bolts to the tank. When looking at the cap as if sitting on the bike, ther should be a small hole in the 9:00 position. If a prior owner has removed this cap for a filter change and put the gasket in incorrectly this could also block the vent line.

Roy