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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: tweather on July 01, 2025, 06:11:08 PM
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Hey y’all, I took my bike out for a couple hours today. As I was coming down my street to park at home, I began to smell gas. Once parked, I saw gas running down the back right side of the tank. However, once the gas evaporated, it appears there’s no further leaking.
Any ideas why this might happen? Should I just assume gasket on the bottom side of the tank?
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Fuel was running down the right rear side of the tank but not just dripping from the bottom rear seams of tank?
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Back right area, underneath the tank is where I saw gas.
In my haste yesterday, I misspoke a bit. Once parked, I didn’t see it running down. I saw it had been coming down, but no indication it was still coming down. I saw it on the air accumulator, if I’m identifying the parts fiche correctly.
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Here's my shotgun approach for a response.
Some common places where fuel leaks can occur:- The seamed joints along the lower edges of the tank. These can leak because fuel has been allowed to age long enough within the tank for its components to separate and allow fungus to be created. Fungus corrodes the metal and eventually creates pinholes in the tank seams and edges where leaks will occur.
- The fuel level sender mounting plate inside the tank. In models later than mid-'85, the fuel level sender is mounted on a rectangular plate connected to floor of the tank by four screws. The interior joint of the plate with the tank is sealed by a rubber o-ring around the base of the fuel level sender. The o-ring can disintegrate enough for fuel to leak past the ring and out of the tank. Some times fuel can even leak at the point where wires pass through the mounting plate to connect to the sender, although type of leak seems relatively rare.
- The fuel tank cap. If the fuel tank is filled up to, or over, the level of the flapper in the fuel tank cap assembly—or if the flapper assembly has been entirely removed—fuel can expand with enough vapor pressure to leak out of the cap and down the sides of the tank during hard riding, hot weather riding, or when the tank is heated by sunlight during hot weather when parked—especially if the cap gasket is old and has been subjected to drying out. BMW created a service bulletin showing that the lower vent hole in the interior rim of the flapper assembly should be drilled out from 3mm to 8mm to lower fuel pressure during expansion. A link to the bulletin is below. I did that modification to my Brick. It lessened the amount of vapor pressure suddenly expelled from the tank when the cap was opened after hard riding or during warm weather. If the tank cap doesn't have that flapper assembly, the tank should probably not be completely filled to capacity and, perhaps, a second gasket should be added to lessen leakage from around the cap.
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Finally getting around to this.
Looks like the culprit is heat, no flapper, and the vent tube isn’t connected. About to connect that, put some gas in her and take a spin.