BST40 carb leak when it's cold

[u/unicyclemos]

Hey all,

I’m trying to track down a weird fuel leak on my 1997 DR650's stock Mikuni BST40 carburetor.

If the bike sits outside in the cold, it leaks fuel. If I park it inside my warm garage, it doesn't leak at all. There is a yellowish, oily gas residue coating the throttle wheel mechanism (where the cables attach) and dripping underneath it. When this happens, the bike becomes difficult to start. It sounds like the starter sometimes seizes up, and I have to put the bike in gear and rock it forward to get it to spin again.

I have an aftermarket tank and a manual petcock that I always turn off, so it shouldn't be a constant gravity flood from the tank.

Thanks for any advice!

[u/Busy-Bread7418]

What might be happening is the cold is causing your seals and covers to shrink causing the fuel to seep in. Might be time for new seals.

[u/jaqrabbitslim → u/Busy-Bread7418]

Agreed, whenever I have carb issues my go to is just to rebuild it with a Mikuni kit. They’re like $30 and it can be done in under an hour

[u/TwistedNoble38]

I would start with a rebuild and go from there.

[u/Amateurknifemaker]

Definitely needs a rebuild kit.