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Yamaha Petcock Partial Rebuild


After getting the prettied up tank onto the XS400 we found a fuel leak.  At first the theory was a leak up around the mount point of the petcock, but further investigation found that it was a passage way on the diaphragm assembly that was causing the problem.

It’s difficult to see from the image, but the diaphragm is all on the outside (right-side in the picture).  This was basically allowing fuel to make its way to the passage and puddle onto the bike.  Taking the vacuum side apart wasn’t too difficult, just four screws.
Basically you’ve got the diaphragm block, diaphragm, spring and the valve cover.
The diaphragm is actually made up of two thin sheets, each goes on one side of the diaphragm block.
The piece put back together, now the diaphragm is covering both sides and should be good to go.  All reassembled it’s hard to see the overall difference, but having the diaphragm on both sides did the trick.
No more leaks, at least not from the petcock.