Fuel Leak. Help!

I have a slow fuel leak on my 99 YZ400F coming from the base of the fuel cock where it mounts to the tank. Originally, I repaired it during a day of riding with liquid gasket, that held for the rest of the day which was all I expected. The second time I repaired it with liquid gasket that was made to withstand fuel. That lasted about a week. So I decided to go to the yamaha dealership and get the actual gasket. I cleaned everything off and put it together. It didn't leak at all during the first ride over a week ago, but has just now started leaking again. Did I do something wrong? Is there a sealant I should be using in conjunction with the O-ring? Thanks in advance for the help.


I haven't seen it with a stock tank, but every once in a while you see one like this where the surface onto which the petcock seals is no longer flat enough for the O-ring to compress adequately. In that case, I have taken a large flat file (medium single cut is the best) and worked the area down a little until it was closer to flat again.

I had this same problem sometime back, but all I did was replace that gasket and all was well. You might try a new O-ring and see if that might help. You might want to check the little spring that holds the valve to the casing. I'm not sure how your exact bike is setup, but my 02 426 had a little round bent medal washer that acted like a spring to keep tension between the valve and the casing. Just an idea.

It happened on my 03 - tried to replace oring and worn parts, still leaked. Then I got smart and priced entire petcock assembly.....15.00. It was a no brainer and no more leaks

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