Hello Thumper Enthusiasts. First and formost, I just want to state the obvious and say what a great and informative site this is. Now, down to my problem.

A few days ago my 98 yz400f began quitting on me after running absolutely normal for approx. 10 minutes - simply acting like it has run short of fuel. when I try to restart, it will only start with the choke on, and will not accept and twist of the throttle for approx. 1 minute until it clears up, and runs normal again. At this time, I can turn off the choke, and proceed normal for the next ten minutes or so. During a crash a few months ago, I bent the choke plunger , and straightened it by hand and kept riding. The plunger is very wobbly. I wonder if it is not sealing properly. i am fairly certain that the float height is correct. Over the winter, I installed an R&D P-38 Carb Pump Plate Kit, and have had no trouble with it. I have ordered a new Accelerator Pump Diahpragm and a new Choke Plunger Assembly.

Someone at Rausch Creek Motorsports Park near Tower City told me to check the little check ball on the side of the cylinder. They suggested lubricating it, and that if it was not free, or if it was dirty, that it might cause my symptoms.

Anyone have any suggestions.

Thanks, Ian :)

Not to scare you but there is an outside chance it could be an ignition problem, but probably not considering the fact it will start with the choke on.

I would suggest two things to check:

The fuel tank. It may have trash in the bottom and is blocking off the fuel supply after you have been riding for a while. That might explain why the bike will only start and run with the choke on but won't rev. After you have kicked the bike a few times and sloushed the fuel around it would clear up and you could take off again. This happens a lot on cars because they have a fuel pump which draws a vacum on the tank, as soon as the engine dies, the vacum is released and allows the debries to float away.

Anyway, check for garbage in the float bowl and make sure the jets are clean (don't use no wire!) sqirt carb cleaner through the jets and verify they are clear.

Hope this helps.


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