Problem with 1998 yz 400

I am having an unexplainalbe problem with an 1998 yz 400 it seems to run fine but then will just die, you rev it up a few times and then out of the blue when the engine comes down to idle it will just die. Then its a pain to start. We have put every diffrent combinations of jetting in it but i am starting to think it might be something else. Any ideas would be great. :thumbsup:

When it dies, check the spark.

This will not be anything new, but whenever I approach an engine problem, I try to narrow it down to one of three categories: Spark, Fuel, or Compression. That being said, I doubt your problem is compression since it runs for awhile to begin with... This leaves fuel or spark, so I agree, after it dies the next time, check the spark. Check the fuel lines to ensure that there is not a blockage. If the lines are restricted, the motor will run until the bowl empties, then no more fuel until the bowl is refilled, and if the lines are blocked it could take a little while. The petcock is easy to take out, and doing so allows you to inspect the pick-up screens from the fuel tank. Once you've narrowed it down to one of those three categories, problems are a lot easier to deal with.

Its a long shot but I have seen pluged gas cap vent hoses cause problems like what you describe. If the tank cant breath the carb cant suck gas and it will die.

Thanks for the suggestions guy but we got it firgured out. It turns out it was the jetting. The o-ring and washer were missing of the fuel screw and we also had to drop the pilot two sizes. It was weird it ran awesome all winter but we didn't think it would need that much of a change. :thumbsup:

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