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My engine wont stay running while it is cold. If it hasnt ran for about ten minutes it will sputter and want to die if its given any throttle, almost like the choke is on. I jetted up the main one size when a pipe was added. I also run with no airbox lid now so should I jet the main up again? Should a different change be made to the carb, like the pilot jet? The main jet is the only one Im familiar with so any info will help.
Engines dont run right cold, you need to sit there with the choke on for a bit feeding it a little throttle for a bit, all bikes do that.
the main jet doesn't effect the idle, that's the pilot jet. A bigger pilot would help. You can let it idle with the choke on or at half open, also till she's warm also.
I second on the pilot jet. Ive done all my own jetting and from experience, its sounding like your pilot is running too lean. Get yourself a 400EX service manual from your dealer. It has all the pictures in the world on how to tear every inch of your quad apart, including the carb.
Good luck and let us know what you find out.