my 99yz400f wont run with the choke off

9 replies to this topic
  • 1999yz400fRider429

Posted January 28, 2008 - 02:11 PM


i have a 1999 yz400f that i just picked up for 1000 bucks and it was running rough, i cleaned out the carb and it helped but not much, it has a hi flow air filter but other than that it is stock, the problem is when i start it with the choke on it runs ok but backfires, when the choke is shut off it idles for a few seconds then dies, what could be the problem?

  • William1

Posted January 28, 2008 - 03:03 PM


Clogged pilot jet, clogged carb passage ways for starters.

  • frdsrck

Posted January 28, 2008 - 03:06 PM


Yep, I had a problem like that too. Ended up that there was dirt in fuel. Could also be water.

Start by cleaning out the carb, and draining the fuel tank if you find any dirt/water in the carb.

  • 1999yz400fRider429

Posted January 28, 2008 - 06:08 PM


thanks, i will try that

  • 1999yz400fRider429

Posted January 31, 2008 - 11:45 AM


i tried that and it definatly improved but it still wont stay running and it still backfires a lot

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Posted January 31, 2008 - 08:33 PM


On the slide in the carb there is a rubber gasket on the backfire plate...most likely this is the culprit! Replace the gasket and make sure the plate isn't cracked!

  • 1999yz400fRider429

Posted February 11, 2008 - 02:24 PM


i messed around with air jets and it runns 99% better but i had my buddy look at it and he noticed the accelerator pump leaks when i twist the throttle (not much) you think thats why it still bogs a little when i twist the throttle fast.

  • William1

Posted February 11, 2008 - 03:43 PM


Now you are talking about a different problem. When you say the AP leaks, where is the leak coming from?

  • 1999yz400fRider429

Posted February 12, 2008 - 11:55 AM


the side, it just jets out the side of the accelerator pump.

  • William1

Posted February 12, 2008 - 01:40 PM


Ah, the diaphragm is probably bad or just the screws are loose.

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