Is my yz400 in need for a rebuild?

4 replies to this topic
  • bubbaisthebest

Posted January 18, 2009 - 02:22 PM


I got a 1999 yz400f during the summertime, and up until about October, it has run great. Now it is fouling spark plugs, 4 in three months, and it's not starting in one or two kicks like it usually does. I'm thinking that it's time for a new top end. Any thoughts would be appriciated. Thanks

  • Smokeslider

Posted January 18, 2009 - 04:15 PM


Have you checked the valve clearances yet?

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Posted January 18, 2009 - 05:42 PM


sounds like u have a jettin issue if ur foulin ran great durin the summer time where its noticeably warmer. and now its gettin pretty chilly out now..yammies can get very touchy bout how its jetted durin weather changes

  • DeeCee

Posted January 18, 2009 - 07:40 PM


First up I would check the valve clearances and give the carburettor a thorough clean out.

Hard starting is often a sign of tight valve clearances. The plug fouling could be due to your mixture being off.

  • grayracer513

Posted January 18, 2009 - 08:09 PM


It's very likely that the problem is jetting related due to the cold weather.

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