99 yz400f wont start

4 replies to this topic
  • dmcswain08

Posted December 12, 2014 - 02:41 PM


I just bought a 99 yz400f and I can't get it running it has spark just rebuilt the carb seems to have good compression. But still can't get it to start. It will cough for me some but that's all. Anyone have any clues to what i might do?

  • Krannie McKranface

Posted December 12, 2014 - 02:56 PM


1. Service manual downloaded from Yam website

2. Do a leak down test of the motor

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  • grayracer513

Posted December 12, 2014 - 03:53 PM


Did the bike sit for an extended period?  Could have a blocked pilot jet (compressed air will not clear these). 



  • dmcswain08

Posted December 12, 2014 - 07:20 PM


Won't let me download that manual. But what kind of leak down test I'm not familiar with it. And yes the bike sat out in the weather I think the guy told me it just had a broken fuel line and he had a hard time starting it. So he had to pull start the bike. So I pulled the carb and cleaned with carb cleaner.

  • grayracer513

Posted December 13, 2014 - 07:54 AM


The problem is that just using carb cleaner won't clear a blocked pilot jet.  Remove the carb and review the link I posted for cleaning carbs.  The jet may even look clear and still be partly obstructed. 


A leak down test involves applying compressed air through the spark plug hole and determining what percentage leaks out of the combustion chamber.  Obviously, some will go past the ring gaps, but if it's too much, you can tell how much and usually exactly where it leaks.  If you "appear to have compression", you probably have enough that it should run.

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