03 Yz450f wont run please help

Yamaha YZ450F 2003

2 replies to this topic
  • jaredwhite1212

Posted October 24, 2013 - 06:24 PM


Hello everyone I am new to this forum and have heard it is one of the best, which is why I came here to ask you all for help.  I have a 2003 YZ450F that was just rebuilt.  The bottom end was redone bearings and all.  The bike just will not run.  It is dead on timed, it has spark (or when the plug is out of the cylinder and i kick it over it does) and it is getting fuel.  for some reason even with those three things, it barely will ever even pop when i kick it.  i am all out of guesses and am hoping you guys can help me.  It has a lot of compression too. Anyone ever have this problem before or have any advice on things to check?  all connections seem to be good.  I have tried everything and am all out of ideas.





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  • grayracer513

Posted October 24, 2013 - 06:41 PM


If the bike sat for a time, especially during the summer, it may well have a blocked pilot jet.  All it takes is a dried film of varnish, even one that leaves the main orifice mostly open, and the bike will barely run.   My own '06 pulled this on me after the top end work I just did.  The orifice in the number #45 pilot jet in mine is supposed to be .018".  It was nearly impossible to start, and was in fact impossible to keep running.  A .016" drill would pass through the jet, but the .018 wouldn't.  That was all it took. 


Always worth checking.  The fuel you see on the plug may well be from the accelerator pump and or starter circuit.

  • jaredwhite1212

Posted October 25, 2013 - 06:10 AM


ok thanks for the reply. i will check all of the jets in the carburetor as it did sit for a while.  I was also told that it could be a throttle position sensor gone bad, but i have read forums about people unplugging those and the bike would still run

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