07 yz450f wont start after rebuild

8 replies to this topic
  • personius264

Posted October 16, 2014 - 08:20 PM


Hey guys this is my second top end rebuild but this time I've been having trouble with my bike not starting after I did a top end. When I try to start it all that happens is a backfire after every ten kicks or so.


I have a brand new cylinder,new timing chain,new valve springs, and I had the valve seats re cut by a reputable machine shop. I have stage 1 hot cams that I installed on my last rebuild with no issues. I re shimmed the valves and i'm within tolerance according to the manual. I'm pretty sure I have the timing right and I know i'm at tdc. I'm going to check the carb again tomorrow, but did I miss something? could this be an ignition issue?



did some searches but no luck for me.



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  • grayracer513

Posted October 17, 2014 - 07:02 AM


If the engine sat for a few weeks while the rebuild was done in the weather we've had lately, there's a good chance that the pilot jet is clogged or partly so.



  • personius264

Posted October 17, 2014 - 10:20 AM


Ok thanks. I'll go through my carb again and I'll let you know my jetting while I'm in there

  • personius264

Posted October 17, 2014 - 08:03 PM


I cleaned the carb today but before I put it back together I wanted to to give you my jetting.
I'm in California at 30 feet elevation. Main 65, pilot 42, pilot air 100, starter 72, leak 50,
my needge clip is at the 6th slot on the needle. I have an after market accelerater pump, dr.d full system, and hot cams.

Could it possibly be as simple as moving the clip on the needle? Before the valve job it ran the best in the 6th slot, could it be off due to my valves sealing better now?

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  • LoonRider

Posted October 17, 2014 - 08:12 PM


Is it just be, or does that exhaust lobe look a tooth off...

  • personius264

Posted October 18, 2014 - 01:39 PM


165 main jet. Sorry that was a typo.

  • personius264

Posted October 19, 2014 - 09:34 PM


Cleaned the carb,and moved the needle clip to the stock position. No backfiring,but it still won't start.

  • peaceofwork

Posted October 22, 2014 - 07:20 AM


hey leave that needle clip ware it was if it ran good before the rebuild.  it is your pilot jet, pilot jet passage or fuel screw and fuel screw passage if it really is a carb problem . most of these bikes are a little tough to start the first time after being apart a while, double check your cam timing. in my un-expert opinion lol

  • personius264

Posted October 23, 2014 - 04:42 PM


I found the issue20141023_144711.jpg gonna get some shims tomorrow.

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