06 YZ450f backfiring while trying to kick start it

I just got this 06 yz450 and im pulling out my hair.

I was told it has a fresh top end, and it looks like a new head. 

so I live in South Lake Tahoe 6300 ft elv. so not having the opportunity to ride it when it was bought for me by girl friend. when i tryed to start it, it backfired 3 or 4 times before it reluctantly fired and ran very lean.

So my first thought was valve timing  or a shim was tweaked,, nope that wasn't it so i tried carb. found a disintegrating accelerator pump diaphragm I thought ah ha i found the problem, nope.

So Im not sure where to go with it. 

You pretty much have to go through everything to be sure there is not a problem that can turn castistrophic.....


You symtom could be just poor starting ritual,  a fuel screw that is not adjusted, a clogged pilot jet from sitting (most likely)  or a skipped or improperly timed cam set.


I would check the valve clearances, inspect the chain and cam timing, and inspect the stator pick up for debris.

Edited by KRANNIE


a clogged pilot jet from sitting


Definitely at the top of the list. 

All of your suggestion are exactly where i started looking for an answer (except stator debri. That was great advice.) unfortunately i couldn't find a problem in any of those areas.

I called the machinist at Thumper  Racing (he was the guy who did the valve work on my head), back and he said, Although rare its possible that one or even both cam gears could have slipped on the cam shaft if a valve got bent and  the right forces where applied when motor blew.

So tomorrow im bringing my bike back to Thumper Racing after i finish reassembling it, So he can check my cams (i forgot what he called that procedure)

since i wasn't there when bike blew up, which is why it has "a new top end", 

Since the &%$#@! i bought my bike from is on a walk about (permanently) any info on what happen when it blew up is going to be impossible, the cam gear or gears slipping on the cam shaft sounds plausible.

And now a pattern starts to emerge.

I will update, thanks.

He may be onto something.  The process he mentioned is alternately referred to as either "degreeing the cams", or "checking lobe centers".


You can actually get a fairly close idea visually by comparing the lobe position while the marks are aligned with these pictures:





Thanks for that amazing pic, 

After visual inspection, I think the cams are ok.

The last place this problem could be is electrical.

I have a riding buddy who has access to a duplicate bike like mine.

So hopefully he was able to pull the CDI box from that bike (which run perfectly) and then tomorrow we will try that CDI box on my bike.

Keeping  all fingers crossed.

Will up date.

So my problem was a bad stator,  

It had a weak spark, not strong enough to keep bike running at idle.



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