Please HELP 07 yz450 WONT START

With the spark plug out it gets easier and with the head and jug off it spins free so I think it has to do with either the timming and compresson or the valves. I dont think the guy that rebuilt the head new what he was doing so hopefully when the head is torn apart ill find somthing.


Good start.  I have a 07 YZ450 and a 12 WR450.  Basically the same motor on both, but I haven't experienced any of those kinds of errors to learn from.  My YZ kicks over normally / like what you described on the Youtube videos.  Bringing it to the forums, it may greatly help the next guy.


Edited: Thought another's post with pictures was OP's.

Edited by mch

once i get it all back together( soon i hope) ill take pics and such and hopefully itll be running.

Feel free to use my pictures above as a reference, my bike is up and running. Going riding tomorrow! Make sure your valve crearences are within spec, and don't over torque the caps (7.2ft lbs)

UPDATE! Okay so after a long wait on parts, i got a new timing chain, stage 1 hotcams, new piston rings, new head gasket, and i had the cylinder bore honed i put it all together and forgot to snap pics of the timing but it started and ran :ride: . I counted 13 pins across the two cams and they were right on the marks to line up with the head. it fired right up after about 8 kicks( which were easier since the hotcams decompression pin sticks further out than my old one) and i shut it off after about 15 seconds because it was idling high. Restarted it with one kick and then it shut off after about 5 seconds, then could not be kicked again, the kicker was stuck. i decided to pull the clutch basket out and check out the kicker then figured out that's no the problem because with it not connected the motor would still hit a spot where it wouldn't pass when turning the crank. Pulled the plug out and looked with a bore scope and found a valve laying on top the piston broken :banghead: . I took off the valve cover to find the only where the only new valve was the head had collapsed into the engine, im not sure how yet but looks like the head gave out and dropped the whole valve assembly spring and all into the combustion chamber.  My guess is the same idiot that put the bike together himself also put that valve in, i couldnt see cracks or anything that would tell that the head was bad and i did a leakage test on it and found no leaks. ANyway long story short, itll be awhile but i plan on keep rolling on with the bike and fix it. Good news is i found the clutch basket was all new and performance(gotta look on the bright side right?haha), I also got to hear it start and run for about a total of 20seconds :rolleyes: haha better than not at all, even if it shit the bed. I plan on getting a big bore kit and gytr performance head and performance valves, i had got a jet kick for it but didnt mess with it since the bike wasnt even running yet i didnt wanna complicate things further. Ill post some pics of the carnage and when i rebuild it again ill be sure to take many pics. THanks for all the help and suggestions! :thumbsup:

well that explains the locking up situation. how did it even start with a broken valve? mine with just a valve out of spec wouldnt start because of lack of compression, let alone one completely missing. did it sound horrible?

:facepalm: Obviously, it didn't start with a broken valve.  The valve broke and then it stopped.

There was a tap noise, my guess is the guy forced the valve in the guide and thats why previously it would hit a hard spot when trying to kick it over because the valve may have been sticking and it stuck open while running which the piston smacked into and then poof no more engine. If it was a timing issue i doubt it would have started and it having new cams and chain i highly doubt it jumped time since the other valves look fine and even with the tensioner out theres so little slack on the new chain and cams i dont know if itd be even possible to jump time.

UPDATE!! I hope to be ordering parts soon to get the bike back together! I hope to either be making a video of the reassembly of the bike and engine or lots of pics! hopefully will be posting somthing in the next week or so!

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