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SincYZ450f

Top End Rebuild, Won't Start!!!!!

23 posts in this topic

Recently did a top end rebuild on my 07 yz450f. Put it back together and now it wont start only gives the odd backfire. Has good spark.

Any ideas wat my problem could be

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Ye did that i think everything seems to be fine. I didn't mark the timing chain when i took it apart but did the timing by the manual and it seems to be fine. Seems ok to kick (not hittin any valves, good compression)

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Which mark on the crank should be used 4 the cam timing. The marks on the crank are' H I ' I used the 'I' mark??? Is that rite

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Which mark on the crank should be used 4 the cam timing. The marks on the crank are' H I ' I used the 'I' mark??? Is that rite

yes mark on case aligned with the I on flywheel.

the best way by gray racer:

Remove the spark plug and use a screwdriver to find TDC, or eyeball it by lining up the flywheel key with the bore centerline and time and assemble the chain and cams with the whole run of chain visible. Foolproof, but be sure to double check the timing by pushing the flywheel and cover on by hand and using the real marks just before torquing everything all the way up. You don't want to have to take any more apart than necessary if you screw up.

and yes it works !

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It could if the fuel turned on you. Two weeks isn't much but you might try some fresh gas and think about cleaning out the carb. I ha done case where the fuel turned into something non-flammable in a week's time sitting at the shop, but that is unusual.

Fresh plug?

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When I trouble shoot someones bike for "No Start" I take the plug out and put the plug wire back on the plug and ground it to the head so I can see the spark happen when I kick it over......I want it to spark on the compression stroke.....With the plug hole so far down inside the head I don't know if you could use this method (maybe with some help) By holding a finger over the plug hole and have someone kick it through.....The compression will blow your finger off the hole right when the plug sparks.....At least you will know your timing is close.....If your happy with the spark being in the right place the carb would be my next place to go (Fuel)

Good Luck

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My friend had this same problem after re-building his 03 YZ450. It turned out to be a shim that wasn't centered on top of the valve stem & that wouldn't let the valve close completly. We know the local Yamaha mechanic really good & he came over after work & spotted the problem in about 5 minutes. I guess the shims sit on top of the valve stems in a little dished out seat. When my friend put everything back together that shim was off to one side & was sitting at an angle instead of flat in its seat. Hope this helps.

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2nd on the shim! Check your clearances, mine was off on one exhaust valve, and i had the same problem. Reseated and fired right up.

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I just did a quick valve adjust, and the same thing happened. Kicks over fine, just backfires after about ten kicks.

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Over the years I have had a learning opportunites by screwing up a valve adjustment in every possible way. Got the timing off, bent the valve, Timing chain was loose and skipped, bent the valves. Over tourqued the cam caps, seized the cams. Good, expensive lessons.

Double check the valve timing. If you are sure, and I mean SURE it is right, then pull the main jet and clean it. It's easy. Should start.

One more thing, did you use assembly lube?

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