Hey guys, I have a 08 yz450f. I just replaced the head gasket and valve cover gasket. ive read every forum about timing and done exactly what everyone has said to do. bike still will not start. ive got the TDC line matched up perfect and the dots at 3:00 and 9:00 lined up with the motor. still nothing. thought maybe I was 180 off but I wasn't. Ive tried just about everything I can and I cant kick start it, pull start it, or anything. any suggestions? this is my first Yamaha ive ever had and im Kind of starting to think it was a bad idea to get rid of my Honda. help me please!!!!!

Was it running before replacing those gaskets?

Haven't had a Yamaha, but when my honda would get til it wouldn't start it was the valve clearances every time. You should probably check those, for honda the intakes are .004-.006" and the exhausts are .011-.013" (the exhaust numbers might be off by .001 I don't remember, but that small of a difference isn't enough to make a difference in starting). My exhaust valves haven't moved at all on the honda, it's always the intakes and the right side intake always went first. Obviously a yamaha is going to be different, so I would check and see what the clearances should be for your bike and see where your valves are.

You need fuel/air, compression, and spark to get an engine to fire as long as everything is in time. If the valve clearances are in check, I would clean the carb, and make sure your getting spark by grounding the plug and kicking it over to see if it sparks.

Before I replaced the gaskets it would only start if I pull started it, and it blew white smoke so that's when I went for the gaskets. they re used the head gasket. it was shot and so was the valve cover gasket. now I cant even pull start it.

If the bike is in time im guessing it's one of these problems...


valve clearance, piston rings, carb needs cleaned, or no spark    This is the order, from most likely to least likely, that i think the problem might be. Not necessarily the order i would check them off the list as not a problem anymore. Probably the spark plug and the valve clearances would be the easiest to check first. Then just clean the carb to check if it was the problem i guess. Finally, the piston rings would probably be the toughest of the four to check.


I doubt it's gonna be the spark since it was running before the head gasket install. I would also double check that the intake is all sealed up and not sucking any extra air.

What should the clearance be on the valves? .008 and .010?

quick google search tells me that the intakes should be about the same as my honda .004-.006" and the exhausts a little tighter than the honda at .008-.010"


again this was just a quick google search so nothing definitive here, but the numbers seem reasonable.

Clearances are .004-.006" on the intake and .008-.010 on the exhaust. 


If the bike sat for any significant length of time while repairs were made, the probability is that the pilot jet has at least partly clogged with dried fuel varnish.  Inspect and/or clean it per the post in Common Threads.  Also check the spark.

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