2007 yz450 - no compression

I'm here at cahuilla mx track and when I try to kick the bike over there is almost no resistance on the kick starter. There also might be a slight ticking noise when I push the kick starter by hand. This bike has maybe around 40 hours since new. The last time I rode the bike felt fine. Oil and maintenance has been meticulously maintained. I have never checked valves or replaced timing chain.

maybe the auto decompression is stuck? Pop the the valve cover off and take a peek, 15 minutes to check...

Other than that, it's a crap shoot...

They don't stick, and they can't dump all the compression, anyway. If the bike was healthy, there will still be a solid spot in the rotation to mark where the compression stroke starts, and that's with the auto decompression active.

More likely, the engine slipped time and either has a valve open through the compression stroke or it has bent a couple of them.

Then too, since it has sat for a while, it could be rusty valve seats or dry rings causing it. If the timing looks alright, try to get it to start. It may clear itself up after it runs a while.

Thanks for the input. I'll check timing and report back. This isn't a worse case scenario is it? The reason I ask is the bike ran great before, then maybe a month passes and now it's dead. I did run vp u4.4 last time, but I jetted for it.

Edited by motorcross242

So I checked timing, and everything looked perfect. I then go to check valves and the shifter side exhaust valve was stuck in the open position. I rocked the bottom timing bolt (crank bolt?) so the lobe would rock on the bucket cap and click the spring uncompressed and everything is good. I cycled the cams probably a hundred times softly then checked valve clearance. Intake was .10 mm and exhaust fit the .20 fine. Maybe on the tighter side but I wanna ride before I do anything else.. Put everything back together and put in pump 91. Jetted back to stock and the bike fired right up. Rode up and down the street and everything seems to be just fine.

The only thing that makes me nervous is I did hear a faint click at the track, hopefully trying to start it with the exhaust valve open didn't do any harm.

Is this possibly due to the u4.4? I mean this is not just a coincidence. I change my oil and air filter every ride.

Edited by motorcross242

I know U4.4 can gum up the carb...so maybe it's possible some unburnt fuel gummed up the valve? Gray will know....

Does anyone see any reason to not ride the bike the way it is? I'm paranoid there may have been damage to the piston from valve tap?

Also, is it possible the tap caused the valve to bend? If the valve is bent, wouldnt the valve be way out of spec?

Should I take everything apart and inspect? Of course, this would have to be done by a professional. Gray's shop?

A valve stuck open will burn and cause all sorts of problems ... Might be worth it to pop the head off and do the valve seats, maybe replace the valves too.

What exactly do you mean burn if a valve is stuck open? I believe this is what happened: Ran the bike with u4.4 last time at the track, jetting felt right on, maybe a little on the rich side. When I shut the bike down, the valve was left in the open position. After sitting for around a month, the residue (lead?) from the U4.4 kept the one exhaust valve stuck open.

My only concern is the clicking I heard while kicking the bike over probably like 5-10 times, is this grounds for doing a new top end and valve job? Or should I just ride on like nothing happened?

The yami engines are a little noisy. As long as it isn't making a knocking sound I'm sure the valves are fine. You said they were in spec so I doubt there is any damage. If it has good compression and runs good, I would not take it apart.

What exactly do you mean burn if a valve is stuck open? I believe this is what happened: Ran the bike with u4.4 last time at the track, jetting felt right on, maybe a little on the rich side. When I shut the bike down, the valve was left in the open position. After sitting for around a month, the residue (lead?) from the U4.4 kept the one exhaust valve stuck open.

My only concern is the clicking I heard while kicking the bike over probably like 5-10 times, is this grounds for doing a new top end and valve job? Or should I just ride on like nothing happened?

Does anyone else have an opinion before I go riding?

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now