03 YZ450F valve troubles

I have a 2003 YZ450F that I bought used a couple months ago. This is my first bike and I am hoping I didn't buy a lemon. It has always been very hard to get back started if it stalls out on the trails and last weekend I noticed a loss of power and some more then usual noise coming from her so I shut it off right away, now the bike will not run unless you give it gas the whole time. I have completely rebuilt the carb and checked the operation of the hot start lever and all that. So today I pull the valve cover off and start checking clearances

I find it odd that some have worn tighter and others looser

Is it worth re shimming these or am I looking at new valves, I know that with these titanium valves once they start they don't stop, I just am curious how long they would hold up if I just re shim them


Edited by 2whls4life

Looks like you have valve seat damage on your exhaust valve with .10. It may not be sealing properly anymore and thus the reason for the hard starting. When the seats wear that's when you get a clearance that shrinks.

You can pull the tappet and check to see if there is a shim, measure it and it you can use a smaller one to get the .20-.25 clearance that you want.

I would personally pull the head and check the seats to make sure that they are sealing properly.

Thanks for the response, if I could shim it and have it seat properly would it last another say 50ish hours or whats the life of it at this point, any ideas?

I ride mostly in the woods, I don't race it or plan to.

(Im just wondering if it would be a better option to replace the valves, springs,etc and have the head machined to match, or could I get some decent life out of it still just by shimming it)


Shim it and then watch the clearance closely over the next few rides. If it holds at the new settings, you're OK for the time being.

Ok, i'll shim it this weekend and see how it goes, thanks

well today I pulled the valve cover and and recorded clearances

Intake: left: .08mm center: .18mm right: .14mm

Exhaust left: .20mm right: extremely tight

I pulled the cam covers off and lifters and I find this:


What would cause a valve cotter to do that? This would explain my problem and why I could not even get a .03mm feeler gauge in there

What can I do?

Please help!

Hard to say what would cause that, since you don't have manual decompression. Getting the cotter back in place without removing the head will be pretty tricky, but a good mechanic can probably do it for you.

I would purchase a new headgasket and take that head off for further inspection, I think it's just not worth trying to ride a bike that could kaboom at any moment. Until you know for yourself that this is not the case then great, but until then you should remove the head and check it out!

Well what exactly holds the valve cotters in place? Just the pad and the lifter?

Must have been really out of adjustment right before this happened to allow it enough play to pop out like this.

Do you think I should pull the head to check the valve seats?

^^^^ beat me to it haha, sigh I guess ill be pulling the head now haha

But I will feel better about it like you said, I enjoy all this anyway. I love knowing how everything works, and the condition of all the parts.

BTW the previous owner claims that the bike has stage 1 hot cams in it, any way that I can verify this while im in there?

What holds the cotters in place is the valve spring and the tapered shape of the cotters and the bore in the retainer. The lifter and lash pad have no part in it.

When I pull the head I will just use a valve compressor to take the retainer off and put the cotters back in place. Do you think the retainer wore or failed in a way that would let them do this?

I just don't want to reset them and then have it happen again

Has anyone ever seen this happen before?

This is what I get for buying an older used bike haha, I trying really hard not to have to rebuild or replace the head on this bike. Especially since its my first ever bike and I haven't had much chance to ride it yet :p

Buy a new pair and check the condition of the valve stem.

Ok thanks for all your help, quick question:

Are you/did you used to be a mechanic? Just wondering where all of your knowledge comes from

Thanks Ill keep you updated once I get the new parts (local shop doesn't carry very much so prob gonna have to order...oh joy)

I'm going to guess engineer who did mechanics as a hobby! That's my bid

Guy knows his stuff

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