Valve clearance opening up

I have an 06 yz450f. Last time I checked the valves the clearance was right at the top of the correct range. Today when I checked again the clearance has opened up past spec. Now I have read all I could find and understand valves could get "tight", but I am a little mixed up as to the issue I may be having. Any help will be appreciated.

I have a YZ450F 2007. Both my intake outboard valves are getting more and more loose. The center one and both exhaust did not move ever since i bought the bike brand new.

I just keep adjusting them. By now, I have a 2.00 shim installed on both intake outboard valves. Center valve is stock at 1.75 shim.

Now do i know what it means?? no, not a clue. But your right though, in theory valves should get tighter.

I hope someone can explain us why they are getting looser and NOT thighter......

If you have carbon built up on the seat then the clearnace will increase.

If you took the last set of readings with the engine at a significantly higher temp, the clearance could open up too.

Another possiblity is an error in taking the readings.

I'd say carbon build up is the most likely. Pull the head and take a look if it is outside the service limits.

Thanks for the reply's so far. Yeah I was kind of wondering about build up on the valves/seats. I did notice on one intake valve that the shim sat deep into the slot on the top of the valve and in the spring retainer. Now this seemed odd to me as it was a larger sized shim and was basically flush with the top of the spring retainer. Looking at it the two keepers seemed in place with nothing else out of the ordinary. Could the top of the valve stem/stems be getting worn down?

My 07 YZ 450 has been getting loose as well. I re-shimmed my intakes with 1.90s a couple weeks ago - exhausts were dead on. I would guess that I'm experiencing some carbon build-up on the valves and seats because my bike has been running a little rich the last few months but it's just a theory. With the stock valves being titanium I wouldn't expect there to be much wear on the valve tips, but the only way to know for sure would be to check the valve stem heights. I'm not too worried about the clearance yet, just as long as I keep on it after a few hours of riding. But if the clearance starts to get way out of spec then I would draw some concerns.

What would cause the wear on the end of the valve stem? The shim sits directly on top and the lifter/bucket goes over that. The only friction should be between the lifter and the cam and that is well oiled. The shim isn't spinning on the stem.

BTW, I helped a buddy pull apart his son's head yesterday. The center intake valve was loaded up with deposits to the point it was leaking. Regretably he didn't check clearanced before pulling it apart.

In my experience with my bikes it is common for that center intake valve to get more build up than the rest of them.

Actually, the cams and lifters are set up slightly off center from each other just so that the lifter does spin in its bore as the cam runs over it to distribute wear over a larger area. I'm sure you've seen the polished spot in the center of very old shims where the size is hard to read. But actual wear at that point is minimal, and having more than one valve do that as a result of stem wear would be very unusual.

Well finally managed to tear into the bike today. Here is what I found. Carbon build up was not bad, in fact the center intake valve looks brand new. I did find on the one valve (that the shim sat down in the slot further than seemed normal) that in fact the top of the valve head (above the retaining clips) has worn down enough that it is obvious to the naked eye. Especially in comparison to one of the other intake valves. I think I am going to replace that valve and watch it and the rest of them closely.

Very interesting findings. I currently have my motor out of my bike which is a wr450. I did my poor man's leak down test (gasoline in the intake manifold) and it passed. I know its not leaking.

Yet I had to go up a size on two of the valve shims. Maybe the top of the valve is worn... Curious....

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