No compression when warm

The decomp system in a YZF is just too damn simple to cause this problem. It's usually sticking valves.

I agree with Gray (again) a valve may be hanging up slightly, if you get it hot then take the valve cover off then check the valve clearences you will most likely find your problem.

What oil do you use, I use Amsoil 0W-40 Synthetic Motor Oil (AFF) I think it is the best lubricant available and consequently have had no engine, transmission or clutch problems in 400 hours of use between my 02, 03, 04, 05 & 06 YZ's

The challenge is now to get those valves clean without having to tear the heads apart... :applause:


I read that these fuel additives clean the intake valves but not the exhaust valves. Since the problem lies in the exhaust valves I'm concerned that this will not do anything.

Can I remove the header and spray some Yamaha Ring Free directly on to the exhaust valves?

Thanks for the assistance.

Well, you're partly correct. The correct part is that when used as a fuel additive, these kinds of compounds (which are almost all virtually the same as "Techroline", BTW) don't get a chance to do anything to the exhaust valves. Thus, the "soak and shutdown" technique I described. And yes, you could squirt some directly on the exhausts. Be careful of getting too much fluid into the cylinder.

However, the likelihood is greater that it's an intake doing this. If so, the carbon causing the problem will be deposited in one of two locations; either around the outer edge of the valve near the point where the valve face and the seat meet each other, or on the stem where the end of the guide is when the valve is closed. It will probably be heavier on the two outside valves, since more fuel runs up the center than the outer ports. You may be able to remove the carb and pick this off using something like dental picks (with the valves closed) and blow out the pieces with air. The engine can digest very small pieces of carbon without a problem as long as you get most of them.

Hope this doesn't ruin your day but my 03 450 had the same thing happen.....upon removing the cam cover I was horrified to find that the right side intake valve bucket

was being held open from a smeared over cam bearing......was a real mess caused by lack of cam bearing clearance and the cam gauled the head bearing surface (no removable cam bearing the head is the bearing ) I ended up sending the head to

Engine Dynamics in Petaluma...Mike Crowther is the owner and he did a great job of repairing the head.....Check the intake buckets...probably seized in their bores, at least one anyway....


Yeah, that's a possibility worth looking at, too. :crossedfingers:

Start with the basics and check your valve clearance!As engines warm up the grow!Get larger!If the valves are at the minimum clearance or just under which still lets the engine run,once it warms they become tight and could possibly stay slightly open causing no compression !

Everything seemed clear so far looking from the top side of the head. I'm going to go through a tank with Yamaha Ring Free in "shock treatment". Everything started normally for my garage test and the run up and down the street. I killed it and started several times and no sign of the infamous loss of compression. If this doesn't do it I'm taking her apart. Thanks for the info and I'll keep everyone up to date.

If this only happened a few times, you could have some type of buildup in the cylinder, carbon or the like. If it flakes off it could temporarily get lodged between the valve and the valve seat, letting air pass through and not letting compression take place.

I would definately check valve clearances.

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