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Wiz636

Auto-decomp Device Sticking - 2008

4 posts in this topic

Lately I have been experiencing my auto-decomp device sticking occasionally so that when I go to kick the bike over it just cycles through without ever building compression or allowing me to find TDC. At least that is what I am assuming is happening as I have not torn it down yet. Anybody else experience this? I have about 350 hours on the bike (I have an hour meter but have not looked at it recently). I have about 50 hours on a top end rebuild and the valves were in spec at that time but I have not checked them since. Once I can finally get to the compression stroke it fires right up hot or cold. It does not happen every time...probably about 30% of the time I try to start it.

I run Amsoil MCF exclusively and change it every 8-10 hours.

Is there a fix for this or is it just time to replace the exhaust cam?

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Hey, Pete, good to hear from you.

The way the auto decomp works, even if it did stick, it couldn't eliminate the whole compression stroke. More likely a valve with a bit of gummy tar built up on the stem where it enters the valve guide. You might even "fix" this with a little high grade injector cleaner added to your tank at about 1-1.5 times the labeled ratio.

This still your '08? Lot of time on that one, eh?

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Yep, same bike...I have it perfectly dialed in for me and I still love it as much as when it was brand new. This is the first mechanical issue I have ever had with the bike after years of racing it...although I am pretty diligent with maintenance.

So potentially a sticky valve? That makes sense. I am due for a new timing chain so I guess I will just pull the head and all the valves out for inspection. Aside from measuring the valves themselves to see if they are still in spec what should I be looking for? Maybe it would be a good time to just replace the valves?

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I know what you mean about the bike. My '06 is so dialed in right now there's no way I'd get rid of it. (although the new WR450....)

If the valves haven't needed much adjustment, you won't see much. If there is no detectable wear on the face of the valve, then you have to take your best wild guess as to how much longer they'll last before they start sinking. If you do find an edge on the face that's clearly worn into the surface, you should replace them.

Wire brushing the carbon off with a bench grinder is OK as long as you stay off the valve face itself. Once clean, check the fit in the guides and you should be good.

Also, look for any of the lifter bores to have formed a ledge at the top that could catch a lifter and keep it from rising. This is rare, since there are only one or two spots in the head where this is even possible.

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