valve contactin piston

6 replies to this topic
  • Vanilla_Gorilla

Posted July 26, 2002 - 05:26 AM


after reading numerous posts, i see that many people here think that the #3 exhaust valve can contact the piston when the decompression lever is used with the motor running. I will simply say this, the valve will NEVER EVER contact the psiton this way. it simply wont happen.if you pull your decompression lever (on the engine) up all the way (maximum lift), and kick the enigne over slowly, the piston will not touch the valve! I have also provent his theory, accidentally, when in a crash and my decomptression cable got cought in another riders handlebars, ripping it off my hydraulic lcutch mount and fromt he engine, killing the engine. after i reinstalled the new cable, the engine ran just fine, no problems.

  • John_Lorenz

Posted July 26, 2002 - 05:34 AM


Ok so next time your riding use your comp release All Day, just like a 2 smoke and let us know what happens.

Pull it in while traversing a down hill or a reall long straight then pull it in to go into a corner.

I think the real reason why posts have been against it is, one the Manual clearly states not to do it. I think it is a matter of the Valve and the Compression Release itself may cause issue when running.

Holding the lever open when running may cause damage, That is what the manual says. Honestly I believe it

  • Vanilla_Gorilla

Posted July 26, 2002 - 05:40 AM


i think the only damage it could cause is the actual decomp lever assembly may be damaged from the reciprocation of the valve pounding on it, but just as far as the valve contacting the piston, it wont happen.

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  • DaveJ

Posted July 26, 2002 - 06:14 AM


It would be very wrong and dumb for Yamaha to design a device that would open a valve up far enough to make contact with the piston.

One minor spill (EGO, you know what I'm talking about) and the engine would be toast.

However, this not a decompression device like the old TT500 days. In other words, it should not be used for braking. It's way too fragile.

So yes, I would say, and have proven, that the de-comp is fine on bump starts.


  • John_Lorenz

Posted July 26, 2002 - 06:29 AM


I agree in both aspects, but Nilla brings up what I guess I failed to say.

I think the damage would be to the actuating arm and assembly of the Release. As the Valve is being pounded by botht he CAm and the lever. Thsi to me were to probabilty of a bend would happen

Agree also it would be completely stupid to not incorperate this thought inthe design.

First rule of development
A common mistake people make when designing something completely foolproof is to underestimate the ingenuity of complete fools.

[ July 26, 2002: Message edited by: E.G.O.**** ]

  • Shawn_Mc

Posted July 26, 2002 - 07:00 AM


I use the compression release to kill the motor. Always have. Keeps the plug nice and dry. Also, if youve got a run away motor and the kill switch aint cuttin it, the compression release will always work. The compression release only lifts the valve enough for it to become a huge leak, not what I would call an "open" valve.

  • Vanilla_Gorilla

Posted July 26, 2002 - 03:57 PM


yeah, it lifts the valve 3mm or so.

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