2002 YZ 426 Decomp question

5 replies to this topic
  • Mike Schubert

Posted January 11, 2008 - 11:56 AM


If the compresion release on an '02 YZ 426 is used while the motor is running at idle is the damage immediate and catastophic or do you just get something like a slightly bent valve that will sooner or later beat it's way thru the head / piston? This happened to my bike but it seems to be running fine despite the warning. Thanks.

  • Dustin_Mabro

Posted January 11, 2008 - 12:05 PM


I accidently pulled the decomp lever on mine as well...runs just fine. But I'll never do it again. I'm not sure exactly what it'll do.

  • Dano426

Posted January 11, 2008 - 02:11 PM


The decompression lever is used to open one of the exhaust valves so that it is easier to kick over. I do not know for sure, but I think that you would just have loss of power. At idle, I wouldn't worry about damaged valves.

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  • skwerlee

Posted January 11, 2008 - 02:13 PM


I've done it several times...does nothing but lose power until it dies. My dad's old 76 Yamaha XT500 is the same way.

  • grayracer513

Posted January 11, 2008 - 03:38 PM


Many of us back in the day used to use the compression release as a kill switch. Back then, the buttons you could get over the counter weren't that reliable, so lots of us just left them off.

The engines were built a bit differently then, too, of course. They way that it is in a YZF, the valve lifter raises the right exhaust valve off its seat maybe .060" at the most. The valve WILL NOT hit the piston under any circumstance as a result of this, and you won't bend a valve. However, because the valve lifter works by stopping the lifter at its outer edge, there is a rocking force applied to the lifter that is not normally there, and at high engine speeds (say 3000 or more) there is potential for the lifter to be broken by this force. It won't be immediate, and at low speeds, there isn't much risk, but I wouldn't make a habit of it.

  • Mike Schubert

Posted January 11, 2008 - 05:18 PM


Thanks - You guys are the best. I plan to make the cam change soon.
My '77? MX 400 had a CR but of course no valve train to be concerned with. I miss those days.

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