07' 450 Auto-Dcomp Sticking Open?

2 replies to this topic
  • lftdjeep426

Posted August 10, 2009 - 06:20 AM


I just put a fresh piston in my 450 3 weeks ago. Since the new piston I races one motocross race the following weekend and it ran flawlessly. Yesterday I took it out to ride and when I unloaded it and kicked it there was no compression. I could "kick" it through with my hand easily. I put it in gear and rolled it forward with little resistance. I rolled it back and forward again and the lobe must have caught because all of a sudden the rear locked. I kicked it first kick and it fired. Never did it again the remainder of the day and I put 2.6 hours on it.

Anyone ever experienced this? Could I have done something wrong in the rebuild? Should I look into getting a new cam?:thumbsup:

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  • grayracer513

Posted August 10, 2009 - 08:05 AM


You said you put a piston in it, but did not mention doing any work on the head. I'll assume you didn't. The most likely culprit in temporary loss of compression such as this is valves sticking in their guides due to a bit of carbon wedging into the bottoms of them, or being held off their seats by carbon, rust (if left to sit in a moist climate with the valves open), or in this case, a stray bit of gasket or carbon from the rebuild. Normally, when this goes away, it's gone, but it can recur if it's caused by carbon built up around the back of the valve.

The automatic decompression mechanism cannot cause a complete loss of compression regardless of what might happen to it. When it is active, that is, at speeds lower than 700 RPM, a pin extends from the cam in such a position as to lift one exhaust valve off its seat during the first 40-50 degrees of the compression stroke. The valve closes again at about 70-80 degrees BTDC, leaving enough of the compression stroke to generate around 100-120 pounds of compression and start the engine. The second the engine exceeds 700 RPM, the pin retracts, and the valves operate normally.

Even if the pin were to stay stuck in the extended, compression releasing position, there would be a normal amount of cranking compression, and the engine would actually run. It would be exceptionally noisy, however, and you'd shut it off immediately.

  • lftdjeep426

Posted August 10, 2009 - 08:18 AM


You said you put a piston in it, but did not mention doing any work on the head. I'll assume you didn't.

I did have the head put on a flow bench and everything checked out so no head work was done. Thanks for the explanation of the decomp cam. Hopefulle this was only a onetime event. It really bumms me out that I have started having problems with this bike, i.e. bad wiring harness led me to tear everything apart to find an answer as to why it wasn't running which is when I put a new piston in (40 hours on the bike at that time). I've probably got 10 hours on this piston in the last three weeks...... I'm about to start my race season in a month and these little things happening are not very confidence inspiring.

Is there any possibility of something in the head not getting oiled properly and causing the valve to stick open? How common is this? :thumbsup:

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