09 Auto decompression ?

5 replies to this topic
  • Gunner354

Posted February 18, 2010 - 10:29 AM


My son in laws bike is very easy to kick over (auto decompression working properly?) My bike is much harder to kick over most of the time, but at times it's very easy like our other bike. Not sure if the very easy or the harder to kick over is normal.
I like to slowly kick the motor until I feel a resistance, then get the kick starter to the top and kick a smooth full kick. It has a decent amount of resistance when I kick it. Starts first kick most of the time.
Gray please expalin why the difference.

  • grayracer513

Posted February 18, 2010 - 10:37 AM


Both YZ450's?

  • Gunner354

Posted February 18, 2010 - 10:44 AM


One bike is very easy all of the time and one bike is harder to kick over 95 percent of the time. The other 5 percent it's very easy like the other bike. Is the auto decompression possibly not working properly or not resetting?

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  • grayracer513

Posted February 18, 2010 - 10:54 AM


Again, are they both YZ450's?

  • Gunner354

Posted February 18, 2010 - 12:00 PM


Yes both 09 yz450

  • grayracer513

Posted February 18, 2010 - 12:15 PM


There are two possible reasons why one would show more cranking compression than the other:

One is out of time. One tooth retarded on the exhaust cam will reduce compression to the point that it nearly (or actually) won't start. One tooth advanced will make the bike extremely difficult to crank. If both bikes run well, this is not likely the cause.

One seals better than the other. Not unusual, especially is one is newer. A leak down test will answer this question definitively. The causes could be that the rings aren't sealing as well on one bike, or that the valves aren't seating and sealing as well, which can in turn be a result of nothing more than a carbon build up on one or more valve stems.

On the question of why this changes occasionally, it's not unusual for an engine to catch a particularly good seal some of the time and just appear to have more than the normal amount of compression. This can also be temperature related.

Then there is the situation that occurs on a stall out now and then wherein the decomp pin is trapped against the lifter by the timing of the sudden stop:


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