06 yz450f compression issue

6 replies to this topic
  • dmbperson

Posted August 22, 2008 - 10:23 PM


i just put in a crank, cylinder, piston, rings and cam chain. I got it all back together and when i went to kick it over i have little to no compression, what could have I missed, I checked the timing again, how does the auto decompression work on this bike?? any help would be great.

  • desert-rat660

Posted August 23, 2008 - 06:36 AM


check the shims under the valve buckets, u may have one or more that arent seted in the retainer correctly, it would cause the valve to be held open a bit

  • grayracer513

Posted August 23, 2008 - 07:43 AM


Check the valve clearance. It could be a tilted shim, as suggested.

The auto decompression mechanism consists of a spring loaded flyweight on the exhaust cam. When this flweight moves in response to speed, it rotates a shaft in the cam center that retracts or extends a pin located near the base of the left exhaust cam lobe. The pin is extended at speeds under 700 rpm, and is positioned so that it lifts the left exhaust valve off the seat at around BDC as it passes over the lifter, and rolls off the edge of the lifter at about 60 degrees BTDC, letting the valve reseat. This shortens the compression stroke by about half, leaving enough compression that the engine can start, but cutting it down enough that it can be rotated without releasing compression.

The AD unit is dependent on cam timing. If the exhaust cam is one tooth retarded, the engine will have very little compression, so you may want to recheck that as well.

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  • dmbperson

Posted August 23, 2008 - 10:14 AM


Thanks for the input from both of you, It ended up being the right exhaust valve shim was cocked and causing the valve to be open at all times, I was pulling my hair out tying to figure that out, I was glad to see it was just an over site, and a simple adjustment. Thanks once again for all your help!

  • grayracer513

Posted August 23, 2008 - 11:26 AM


It ended up being the right exhaust valve shim was cocked and causing the valve to be open at all times, ...

Kind of interesting....that's the 3rd or 4th one in two weeks....:thumbsup:

  • desert-rat660

Posted August 23, 2008 - 05:51 PM


no problem

  • ogrione

Posted December 21, 2011 - 06:28 PM


thanks for the post....I just had the same problem! shim cocked after disassembly
compression test read zero I was scratching my head

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