01 yz 426 low compression

5 replies to this topic
  • Hitman

Posted November 28, 2005 - 10:02 AM


Just bought a used 01 yz 426. It has little compression it had set up about a year from last ride. Took off carb held my hand over intake and compression is definately being lost past the intake valves. I pull-started the bike and it ran about two blocks or so till i went to let it idle and it died (did this twice keeps fouling plugs) Anywho took the valve cover off and the I cannot get my smallest feeler gauge .0015 between the lobe and lifter on the intake side (exhaust checked ok .010). could it have gotten so tight that it is causing compression loss.

and timing chain is really loose, took tensioner out and seems to be working properly is this fairly common for the chain to stretch that much. can i change just the chain or do i have to change all three gears.

  • s3706

Posted November 28, 2005 - 01:22 PM


If the tensioner is working correctly and the chain is still loose replace the chain. Make sure you have it at TDC when you are checking your intake and exhaust clearance. If they are out of spec re-shim until they are in spec. If the clearance is to tight it will make the bike hard to start and will not idle correctly. I would replace that timing chain before you trying starting it any more because if it jumps timing you will end up with a BIG repair bill. I hope this helps you. :applause:


Posted November 28, 2005 - 01:45 PM


Did you check the piston/rings? If your valves arn't leaking then its probably that.

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  • odlaw19

Posted November 28, 2005 - 07:37 PM


Check that the shims aren't out of place under the lifter. i had this problem when reinstalling the cams on the weekend. Remove the lifter and check the shim is in the center, otherwise it will hold the valve open.


  • felony

Posted November 30, 2005 - 07:20 PM


i just replaced my timing chain today,wow what a big difference in snap.before it would want to cough hiccup and stall frequently.do that and check your valves and you'll be amazed at how good it runs

  • grayracer513

Posted December 01, 2005 - 09:24 AM


If yo can't get a .0015" (.04mm) gauge under one of the valves, you have a problem right there. Adjust them before doing anything else. If you find that the valve needs a shim .20mm (.008") smaller or more to bring it back into spec, you need to replace the valves. Another indication of this would be if it needs readjusting after only one or two more rides. That clearly shows that the hard coating on the valve is gone, and it must be changed out.

It's a good idea to replace the timing chains periodically (once a year or so, depending) just as a preventative maintenance item. They are very cheap, easy to change, and at the least, will keep the crank gear from wearing excessively if kept fresh.

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