99 YZ400F compression problems...help!!!

1 reply to this topic
  • Tripwire

Posted December 06, 2004 - 07:17 PM


I have a 99 YZ400F that is basically stock and has fairly low hours. Today as I was starting it I noticed that it felt like the compression release would not "release" and I could kick it over without the lever actuated (I would never have wanted to/could push it past TDC without the CR lever before). It also made it run poorly at moderate RPM's (I was hesitant to rev it naturally). :cry:

I adjusted the CR cable and that didn't really seem to help. The actuator arm presses against the engine block stop so it is going out as far as possible from what I can tell :cry:

I then checked my sparkplug to see if it was loose and it was somewhat loose but after tightening it appropriately it still exhibits this problem. I just bought the bike and have not had the valves checked but it has been running fine with plenty of compression and had compression during the first few kicks today so the onset was very sudden. Also, there are no unusual noises in the engine.

Anyone have any ideas as to what could be the problem?

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  • grayracer513

Posted December 06, 2004 - 10:29 PM


Check the valves first. They could have gone tight on you. If you find one quite loose, tap the follower a few times lightly using a hammer on a piece of wood dowel and then check it again. If the valve was sticking in the guide, it should be free, and the clearance will be closer to normal. This can happen once in a while, and it usually is temporary.

BTW, you can tell when the decomp shaft hits the follower if you pull the lever in slowly as the engine runs or turns. When it makes contact you can feel it in the lever.

If that's not it, remove the plug, run the piston down in the bore a little ways, and squirt an ounce or so of oil down the plug hole. Kick it through several times to work it in. Put the plug back in and try again. If that did it, your rings may have gotten stuck in their grooves, or be weak or worn.

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