'99 yz400 low compression - please help

5 replies to this topic
  • blibzool

Posted July 08, 2004 - 10:56 PM


I just bought my first bike, a '99 yz400f, two weeks ago. I rode it three or four times and it seemed to run great except for a lot of backfiring on deceleration. Now after sitting overnight i can't get it to even try to start. I usually had some trouble starting it but figured that was do to my lack of experience. Now it seems to have low compression. It's not zero compression but i can now kick it all the way down with out using the decompression lever most of the time. All i can get out of it is an occassional loud backfire after many kicks. I use the starting drill outlined on this site. I checked the plug and it isn't wet and i don't think it's fouled but i'm not sure if i would know. The bike didn't seem to smoke at all when it was running. I searched this site and saw some stuff about stuck vavles - is that a possibility or does it sound like it needs a piston and rings? What should i do? Any advice is much appreciated.

  • Hamish

Posted July 09, 2004 - 02:06 AM


had a similar problem with mine twice (after engine re-assembly), was a valve being partrially held open, possibly by some dislodged carbon from the combustion chamber. Mix up some 2 stroke fuel at 50:1 or more, push the bike down a big hill with the decomp lever pulled in and bump start it. Run it on the 2 stroke fuel for about 10 minutes. Obviously if it's something more serious then this plan of attatck wont work, but it's definately worth a try.

  • blibzool

Posted July 12, 2004 - 08:47 AM


thanks for the advice. I'm going to try it as soon as i get a chance.

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  • bushy

Posted July 12, 2004 - 09:38 AM


If all else fails... use a bigger hill!

  • yathump

Posted July 12, 2004 - 09:55 AM


My 426 did the same about a month ago i finally got it running by pulling the plug and kicking it over about 40 times i put a fresh plug in(just because) and 2 kicks later it came to life.
Good luck

  • Freestyle666

Posted July 13, 2004 - 12:10 PM


Mine has just done the exact same thing noticed loss off power and when i try to kick the bike off just a bit off compression allthough when decomp lever is pressed it dos relieve a bit off compression any one no how to sort this with out taking to the local dealer cheers Lee :thumbsup:

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