99 yz400 knocking help.....

5 replies to this topic
  • busa4

Posted August 30, 2006 - 06:04 AM


this was my friends bike and he was rinding the bike for an hour with no problems. he took a 30 minute break and jumped on the bike again and rode for another 20 minutes and then the motor started knocking. i bought the motor from him and dissassembled the top end to find the intake valves just slightly tapped the piston. the motor still ran and the valves are not bent. i dont see any excessive wear on the chain guides and the tensioner was rock solid. the chain had no slack when i started to dissassemble. the only thing i can think of is chain stretch or valve adjustment. what do u guys think?

  • tom nolan

Posted August 30, 2006 - 07:02 AM


It seems like the cam chain might have slip a tooth. Did you verify the cam timing before you disassembled it?

  • busa4

Posted August 30, 2006 - 07:15 AM


i should of but i didnt. i assumed it had connecting rod bearing failure. wouldnt slipping one tooth change the timing so much it would have damged the valves and the head?

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  • tom nolan

Posted August 30, 2006 - 07:28 AM


I had a con rod bearing go bad on me, however, it didn't effect the valves. I noticed fine particles of metal in the oil filter and on the magnetic plugs. Have you checked the filter? OH ! one other thing, when the big end rod bearing starts to go, I noticed that at part power, let's say 1/4 throttle, the engine surges up and down. It is a definite noise you can hear.

  • LJ

Posted August 30, 2006 - 07:59 AM


Check to see if the sprocket gear slipped on the cam. Make sure the lobes are 180 apart when the engine is at TDC. Do a search on the forum and I think you'll find people who have had the same problem.

  • busa4

Posted August 30, 2006 - 08:04 AM


THANKS. i driained the oil and there was no metal particals and the oil was damn near clean. do you guys know where i can get a cheap gasket kit at? ebay has nothing for these motors. i am going to reassemble it and recheck the timing.

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