Frankenbike Paperweight?

3 replies to this topic
  • SpAStiCYFZ

Posted October 02, 2008 - 04:51 PM


i've got 99yz400 with a 426 jug which happens to have a fresh sleeve and
new thumper racing 3mm over yz400 piston. when i put this together for the first time, it ran for about 10 seconds and died then wouldn't restart due to
a massive loss of compression.

when I checked the cam timing again i noticed the intake cam gear had slipped on the cam itself. i figured the cam timing was making me lose the compression, so i put used cams from ebay in her. i also relubricated the piston rings and piston and jug and made sure the ring gaps were not lined up.

now it still has not much more than 30psi of compression. i am totally out of ideas. PLEASE SOMEONE HELP ME FIX THIS TURD BEFORE I SET IT ON FIRE!!!
:lol: :bonk: :lame:
thanks in advance,

  • grayracer513

Posted October 02, 2008 - 07:23 PM


Did you leak check the valves after the cam seized and slipped the sprocket? Probably bent the valves.

Did you torque the cam caps in the correct pattern to no more than 86 in/lb?

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  • desert-rat660

Posted October 03, 2008 - 07:31 PM


the valves probably bent as gray suggested, or one of the shims is cocked under the valve bucket. if the cam timing jumped u should have repalced the chain as it was most likely the culprit

  • SpAStiCYFZ

Posted October 06, 2008 - 06:56 PM


the cam slipped because of the friction in the cam journal. i lapped the known good cam into the head so thats good. the cam chain was replaced with the topend. i took it off and checked it when i tore it down for the second time. none of the valves are bent.

i assume none are leaking unless im retarded. it has over 80% leakdown. its not hissing from the exhaust or the carb. and i dont hear it leaking through
the crankcase either. but it has to be something in the rings/piston. i just dont understand. i had the sleeve sized to the piston. and the piston and rings are brand new. do you guys think the oil viscosity i used could have an
effect on the rings sealing initially. i just used 5w20 toyota shit i had at work. thanks for the input by the way guys.

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