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yz400 stumper...
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[QUOTE="will pattison, post: 61851, member: 19283"] that's why this thing has been so wierd- you guys are all correct, but the evidence isn't there to indicate that a gear slipped on a cam. at any rate, i got the motor built back up again last night, and i have enough compression that i can stand with my full weight on the kickstarter and not so much as budge the piston. that's more like it! here's my theory as to what happened: i don't know how the slipped cam chain fits into the picture, but i believe a blew a head gasket- instantly and catastrophically. don't ask me how. my only other blown head gasket was on my '98 as a result of the head bolts around the exhaust coming loose, and it was gradual. i had symptoms like high rpm cutting out and very hard starting that i didn't have in this case, but nevertheless, i think that's what happened. there is no soot on the gasket, or any other obvious signs of damage, but i can't figure it any other way. now...if i were superstitious, i would say that the beast did it to itself out of self preservation. see, i was having suspicions of looming crank trouble because of of some slight knocking at certain rpms. further, the race i was about to start that day was a fast, high rpm affair with several straights where i would have easily been hitting 80 mph. i could say that this bike somehow knew it wouldn't survive the event without throwing the rod and doing huge damage. so, it coughed up it's own head gasket and threw it's own timing chain just to make sure i had to get in there and find the big end bearing about to let go. IF i were superstitious. as it is, it's just a wierd mystery i'll never solve. i will be racing this weekend though, and i have to once again give some kudos. central yamaha in plano, tx was a huge help in getting all the parts on time at a good price. yamaha itself gets both a thumbs up and a thumbs down. there is no reason this crank should have gone south. this bike had 1/3 the time on it that my 180 hour-old '98 yz400 did when i sold it, and those hours were more gentle to boot. other than the blown head gasket, the only issues i EVER had with the '98 motor were leaky water pump seals. on the positive side, i am totally pleased with the tolerancing on this engine. i put the crank in the freezer for about an hour, then heated the left side bearing with my electric heat gun for about 5 minutes. even without grease, the crank would have dropped in easily. additionally, the cases practically put themselves back together. good engineering. so ends the story, i hope. it's not completely back together, but at this point i have no reason to believe it won't run like a charm. thanks tony for the lesson on compression checking and to all youse other guys for the moral support. wp. ------------------ will pattison engineer, racer ignition www.ignitioninc.com [/QUOTE]
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Dirt Bike Brands - Other
yz400 stumper...
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