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Message: [QUOTE="will pattison, post: 61832, member: 19283"] well...i have a cross country race this weekend, and at this very moment my crankshaft is sitting on the workbench. like all racers, i have the occasional mechanical catastrophe. this engine failure on my '99 yz400 makes 4 over the course of twenty years of riding and racing. i guess one every 5 years isn't a bad average, but that's not doing much to ease my stress level. i'm reporting all this because i have an unusual situation- at least for me. i can't figure out what's wrong. while i'm not going to claim to be the world's best mechanic, there are few places i haven't had my hands in on a modern dirt bike. i've built wheels, repacked linkages, rebuilt shocks, revalved forks, split cases and done countless top ends. to find myself unable to diagnose a mechanical problem on one of my bikes is...well...disconcerting. anyhow, i'm hoping that somebody else will have some insight. here goes... the patient: 99yz400, 55 hours on the motor, about 15 hours on a new set of rings and new head and base gaskets. fmf pipe, 8 oz. flywheel weight. barnett clutch plates. no other parts that are not original. the symptom: idling down the road to the starting line, the bike suddenly makes an odd sort of grunt/cough and dies. no amount of kicking will start it. i discover that i can kick it all the way through without activating the decompression lever- i have about half compression. we pull it. no luck. the bike has been running perfectly since the rebuild this winter, and it was running fine before that. the first examination: - all valves can be moved freely by hand, and return easily to their starting position without binding. - no broken cams. - no visible damage to cam chain. - all teeth on both cam sprockets and on crank sprocket. - the cams are nearly 180 degrees out of time. this is the only obvious anomaly. after much confering with thumper racing and stroker, i replace the cam chain and the cam chain tensioner. when i reassemble the top end and kick it through, the condition is the same- only about half compression. the second examination: - leak test on valves shows the seats are...well...seated. - there seems to be lots of carbon on the bottom of the head and around the perimiter of the piston. - there is about 0.006" lateral play in the big end bearing (there should be zero, right?). - there is about .075" wobble at the little end of the rod. yamaha specs about half that, maximum. - slight play in crank bearings. - ample spark. - ring gap on top and middle rings is acceptable, but at big end of tolerance. - no cylinder scoring other than the usual at the front and back (typical, especially with slipper pistons). - no evidence of char on head gasket. - plug looks perfect, maybe a bit lean. regardless of what it's contributing to the problem, i'm obviously putting in a new rod (actually, it's most economical for me to put in an entire new crank assembly). i'm also putting in a new piston and gaskets. i and my buddies are baffled. any thoughts? ------------------ will pattison engineer, racer ignition www.ignitioninc.com [/QUOTE]

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