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Yamaha MX & Off-Road Dirt Bikes
How bad must you melt a piston before it has to be replaced?
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[QUOTE="ws6transam, post: 1419148, member: 65064"] Thanks FNG, Actually, I had Eric Gorr recut the cylinder head for use with 93 octant fuel already and it certainly made it more useful of a machine. I went round and round on the jetting, and cleaned up the bottom end and midrange with swapping out from a Q8 to a Q4 the needle jet and putting in a leaner low-speed jet. I've moved the needle up & down, put on different expansion chambers & mufflers, fiddled with the main jet (and settled on the stock 440 main), and finished up with a swap to FMF power reeds to get the engine running smooth & starting reliably. Eric Gorr even ported the cylinder for trail use. I've had Garry from EFM Autoclutch custom-CNC an autoclutch for this engine, and I've milled, extended, & TIG welded the magnesium clutch cover in order to give it easy-to-use, dial-a-wheelie control, so that if I stab the brake, I don't accidently stall the motor. Now it'll torque through the tight woods in second gear, and still wail down a long sandy straight at 84 MPH. After running it in stop & go street traffic, doing 50 miles of highway, keeping it together for a cold, rainy 5-hour long trailfest, I thought it was getting pretty bulletproof. Then what I think was a chunk of that failed tank liner plugged up the fuel line during an hour's worth of sandy fire trail blasting and viola! It leaned out, got hot, pinged, and seized. I don't know the extent of whether it was the 93 octane (probably not), or the lack of fuel delivery that locked it up. I do know that at least twice that day I had to back off because the engine had been pinging. Fuel related or fuel delivery related, I am not sure. I always thought that as the day got hotter and more humid, the fatter an engine runs. This bike rips on cold weather days... and the plug in the head is blackish whereas it's normally a dark tan - so I think it had been running richer than normal before the failure. YZ490/WR500's are supposed to be particularly susceptible to temperature, barometric changes, and load requirements so maybe that was part of it. I'm just trying to think of ways to make it more resistant to those kinds of problems. [/QUOTE]
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Yamaha MX & Off-Road Dirt Bikes
How bad must you melt a piston before it has to be replaced?
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