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HELP: bike runs then will get hot and die and not restart???

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OK, this is kind of hard to explain but my bike died out of nowhere the other night after about 3 laps around the track. It would try to start, but if I touched the throttle at all, it would die. I pulled it back to the truck and changed spark plugs...nothing. I then thought it was my needle clip (I had just adjusted this before I went) but it was ok. So I then pulled the pilot jet and it was fine. I got frustrated, put it all back together and guess what....It starts and then ran for another 3-4 laps around the track (may have gone longer, but we only get so many laps). So I went to the races this past weekend thinking it was fixed. I rode a few 5 minute practices with no problems and then in the first moto, I went about 2-3 laps and it died again. I tried to restart it and it would not work. I pushed it off the track and watched the rest of the moto and on the white flag lap it starts and runs so I could cross the finish. I go back to the trailer and change the plug and jetting again, checked for a shorted wire, and everything looked fine. So now I am just hoping for the best. I start it up every 15 minutes or so to make sure it keeps starting and as I am going up for the 2nd moto it dies on me. I get a push start and it starts so I go up to the line and it dies again and will not restart. I sit out the race and when it is over, the bike starts fine again.

When it is running, it seems to run fine, then just pop and die. I am guessing electrical, but have NO idea where to start. If I let it sit for awhile, it always seems to restart on the first kick and run for awhile. Anyone know what I need to do?

edit: I checked the valve clearances and they are all good.

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My guess is fuel...

When you changed jetting you introduced a foreign substance (water or dirt) into your carb. If your bike/airbox is super clean... try starting it without the airfilter and spraying carb cleaner directly into the the airboot/carb while reving the motor. If that doesn't clean it out. You'll have to pull your carb off and do a more thorough cleaning.

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Also check the vent tube from the gas cap, is it kinked? Or is the vent not functioning? Can do what you describe.

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When it is running, it seems to run fine, then just pop and die.

Be careful. A mates crf had a main bearing go. The engine would run for a while and then pop and die. I wouldn't say he had a catastopic failure. but bits of the bearing wrecked a brand new piston. I'm not sure if the barrel was scored aswell. Hopefully it's something simpler then that. But if she dies suddenly then better not run her hard until it's fixed.

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Decompression is spot on and the idle is set pretty high already. I will check the other stuff in the flywheel that RHC metioned in you other post. Thanks.

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See if you can borrow an ingition box from someone. It doesnt sound mechanical to me at all. More like it gets warm and a capasiter is bleeding off or something, or a transistor just quits or something....

My bet: Its electrical.

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I think so. I think it was metal shavings bridging the gap between the 2 wires sodered on the ignition pickup coil inside the left side cover. I cleaned the metal off and siliconed over the bare wires and it ran fine........ until I found out where the metal came from. Another story. But yes, I think that was the problem anf RHC was the one who helped me figure it out. Thanks Ron. Now I am off to fix my broken engine case.

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