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Discussion Starter #1
I've checked everything so far, including new EFI module, new throttle body, new spark plug, new intake gasket, adjusting valve lash, checked compression (~100 psi) and so far no luck.

The engine starts find and idles great at ~1800 rpm for about 30 seconds then starts to slow down as it warms up. It gets down to about 1650 RPM, will often do a small backfire and quits. If I jump on and start riding while keeping the RPM above 1800, it seems to run just fine (maybe a hiccup or so, but not too obvious, seems to be running fine overall). Even when it's warm, when I come to a stop, it'll idle for a little while but get progressively less stable then finally just quit altogether...

I'm about to do a top overhaul to see if that makes any difference, but I hate to do that if there are other ideas that solve the problem...

Some ideas I have:

1) Sticking Valve; perhaps as the machine warms up, a valve starts to stick and causes the machine to idle poorly
2) Stator Sensor problem: Maybe the signal from the stator sensor on the front of the engine is providing intermittent signals to the electronic ignition module causing instability
3) Poor battery: It's an aftermarket battery that looks small (12 amp hours) and maybe with the load from the system it impacts idle, but I'm not sure that's true, and voltage is in the ~14.5 volts when the engine is running all the way up until it backfires and stops

Any other ideas? I'm going to try the top overhaul so at least I'll have an engine that is mostly like brand new, but I'll be bummed if that doesn't fix it.

Thanks

Darren
 

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Discussion Starter #2
Well, I spent way more than I should have to solve this problem. New cylinder, piston, rings, refinished head, new coil, new battery and swapped throttle body and EFI module in my pursuit of the problem...

Problem Finally Solved: The last thing I replaced was the Fuel Pump assembly in the tank. The local shop had told me that were pretty sure this wouldn't be the problem so steered me clear of it. However, I bought a complete aftermarket assembly and installed it. Problem solved. Apparently at low RPM, the pressure wasn't enough to allow it to run smoothly, whereas at higher RPM it must have raised the pressure enough to make the injector work well enough to keep it running. As soon as I installed the new fuel pump assembly and fired up the engine, it ran as smooth as I could have hoped... I've run it for 30 mins since then, on and off at different time spans each time and every time it's run perfectly.

Net-net, it now runs like a top, and I guess the extra money I put into a new cylinder, piston, and machined head will hopefully pay off in terms of an engine life that will last me for as long as I could ever want...
 

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So, how many hours/miles were on the that engine b4 the problems started?
 

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Discussion Starter #4
I just bought it used, but there are ~6,000 miles on the ATV and about 600 hours on the machine
 
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