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Engine Backfiring/Popping

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07 CRF250R with an Athena 280 kit on it. 5 hours of time on the 280 kit, 88.2 engine hours on the bike.

The bike was running fine on race day - did practice, but in moto 1, second lap it coughed a couple of times then died. It now behaves as follows.

The bike will start -seems to want the hot start more than before - and will idle OK. However, as soon as I start to give it throttle it pops/backfires loudly - no flames out the exhaust.

I tried a new plug. Rechecked the valves and they were right where they have been since I had three hours on the bike. Timing marks are in alignment. I disassembled the carb and cleaned it and blew everything out with compressed air. No indication either the hot start or choke were sticking. None of this changed anything.

When taking the carb off I found that the wire bundle from the stator had rubbed between the frame and carb - the bundle sheath was cut through and two of the wires had breaks in the insulation. It did not appear that any of the wires were broken. I placed additional insulation around the wires, taped it up and put everything back together - no change in performance.

Convinced the wires were the problem, I cut out the part with the broken insulation, soldered them, and put heat shrink insulation on - no change in performance.

I am at a loss as to where to go from here - anyone have a similar experience or even a theory on what might be wrong?

Thanks for any help -

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Check the spark plug wire and the spark plug connector. My 08 ktm 250f did the same thing and it turned out to be a cracked spark plug connector, $15 fix.

How did I end up in the Honda forum?

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did you adjust the jetting when you put the bore kit in? you have to adjust the main jet......as ive been told

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did you adjust the jetting when you put the bore kit in? you have to adjust the main jet......as ive been told

He said he's got 5 hours on the 280, and that it was running well.

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I would check the hot start again. It's either the hot start, the valves, or since its getting cooler outside the jetting. Chainge the air filter and add fresh gas before you start messing with anything.

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Thanks for the suggestions. Fresh fuel made no difference and the filter was just installed - no dirt on it. A friend let me try his ignition coil/spark plug cap and Ignition Control Module (CDI unit), neither of those made a difference. We sprayed brake clean everywhere an air leak might exist and got no response.

With respect to the hot start and valves, I don't know what else to check. The valves were and continue to be in spec. The hot start operates smoothly and snaps closed after being used. I took the carb apart and cleaned it and holding all jets up to light observed smooth, round, shiny holes - no evidence of clogging.

I want to emphasize that when the bike's throttle is cracked, just above idle, it does not just run poorly but usually makes some loud explosive noises - like a backfire.

Any additional thoughts/suggestions appreciated.

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This sounds exactly the same as what happened to my 250. The running good in practice and the sudden sputtering and dying. It idled fine but when given gas it sputtered and backfired. Check everything that has to do with the spark plug, moisture getting in anywhere, make sure there are no cracks any where. Like I said this sounds almost identical to the problem I had.

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Are you sure you didn't flip the plate on the carb slide upside on accident? I did that and it was backfiring all over the place. if it looks like a triangle it's obviously wrong, need the semi circle cut out with the hole in it down.

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