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Bad Backfiring off idle

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I have posted this in the CRF250 forum but got no "relief". A local bike mechanic insists I have a fuel problem so thought I would try this more specific forum.

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 in spec. 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. Thanks to a friend I was able to substitute an ignition coil (plug cap) and CDI unit from a working bike. I also tried fresh fuel. Finally, we sprayed brake clean everywhere an air leak could exist and got no reaction. 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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A couple of clarifications, the bike will not run much above idle - the popping/backfiring is loud - not just a pop. The carb has a white brothers carb stuffer in it - has since it was new, with a 48 slow jet, 168 main amd the fuel screw at 1.75 turns out.

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Thank you for the quick reply - I noted from other posts that you had pointed out this possibility.

I had not considered that too much because my timing marks line up and at the same time I have proper valve clearance.

Is it possible for the cam sprocket to move and still permit that too happen?

I will admit that when I pulled the valve cover with the bike at TDC my initial reaction was the lobes looked lower in the rotation than usual - but the measurements did not seem to back that up so I dismissed it.

Thanks for your help and clarifications.

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You timed it based on marks on the gears, so the gears are in time. The cams sit on journals, as long as the base circle is over the bucket, you will have X clearance. The only way to categorically tell if the cams moved is with a degree wheel and dial indicator, unless there is a mark on the gear that matchs a mark on the cam.

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So I tried a new/different cam - no change in behaviour. The bike starts fine but starts backfiring/popping as soon as you give it gas.

Any ideas on where to go from here -

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