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kx250F backfire

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OK I know this has been addressed before and I have tried what seems like all the suggestions. I have an 06 kx250f recently bike began popping on deceleration. Starts easy and will idle just pops and RPM's wind down. Had a crash and cracked header pipe (prob was old crack and got bigger). So replaced that thinking that was part of problem. Well that didnt work. Still pops on decel. Valves are dead on, carb is clean and passages are clear. I have stock idle jet #40 (recently was up to a #48 with no issues till now) and tried moving fuel mixture screw started at 1 1/4 out with no change. Actually moving fuel screw to any position makes to diff at all at one point I removed fuel screw totally and still had same issues absolutely no change. Recently there was an old fuel screw where tip had broken off inside carb but replaced with new one and no change. I don't see the old piece in carb so I am assuming it is out. I unpluged the fuel sensor unit out of desperation to see if any change and none at all. Had a similar issue about a year ago and changed the CDI box with a used one. I am thinking this may be the issue. Does any one have any thoughts. Oh and have meticulously checked for any air leaks and found none. and when it pops or backfires there are flames out exhaust on occasion. I have had my mechanic (who is very good also check my owrk and all good) Any suggestions would be appriciated
 
 

 

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