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CRF Wont Stop Backfiring

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Gotta '08 CRF450R standard bike with DEP S7 pipe that wont stop backfiring on decel. It runs as it should when on the throttle but as soon as you back it off it pops and bangs.

Problem came just after i changed the piston

Any ideas what can cause this??

so far have checked for exhaust leaks (fitted new copper ring)

larger main jet to richen mixture (no difference, jetting is good)

new spark plug

timing is good

checked for dust/filings in stator its spotless engine only done 30hrs

im lost, any help

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fuel screw 1 1/2 turns out

it ran fine before piston change and never touched settings on the carb.

When i took plug out it was really white like it was running lean but when i went out and gave it full throttle and hit the kill switch the plug came out a good brown colour.

runing out of things to check what could i have done during the piston change??

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jetting was fine before piston change it ran perfect. what else can cause it backfiring. i did have the timing off a tooth when i fitted the piston and it wouldnt even kick over because the decompressor doesnt work when off the timing. so think ive got the timing spot on, had a m8 check it too.

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as for hot start thats what i was thinking because the old hot start cable was siezed so its not been used in a while ive never needed it, so got a new cable on at the same time as the piston was done. spent today cleaning out plunger and that. is there anyway to bypass the hotstart just to test if its the problem.

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