125 gy6 spluttering on acceleration

[dazza1974]

Hi all. I have a direct bikes db125 cobra gy6 that is spluttering at the top end of acceleration once I have put a k&n style filter on. I have a standard 125 engine with a naraku crankcase breather and an aftermarket sports exhaust and have rejetted it to #115,originally it was #110. I was using the original airfilter box and then got a k&n style filter. I increased the jet to 120 and after I had put it all back together I started it up and it idled ok until I opened it up and then the backfiring began. I took the #120 jet out and put the #115 back in but it still backfired. it would only run properly when i put the original a/f box back on. I have changed the a/f before and it worked but it was an angled one so wouldnt fit properly and came off. so, what have i done wrong this time? Im baffled by this and just dont know what to do.