Picked up this bike for a couple hundred bucks, first 4 stroke. Guy swears it would start after adjusting valves, carb, and oil. He has FMF pipe, new top end, new jets, new spark plug, coil, and has gone through the transmission. After adjusting valves. cleaning carb, adjusting floats, rebuilding carb(bad o rings). Learning starting procedure, learnig bumb starting procedure i have Fuel Air(clean filter with oil) new Plugs with spark(good crisp blue spark, and gapped) Can find tdc and can get it to backfire through the carb????? This happens when bumb starting and kickstarting. I changed to race fuel, checked all the connections. checked valves again. two outside are tight but are good clearance. Compression is good but i am not sure he rebuilt entire top end. i can still push it though the cyclel but its not easy or fast. I took it by a shop and they told me to forget the decompression lever all together(contradicting all I have read) still backfires through carb, oh and no throttle what so ever. with and without choke. if i use throttle it floods. What would cause the backfire through the carb, as i have cleaned, checked, and changed now it actually blows the carb off the boot to the engine. could he have put the lower case together wrong or switched some electrical part i am just missing? I would think tdc, chain, cams all mechanical is in line. but why the spark on the intake stroke? Please help!!!!