backfire

[lvn]

i have a chinese bike, not sure what brand but runs a big air cooled loncin engine with 5 gears, closest bike ive found is a 250cc (but dont think its a 250) atomik link but thats not my problem.
when i run my bike and get the pipes hot it backfires when i rev it high, and it blows a flame like a foot long, its unreal, is this my air/fuel mix or could it be my valve clearance aswell?

 

[Cactus Jack]

Most likely valve clearances ... IF it's a pushrod engine it'll more than likely need more clearance than the usual OHC engines .003 ~ .004 intake / .004 ~ .005 exhaust ... The length of the pushrods result in greater expansion from heat so they usually need more clearance ... Check them with the engine hot to see if they're tight ...