Motorcycle Repair: 50 CC Engine/Carburation, screw adjustment, air screw

Question
I have a new 2006 4 stroke Mini ATV, with a 50CC honda clone engine.  On initial start up, it is hard to keep running and once warm runs OK.  When I rev it up it bogs, and makes a popping (backfire)noise.  I have tried idle and air screw adjustment, changing plug, air filter removed, and even adjusted the needle both lean and rich.  No change was observed.  I'm stumped!  Any suggestions?

Answer
Mike is there a restrictor between the exhaust pipe and head. its usually a washer inside with a real small hole in it, to keep it from running to good. you have to take the pipe loose from the head to see it. also they were putting restrictors in the airboxes as well. is this a Dinli ?

Eric