having problems with the engine on a cg 125?

hi there am having problems starting my bike. the bike will run for a short time then a a weird sound will come from the carb eg like a poping sound. I have checked the air filter and petrol going in to the carb seem to be ok and the rev counter goes from 1400 rpm to about 4000rpm this goes up and down for a while i tryed tune it on the carb but still the same then theirs back firing sound then it cuts out wot could this be .the bike is a cg 125 replica cheers :O)
{urban dz 125 }

mr c lumsdale
October 2007