Hey Knuck
The kind of backfiring you're talking about is usually caused by too lean of a mixture in your idle setting. The cause of that could be either idle screw setting too lean, not enough back pressure in your exhaust (like from drag pipes), air leaking into either the carb or intake manifold, or a leaky exhaust gasket. You might have an ignition problem too if it's an oldie. I'll let someone else talk about that.
When you mention "old", as in your carb, it makes me wonder how old is it? Really old butterfly carbs can often have loose shafts either from worn pivot holes or just worn out shafts and air leaks past the butterfly when closed making it darn near impossible to get a good idle setting. You can check this out just by grabbing the butterfly with your fingers and seeing how much movement there is. A little movement is OK but how much is a judgement call I can't do from here. The traditional fix for this is to get a new carb or have the old one fixed somewhere. Parts for the old bikes are hard to find but a creative machinist could fix you up. All this if you want to keep the old carb like if you wanted your bike kept all original. Otherwise just get a nice used later model carb. The newer carbs work better anyway.
Other than a worn out old carb I'm saying to look for either too lean of an idle mixture setting or possible leaks in either your intake (especially somewhere around the manifold) or exhaust (especially at the exhaust flange gasket). These are the most likely culprits other than a too lean idle adjustment so check all these possible leaks before you go out and buy a new carb. Parts changin' before you've determined the true cause is expensive and just generally depressing.
Good Luck Dude  | 
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