Dirty carb, restricted needle/ spray bar
Air leak, fuel/ feedback lines/ fuel tank
Loose engine component, backplate/ starter holder/ cylinder head
Clutch/ spur setting too tight, excessive clutch shoe wear and heat

I find that the first and second causes are most common.
Plugging exhaust is an overwhelming backpressure that will eventually shut off any engine, and does not rule out possible air leaks. You are using a fuel filter, aren't you?