Start with the basics: Fuel, Air and Spark. Too little fuel would lead to a backfire or pop. A backfire through the carb is a good tell-tell sign of a lean condition and best to start on the fuel system.
Stumbling is closely related to spark problems. As your revs increase the demand for the spark increases. The problem with ignition systems is where to start? Start with the cheap stuff and work up. Plugs, wires, cap and rotor first. Inspect the make sure all the 12V wires and connections are good and tight.
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