Does it perform correctly when you move the speed control at a moderate rate? If so, chances are the carburetor and governor do not work and play well together under extreme conditions. The classic way to achieve a backfire (which is a gas explosion in the exhaust pipe or muffler) is to run the engine at high speed, then switch the ignition off briefly then on again whilst keeping the throttle open. Sometimes the muffler bulges or bursts under those conditions. Now that cars have catalytic converters, such stunts can become very expensive and are best avoided.
If your engine has an ignition kill function that is activated by setting the speed control at minimum, it is easy to imagine the control over-travelling slightly and briefly shutting off the ignition, then letting it resume.
