Let's look for simpler problems::
1. Spark plug -- clean and regap, or replace.
2. Air filter -- clean or replace
3. Carburetor -- clean, check float for sticking.
4. Water in fuel tank from winter? Drain, put in fresh fuel.
A valve problem (4 cycle only) would possibly cause backfire. However,
carbon buildup could also be a culprit. Pull head, and clean, but you
will likely need to replace the head gasket, and you need a torque
wrench to put it back on correctly. Torque sequence of bolts is
If there is a gear problem, it shouldn't run at all, I would think.