Just a couple of suggestions, I'm assuming it's a standard newer rotary mower with a vertical engine.
1) Is the cooling working ok? If it's not a rotary mower and the engine's mounted sidewards (if this is at all viable!) then perhaps the carby/inlet could be in a place where it's getting over hot?
2) Pre-ignition from carbon or dust buildup in the combustion chamber? Just for a diversion, read my little story
here...
3) running too lean will make carby air-cooled aero engines pop and backfire (and run too hot).
4) carby icing but that's not likely to be an issue here?
