I have a 12.5 lawn tractor engine with over head valve back fires at high speed key way looks good any ideas thanks

Hi,
Sounds like mostly an ignition issue. First try a new plug gapped at 0.030" to see if that helps. Most OHV plugs come gapped wrong for OPE applications.
From there it moves down to a possible module or carburetion problem in the low speed circuit. Does the engine get hotter than you think it should? I realize it is a subjective question, but overheating could lead you to the carb, possibly causing an ignition problem.
If it happens at all operating temperatures, i.e. even when first started, I would say ignition. If it becomes more pronounced after the engine warms, I would say it is carb possibly also causing ignition problems.
Pace,
Clarke
PS I gather it is a 28 cu. in. because of the HP, but if it is a 31 and the age is about four years old, then it could be camshaft if it is also down on power.