I bought a used Craftsman (Sears) 10hp riding mower (model 502 255050) and it only runs at high throttle settings and backfires a bit. Everything else works and it does go at this throttle setting, but not comfortably. Clutch tricky to let out at this speed. (lifts front wheels)

I think the gas in it is old. It probably needs a new sparkplug, air filter and oil change.

Sears still sells the parts, but I haven't been able to locate an owner's manual.

What is the first place to look to cure this problem and what is the correct plug gap?

Any help will be appreciated.

I would do just as you said, replace fuel (probably water in it) replace plug, air filter and oil. Should help greatley, not sure on plug gap check with the parts store.

Don

Replace the gas, there is probably water in it. Clean the plug and get the right gap.

And while you are doing all this, remove the carb's float bowl and clean it.