How old is the engine. Is it the classic Vacu-Jet diaphram carb, (carb on top of the gas tank). Or is it a recent float type carb, (tank on top of the engine).
Old style: First adjust your idle speed till the engine stays running. Warm up engine. Now start adjusting the mixture jet. It only has one mixture screw. Start at 1-1/4 turns out from seated. Turn out till it runs crappy, then turn in at 1/8th turn increments. Wait a few seconds between adjustments to let the engine settle down. You may have to readjust the idle as you find it's sweet spot. Keep going in till it starts to stumble again. Now go back out till it smooths out, (usually only 1/8 turn).
Now nail the throttle a couple of times. If it stumbles badly or stalls, back the screw out a little at a time till you can hit the throttle with no stumble.
Be aware the these carbs are a compromise at best. Adjusted as above, for good idle and throttle response, is a little rich for full throttle, max RPM, power.
New style: Same as above, or you may only be able to adjust idle speed. If you have a mixture screw it's only for the Idle circuit. Adjust it as above. If it runs crappy at open throttle, it's most likely dirt in the main, (fixed) jet. If it's clean and still runs crappy, all you can do is try adjusting the float, not alot you can do here.
Good Luck