"Seems to run good" is not a good way to diagnose this issue. I assume there is no tachometer so you have no idea what rpm the engine is turning. You also have no idea of the internal health of the engine. Unless you perform a compression test you don't know if the engine is capable of "making" full power. An engine that revs freely on muffs or in neutral in a bucket means nothing since there is no load on the engine. The engine may also have a prop with way too much pitch. If this suddenly happened, that's one thing. If the engine is new to you, then that's another. Have you pulled the plugs to see what they look like? Is the engine running on all cylinders. Again, on muffs, lots of people cannot tell the difference if all cylinders are firing or not.
