Exactly, used mowers can be interesting. I would check EVERYTHING over like on CSI. Bought a gently used Snapper Commercial pusher that the lady installed the blade upside down. She said it didn't cut properly. Best 80 bucks I ever spent LOL.If you get in there again, look at the flywheel, there should be a part number on it. See if it is the correct one. The times that I have had a mower like this, the timing was off, usually the key. But since you bought it off someone, he may have had someone work on it that put the wrong flywheel on it.
slomo