Craftsman Riding Mower Won't Start

I have a mid-90's Craftsman 46" riding lawn mower (model:917.273421) with an 18hp liquid cooled Kawasaki engine. When I turn the key I hear a loud "clunk" sound. I have previously dissembled the entire top of the engine (radiator, fan assembly, etc) and the clunk is the starter sprocket coming up and hitting the flywheel. It does this over and over but will not continue to turn once it hits the fly wheel. If I keep going and get lucky enough for the sprocket and fly wheel teeth to match up, it cranks and starts fine. If I jump the leads on bottom of the starter solenoid it cranks fine. I put in a new starter in last week thinking that might be it and it still does it. The battery is only 2 years old, and I have charged it several times. It seems to do better with a fully charged battery but not all the time. I’m really scratching my head on this one. Any help would be much appreciated.

Craftsman Riding Mower Won't Start