The one way bearing is slipping on the starter shaft.

I had one with the same engine that did the same thing. It ran awesome when it was running, and would start right up if the engine was hot, but after it sat overnight or even for 30 minutes or so after running it was hard to start again.

One thing you can try. Take your air filter off the engine, and then turn the flywheel until the opening on the crankshaft comes to the top so that you can see down in there. Then take a can of WD40 or Brake Clean and stick the nozzle down in there pointed towards the rear of the engine and spray a decent amount in there. This will help to get some of the oil buildup out of the one way bearing. I always sloshed some rubbing alcohol around in the crankcase of mine and it made a big difference in how well it started because the OWB slipped much less.

Eric