Scag tiger Cub / Briggs Engine wont tunr over when the Hydro belt is on? Help?

After finishing a fall clean up I drove my tiger cub into the trailer and off to the next job. Upon arrival the starter would turn the engine slowly a couple revolutions and the solenoid clicked. After changing the battery and solenoid. Removing the starter and bench testing and removing the clutch...I found it will turn over and start every time with the hydro belt off. I turned all four pulleys (2 hydro, tension and guide pulley) both directions with zero resistance or indication of a bad bearings.

Any thoughts ? I'm stuck.

I'm thinking the starter might be weak? Not sure how to test a weak starter? The bendix flies out on the bench and installed starts the engine with the hydro belt off.

It wont start with the hydro belt on?

Scag tiger Cub / Briggs Engine wont tunr over when the Hydro belt is on? Help?