starter problem

I have a Husqvarna LHT125 hydro riding mower with a 15.5 hp Briggs and Stratten engine. Using a fully charged 300 cranking amp battery the starter will engage the flywheel but will not rotate fast enough to start engine. I took the starter in to my local repair shop , they checked it out and said it was drawing too many amps and they sold me a "almost new" used starter. The same problem persists. All the connections are tight, no loose wires. I have had to use a power bank/booster to start engine. When it starts it runs great. Any help would be much appreciated.

starter problem