Sears Craftsman tractor cranks but won't turn over
I'm having trouble getting my Sears Craftsman lawn tractor started. It's a model number 917.272461 with a 21.5 hp Briggs and Stratton (model number 407777 type 0174-E1).
When I turn the key (with the clutch and seat safety switches engaged), the starter will spin the flywheel/gear-ring a revolution or two and then everything stops.
The tractor was stored in my unheated garage all winter with a nearly empty gas tank, no stabilizer or anything. I replaced the plugs, two sections of fuel line (the length between the fuel pump and carb was leaking), the fuel filter, the battery, cleaned the air filter, and put new gas in it...but it does the exact same thing regardless. I was able to start it once off a jump from my car, and it ran fine for a few minutes.
If it started once and ran, that means the carb is clean enough, gas is getting to the engine, and the plugs are sparking, right? And if I see the starter engage the gear-ring under the flywheel, then the starter should be good, yes? I would say maybe I got a bum battery but I have charged it and since the one jump start that worked, I haven't been able to jump it again.
I can spin the flywheel/gear-ring by hand and it turns most of the way around without much resistance, then gets tough to turn for a second and then it releases and goes around and does the same thing again. Not sure if that is normal...
Is the starter just not generating enough torque? I feel like if it were to fail it would fail completely...not partially. Is a safety switch shorting somehow when the engine starts to crank so that it cuts out before it can get running? The thing that really mystifies me is the one successful jump start.
I've been looking through a lot of posts here (GardenWeb appears to be the place to ask), and although I have found a lot of useful information, nothing has shed enough light to solve my problem. Any help would be appreciated.