Small Engines (Lawn Mowers, etc.): Riding Mower Engine Wont Start, craftsman riding mower, amp fuse

Question
Dear Mr. Mielhausen,

Here is my situation.  My Craftsman riding mower, a 17 HP 42" deck electric start model bought in 1998, won't start.  It was running perfectly at the end of last season, and has been stored in a dry garage and well maintained.

The first crank of this season didn't happen.  The starter engaged, and the motor did try to turn over (at first), but it wouldn't crank, and with each attempt the motor turned slower and fewer times until nothing at all happened.

I have done the following so far to resolve the situation.

1.  Replaced the battery.  No change.
2. Drained the old fuel.  Replaced the fuel filter.  No change.
3. Replaced the starter with a new starter.  The engine cranked once, and ran fine until I shut it off, thinking I had solved the problem.  But it never started again.
4. Replaced the starter solenoid.  No change.
5. Replaced the positive battery cable, since it showed some signs of corrosion.  No change.
6. Replaced the 30 amp fuse, even though it tested okay.  No change.

The mower belt is disengaged.  I have used a jumper cable to go from the positive battery post to the starter. I have used my truck battery.

Nothing matters.  Turning the key now just gets a click from the new solenoid.  
Using the cables to jump to the starter itself gives me a three quarter turn of the engine, and that's it.  It sounds to my untrained ear like the engine encounters some sort of resistance it can't overcome.

I can turn the engine by hand.  If I take the spark plug out and use the jumper cable on the starter, the starter and the engine turn very fast, for as long as I leave the cable connected.

Any ideas?  The Sears service guy came out and poked at it and said a governor spring might or might not be broken, and fixing it might start the engine.  Or it might not.  I hesitate to spend eight hundred bucks on might or might not.

I'd appreciate any suggestions!  Thanks.

Answer
The Sears service guy came out and poked at it...hmmm.  I can suggest a couple of things.  Being that you say it cranks over fine with the spark plug out, a couple of different things could be happening.  First, check to make sure the oil level isn't ridiculously high...this will help to hydraulically lock the engine.  Second, when the plug is out, and you are cranking it over, check for raw fuel coming out of the plug hole...if there is a carb problem (needle, seat or float), the engine could be hydraulically locking with liquid fuel.  Third, it could be a problem with a sheared flywheel key...if the key is sheared, the engine can actually try to run backwards (fire too soon before top dead centre).  Fourth, there could be an issue with the decompressor...the way to diagnose this would be to put a compression gauge in the engine and crank it over (this may require a boost from your car battery).  If the compression is over 100 psi, it's a good indication the decompressor is disfunctional.  I have typically seen compression readings of around 60 - 80 psi on a fully functioning engine.  If this is the problem, you will need to remove the engine and partially disassemble as the decompressor is part of the camshaft assembly that hangs the exhaust valve open slightly to stop the engine from reaching full compression at low RPM (starting) and it centrifugally retracts when RPMs increase.  Let me know what you find.