A9000

Try purging the transmission, see "purging transmission" in the owners manual.

Make sure you haven't mistaken the mower belt for the drive belt then the problem could be the drive belt is broken or something. The easiest way to identify the drive belt is it's the belt that goes to the transaxle pulley.

Note  my 25 HP 48" modle 917.273241 tractor had a extreme drive belt slippage problem due to a too small of a spring on the idler pulley. If yours has the same problem (transmission pulley isn't turning when the clutch and drive petal are engaged) you can verify that with someone checking that using a mirror while the other person is operating the tractor.

I've had the same problem it sounds like in two Kohler Command engines 15.5 and 25HP and after compleatly taking the 25HP's carburator apart I traced the problem down to a corrosion problem in the fuel solenoid on the bottom of the float bowl. A more correct discription of it I think is a anti dieseling solenoid that stops fuel from entering the carburator from the float bowl so the engine won't run on after it's turned off. Evidently what happens is the water that accumilates at the bottom of the bowl slowly corrodes the steel actuator and brass solenoid sliding surfaces. If it's used alot you don't experience a problem, it's when it sits for along time the corrosion builds up to much for the the actuator to break through it so it stays in the off position, thereafter the engine can be started but it runs eradically and only when using alot of choke.

For a possible solution checkout;

http://en.allexperts.com/q/Small-Engines-Lawn-1746/KOHLER-carburetor-problems.htm