Small Engines (Lawn Mowers, etc.): Kohler Moter, using jumper cables, hp kohler

Question
Im writing this for my fiance so excuse me if I dont sound like I know what Im talking about.  But recently he bought a used Craftsman tractor (Model #917.275661, Ser #031605C003268) with a 20 HP Kohler motor (Model #SV6005, Ser #3500406723 from someone who couldnt get it started.  It started once (by using jumper cables and repeatedly turning the key) and it ran great so he figured it would be an easy fix.  There was just a little rattling noise under the hood every once in a while.  But now he cant get it started again not even with jumper cables.  Originally when he turned the key, the starter would engage but the flywheel would only turn alittle bit.  If he took the sparkplug out it would turn over great.  He pulled of the flywheel and the key was sheared so he replaced it. He bought a brand new battery, silenoid and spark plug, had the starter rebuilt and replaced the battery cables.  This still did not fix the problem.  Now if you hold the key it will turn over real slow until it hits a certain spot then it will turn a full turn then stop again. He has checked the flywheel again and the key is good and tight and the compression is 90 PSI.  Any ideas on what this could be?  Thank you for any help you can give on this.  He has been working on this for over a month and cannot figure out the problem.

Answer
Ok, first off you did very well in explaining, you sounded like a long time mechanic. This motor is hang-up on the compression stroke. I am not real experience on this particular engine, but, a lot of larger engines have what is called a compression release. This allows the engine to turn over easier during starting, by releasing the compression for a split second in the combustion chamber. If it's hanging up on every complete revolution then it's on the compression stroke. And just so you know, the last Kohler I did a compression test on was last week, and it had 155psi, and had a lot of hours on it, that 90psi is weak. With the sheared flywheel key something was hit, is the crankshaft bent, and hitting rubbing on the oil sump case where it comes out of the oil crankcase? With the lower compression there is also a chance it is fighting the timing.........Tom