Need help with a riding mower that won't start

I have a 10 year old Poulin Pro that has a Kohler engine. The symptoms of my problem are simple. After the mower has sat overnight, it is easy to start. Even if I run it only a short time, it is easy to start. However, if I run it for an hour or two and shut it down, I may not be able to start it back up. It appears that the problem occurs only if the engine has been run an extensive amount of time and then shut down. In that case, it won't start until I let it sit overnight. What is going on? Any advice will be appreciated.
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This seems to be a common question on this NG.
An internal combustion engine needs three things to run: 1) fuel (proper gas/air mixture) 2) spark (ignition at the correct time) 3) compression (the air/fuel mixture must be properly compressed)
So what I do is start with the easiest first. Pull the spark plug cap, crank the engine and make sure you have spark. Then I put a compression gauge into the spark plug hole and test the compression. You may need to refer to the manual or check online to ensure your reading is within specs. The next thing I check is the fuel system. A trick I use is to remove the air cleaner and put my hand completely over the intake. Crank the motor and my hand should be wet with gas. The motor should sputter and want to start (just like when you choke the motor).
I suspect you have low compression (when the motor is hot) and after it sits, things cool off, the compression is slightly higher, and it starts. But this is only a hunch- T/S is required.
I hope this has helped.
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I would bet on valve sink.
From Mel & Donnie in Bluebird Valley
http://community.webtv.net/MelKelly/TheKids
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reader wrote:

mower, remove the spark plug, and then check for spark. Don't forget your asbestos gloves.

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snipped-for-privacy@NOSPAMjuno.com wrote:

Which model Kohler?
--
Art

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snipped-for-privacy@NOSPAMjuno.com wrote:

on the fins? Is the engine running faster than it was originally governed? It sounds like it might be getting too hot. Also synthetic oil helps air cooled engines run a little cooler as well as longer life.
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Possible vacuum in gas tank. Try taking gas cap off (and maybe even blowing in the tank). BTW, a 2 hour run is a lot for a 10 year old motor. Try working at the machines schedule rather than your own.
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