Small Engines (Lawn Mowers, etc.): Will not start after trying everything, head gasket problem, strong park

Question
Dear Sir,
I have a Kohler KT17QS, spec number 24167, that is 10+ years old. I have been running it since mid February while waiting on new deck belts. I was just riding it around the yard to get it ready for the mowing season. However, it was running a little rough. I kept trying to adjust the carb to adjust it out and it seemed that that was not getting any better. So I cleaned the gas tank, replaced the gas hose, and installed a carb kit, basically replacing everything but the float. Cranked it up again. No change. So I checked the spark plugs. One ok and the other not so good. Checked the plug wires and found one faulty. Replaced bad parts and turned the key. Nothing, except the sound of the engine turning over. Replaced the other plug and wire. Nothing. Checked the points, firing. Checked the plugs against heads, firing. Put a little gas in the chamber. Nothing. I have a half of a tank of gas in the tank. The shutoff valve is open. And I do not know what else to do. Any suggestions?

Answer
If you have a strong park (can jump 1/8" gap) and it won't start, even with a squirt of fuel, I suspect you have a compression issue.  Because this engine will likely have a decompressor feature built in to the camshaft, doing a compression test will not get you a valid reading.  You need to listen closely to the exhaust and the intake for a "whisping" noise while you are cranking it over.  This indicates compression leaking past a valve and a valve job would be required to fix it.  If you have a head gasket problem, you could be losing compression there (and you should be able to find the leak with soapy water.  Another thing to check is the muffler isn't plugged with something like a wasp or mouse nest.  Hope this helps.