I have a Kohler command pro 27 hp engine. The engine runs but has half the power as it normally does?.

Dwayne, the third link talks about the signs and test for leaking head gasket. Check the attached links,instruction and guides, Good luck
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Check the oil for fuel in it. Oil will be really thin and be over filled. Take out the spark plug and see if excess fuel is pushed out when you crank it. If there is then the fuel shut off solenoid on the bottom of the carburetor is leaking fuel into the engine when it's not running.

valve seats have come out do to hitting something motor starts and runs fine the slowly loses power from super heating and finally stops wont restart until cools down right valve jobs per cylinder 150.00 each

Make sure the choke is coming off. Check the compression.Many times the head gasket may leak when the engine heats up. Also check for spark sometimes the magneto can heat up and quit after running a while. See if it will run if you prime with a little fuel, if so ,some of the Kohler engines this size have a fuel pump that could be failing. Check for vapor lock. It takes air and fuel properly mixed, spark and compression any of these missing the engine dies.

spray some starting fluid in carb if starts ether bad feul filter or fuel pump or need to rebuild carb

YES

run it with the gas cap loose the vent might be plugged then check the pump

Try changing the spark plug.