Mike: You can address both the carbon buildup and sticking valve issues with the motor intact.
To resolve carbon buildup: warm the engine up well and carefully spray water into the intake while manipulating the throttle. If there is a good deal of carbon, it will be expelled out through the exhaust. Some Forum members (who include Dennis Frisk, if memory serves) report a satisfying spray of sparks coming out as well.
To resolve the sticking valves: I've had the best results from using the Low Ash oil available at your local Cub Cadet dealer or one of the Forum's sponsors (I know Cub Cadet Specialties carries it). I used to have some really aggravating issues with valves sticking until I switched the Low Ash oil, but I have had none since. You can also hit the valves directly with Sea-Foam or other solvent if you care to remove the carburetor and/or muffler/pipe to get at them.
That said, I'm not sure that either of these issues are really the problem with your engine. If the valve is truly stuck, you have no compression and the motor won't start until the valve reseats itself, usually after the head has cooled down a bit (about a 1/2 hour). I'm not sure what symptoms excess carbon cause in an air-cooled engine, but I imagine pre-ignition could be caused a "hot-spot" developing in the carbon. Harry Bursell maintains that carbon particles score a great many more cylinder bores than they're given credit for. I know that in water-cooled automobile engines, the carbon can build up on the back of intake valves to the point that it seems no air can get through the manifold!
I do sympathize with your plight though, my engines often react very differently when they are hot as opposed to when they are cold. And I've had some that I dreaded restarting hot. I remember when the rust in the gas tank of my 149 had so clogged the filter that it would die and NOT restart until I had given the carburetor bowl time to refill. Charlie helped me understand that I really needed to clean the tank properly (see the FAQs). I wound up securing a really clean replacement tank from Forum member Tom Hoffman.
Something that helped ALL my starting problems was setting the timing correctly, which may not be an issue with your motor.
Intuitively, it seems to me that your hot restarting issue (given your choke/throttle actions) would be a fuel/air delivery problem. Are all the filters clean? Does the engine react differently with the air cleaner off? Have you pulled the plug and looked at it before you try restarting it to see if the engine is running lean or rich?
Another thought. If the head gasket is leaking, it can cause all kinds of problems, with running and starting. Even though the tractor "runs," it runs a lot better with a well-sealed head.
I stopped oil consumption issues, and greatly aided my starting and running issues with a new head gasket. Of course, the head has to be flat too in order for the gasket to seal properly. I believe there is a FAQ on preparing a head for reassembly.
I know you don't want to take the engine apart, but the experience of many Forum members and myself confirms that it is often the most direct route to figuring out what is wrong with an engine.
