Small Engines (Lawn Mowers, etc.): JD STX 38, Kohler 12.5 HP, selonoid, soleniod

Question
Hi Eric,
This question concerns a JD STX 38 (not sure of the year) model # CV12.59 (1215) It has a Kohler Command 12.5 OVH.
A friend decided to use this tractor after a couple of years non use. He got it started it ran for about 5 min and then died.
I went over to help, tried to start with no success. I changed the plug, put in a new fuel filter (paper type) and fresh gas. It started, but did not run well. I pulled the carb and cleaned it best I could and reinstalled. The motor ran, but again not so well. It is missing and at times there is white smoke coming from the exhaust.
This unit has a fuel shut off selonoid and I found that if I disconnect the wire the motor runs fine for a short while then starts to die. When I reconnect it runs fine for a short while then wants to die. If I do this repeatedly I can get the engine to run pretty well. Does this point me to the choke? I believe that this model has an auto choke, but I have not spent enough time with it to figure out how it works.
If you have any questions feel free to email me off line.
Thanks
Stan

Answer
The fuel shut-off solenoid stops fuel flow through the carbs main jet.  Some early models were prone to a weak ground causing the soleniod to shut off the fuel.

If you have a volt meter you should be able to measure 12 volts DC on the wire with the key switch in the run position.

Did you clean the carb with an air compressor and carburetor cleaner?  Have you changed the oil?
Let me know.
Eric