If you are talking about the thermostat for the AutoChoke system, it is very easy to check. Take the air cleaner cover and air cleaner filter off. Look to see if the choke is closed, it should be. If it is open you will have to remove the shroud and see if the thermostat, mounted between the muffle and the block, is holding the airvane open, not allowing the choke to close. Carefully try to move the arm back to allow the airvane to move and the choke to close. Once the arm is all the way back, is the bendable part of the arm between the marks on the airvane? If not adjust the arm. Now reinstall the shroud and start the engine. When it starts the airvane should open the choke and the engine should run smoothly. Allow the engine to run for about 10 minutes to heat up the thermostat. Shut the engine off and choke to see if the choke stays open, which it should if the thermostat is working properly. This just means that the MAY be working, but you must now allow the engine to cool down. When it cools down the arm should allow the spring to close the choke, for the next cold starting situation. If the choke does not close after cooling down, and you set it correctly as stated before, then the thermostat is bad. If the choke was closed at the beginning, then you would start by running the engine as stated to test for proper operation.
From your lack of a complete description of what is happening and not answering some of the questions asked by the forum techs, it is very difficult to supply you with good answers. From the little info you do supply, I doubt that the thermostat has anything to do with you problem. I would be checking to see if the vent on the gas cap is plugged, is it losing spark, did you set the float level on the carb when you cleaned it and have you checked the valve clearance? Anyone of these or combination could be causing your problem.
