Hi sewallgood,
I will try to give you a few suggestions to help solve this problem. I see you have replaced the piston and rings. Did you hone the cylinder wall before installing the new piston and rings so the new rings can properly seal ? Have you checked the compression after installing the new piston and rings? Did you have instructions on how to properly clean and install a new carb kit, as it is very important to get it right on? Have you cleaned or replaced the air filter? When working on the carb, did you replace the very fine mesh filter that is located inside the carb? What can cause the engine to run just a few seconds, can be the fuel tank not venting. As fuel is being removed from the tank, air has to replace the volume of fuel removed. This is usually done by the fuel tank cap that has a 1 way valve that lets air in without fuel leaking out. Some others have the valve installed on the fuel tank itself. Check the valve for obstructions or if damaged. Being careful try running the saw with the cap loose and see what happens. There could be an air leak somewhere, affecting fuel mixture, (gaskets, loose bolts, etc.). Something that not too many people check is carbon build up at the exhaust port that will restrict exhaust gasses causing too much back pressure. This can stop high RPM from the saw. Remove the muffler and clean the carbon from the port. Set the piston at top dead center so carbon does not go into the crankcase and use either wood or plastic tool so piston and cylinder wall are not damaged.
To adjust the carburetor turn both mixture needles in (clockwise) until lightly seated so no damage is done to seat or needle. Now turn the 2 needles out (counterclockwise) about 1 1/8 turns. While engine running and warmed up, adjust until running good. Do not adjust high speed mixture needle too lean turned in (clockwise) too much, as this could cause not enough lube or cooling and damage the engine. Do not adjust lower than 1 turn from a lightly seated position.
Adjust the spark plug to .030". I hope I have given you a few good suggestions and that it is understandable. Any questions welcome. All the best and good luck.
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