MY TS will start good when cold, as it warms I get almost no power and hot exhaust puffs out of the screen located on top of the muffler. It is hot and had melted part of the plastic behind the seat and the bottom of the bed. If I put water on the engine to cool it off it runs ok for a while. I have hand pumped fuel into the carb line to be sure it has fuel, a couple times I pulled the govenor and held it wide open and it ran ok, I get black smoke out of the muffer vent I mentioned earlier. (as a side question what is the purpose of the screen on top of the muffler?) Also I pulled off the shields around the head looking for packed hay or grass that might have caused a overheating condtion

I am thinking maybe the magneto? Hate to take it apart if there could be somthing else.