Just a few tips. Whether you run a cold plug or hot plug, you still can not run the engine at a lean setting without eventually burning it up. A hot plug may help the engine run better lean, but lean running is not good, and this sounds more like a fuel draw issue to me. When using the pitts style muffler with two exaust outlets, you may find that you need to plug one outlet off to create enough backpressure in the exaust to properly pressurize the fuel tank for a good fuel draw. For some reason some engines just draw better than others. Some engines will even run without a pressure line, and some have to have it. Then there are some that will not run properly outside of using the stock exaust pipe. Make sure to blow thru the nipple on the pitts to make sure it is not plugged, and try blocking off one exaust outlet with a cap of some sorts. I use a rubber exaust extention piece with a dowel plug.

If you are using the Evolution TPS engine, make sure you have the needle valve limiters removed(Little Blue Anodized Collar on the High End Needle). They will not allow you enough room to properly dial in the engine if you have changed out the exaust for an aftermarket type. Always remember that the engine should never get so hot that you cannot touch it for a couple seconds even after running it at full for a few minutes.Good Luck