most of the new saws are set way to lean from the factory and you cannot turn the jet out because of the limiter on the jet . If saw runs fine until it get heated up then the carb is to lean , as a saw gets hotter is needs more fuel to help with the cooling of the engine . So take a 660 stihl that is used to mill with , in stock form I will set the high speed jet so saw runs at 11,500 rpms at wide open throttle , yes I know that sounds rich but put a tach on that same saw at the end of a long cut milling and it will be over 13,000 rpms , that is from heat in the engine . If you set that saw to factory specs it will die or blow up from heat . Remove limiter and adjust carb to where it runs proper when hot and put limiters back on jets .