Is this a unit you have had for awhile, and if so have there been any previous related issues such as a reduction in RPMs or in power? There is a fair chance you may have a partially blocked exhaust port, or even more likely a blocked spark arrestor screen. Remove the muffler and check the port for carbon buildup, and also check the muffler and spark arrestor screen. If the port is carboned, raise the piston past the port opening and CAREFULLY scrape the carbon away. If the screen is partly blocked remove it and, using a propane torch, carefully burn the carbon off the screen. Best to do this outdoors on a calm day, and you may want to hold the screen with a pair of long nose pliers. If in doubt about either procedure, try starting with the muffler off, but do not run long without the muffler if it starts.
Hope this works for you.