My Craftsman walk behind has been tuned up pretty good.New oil,filter,spark plug,rebuilt carb,fresh gas.I notice when it runs the muffler will sputter at times.Dont understand why.
Could be several things you can't really do anything about. Most likely it is plug misfire. I read a few years ago where champion claimed a 25% misfire was acceptable. Fuel quality could cause a misfire, and it may not be bad gas, just today's junk gas.