The muffler on my Briggs Stratton four-stroke single cylinder OHV air cooled engine on my pressure washer started getting red-orange hot on the exterior of the muffler after running a minute or so, then it started to kind of sputter while bluish flames started popping and firing out of the muffler too. So I shut it off. Obviously something wrong, but what might be some first things to check?

 

The fuel/air mix is lean = dirt in carb jets or some restriction to fuel flow, like a cap vent or filter. Usually it's a carb cleaning to fix it.