I have a new Briggs and Stratton 8000 watt generator powered by a Vanguard 15 hp overhead valve engine.Since it came from the factory it has a popping sound from the muffler as it runs. It seems as though there is a misfire. The spark plug has been changed and the spark is in spec. Service dealers can't find the problem. They say it should run smooth without the popping sound. Any advice would be helpful.

Well, if it is new and the service dealer can't find the problem, are they an authorized briggs dealer? If so , stay on them and if they do not give you results, tell them to have the area rep come by and check it out. Stay on them, use your warranty. DO NOT go into the engine yourself, or remove any components, doing so could void your warranty. Carry your sales receipt in with you, and have a warranty inspection done.