305cc Briggs & Stratton.Backfires through carb when trying to start. Has spark and fresh gas . Used 10 days ago and it worked fine.

Try removing the head and cleaning the head and valves of carbon. On most B&S engines, the teflon coated head gasket is reuseable. Repeat this process every 3 seasons. Use full synthetic 5W-30 engine oil. It will help prevent carbon build-up and allow the engine to start easier in cold weather.

12 running hours on it. New plug. New gas. Used 10 days ago,worked fine. Won't start now.

If it ever sat for long with old gas in it... like over a summer... then the carb needs cleaned well.