At one time, I understood the Briggs carburetors rather well. But have never been inside the ones with no external speed control nor external choke control. Mine is a 4.5 HP vertical shaft that uses the brake to kill the engine and has a red priming bulb in the side of the carburetor. Mine "searches" with varying speed of the engine, a classic symptom of lean fuel mixture. No visible means of mixture control. My thought would be to clean the jet. The cleaners poured in the tank have not helped. So where is the jet and what is the easiest way to access it? Model 10H902, Type 0285, E1 0421855. It is a flat head engine.