Had the same problem on my BME 50. Got one of Troy Built models carbon fiber cover plates for the carb. Cleared up my engine. As a quick test, take some managers tape (two faced tape) and put two small pieces on the cover plate but don't block the little hole. Then take a small piece of lite ply and put it on the tape. Fly the plane and if the problem goes away or gets better buy or make a nicer cover. Had another engine that needed both the velocity stack and the cover.