I had similar problem and tried every resource I could think via internet…I asked knowledgeable friends …checked valves, replaced carb, shear key,plug…idled up and sometimes blades would engage.and thinking it might be water in the fuel I changed gas and oil….that’s when I discovered that my oil didn’t look right. Apparently the previous carburetor had let gas get into the oil and that’s why the blades wouldn’t engage !!