I have a 2 year old self propelled Troy Built 6.50 hp Briggs & Stratton lawn mower. It will only start if I spray starter fluid in the carburetor, and only for about a second. The primer pump doesn't seem to pump as much fuel as it use to. My son and I changed the spark plug, added fresh oil and a new air filter. We even changed out the fuel. My next step is the shop:( Any Ideas?