My riding mower is extremely difficult to start and most of the time won't start at all. Its a briggs & stratton 17hp ohv 501cc single cylinder. And when I do manage to get it to start it acts like it wants to die then runs decent for another 5-7 seconds then acts like it wants to die again, over and over. Turn it off and it takes forever to get it crunk back up, if at all. Its getting fuel, spark is good, compression is good, took the valve over off and rotated engine and valves aren't stuck. The plunger in the bottom of the carb is working fine too. Anyone know what might be causing this?