My riding mower is extremely difficult to start. 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 arent stuck. Anyone know what might be causing this?