I have a Murray 42" mower with a 14.5 HP Briggs and Stratton OHV model number 287707 engine. I cannot get it to start. I have replaced the battery (300 amp cranking power), replaced the selinoid, changed the oil, drained and replaced with new gas, new fuel filter, adjusted the valves, cleaned the carb, changed and gapped the spark plug and it still will not start. The crank starts to turn and then stops. Help! What do I do?