My 12.5 hp b&s engine will not turn over. I've tried to turn the engine over manually (turn the air screen on top of engine) and it turns about 5 degrees but it is very hard to move it. I've checked the battery, the wires between the battery-ground-solenoid and starter all wires checked out good. When the key is turned to start you can hear the starter trying to move the engine. What is wrong?