Engine: Onan 14 HP Mowing on a hill and had to turn mower quickly to keep from flipping over. Engine shut off by itself. When I tried to crank it back, it would not run smooth. Very rough idle. Gave more gas and then the mower started to backfire. Tried to keep it running but had to crank it several times. Still wouldn't run smooth. Took spark plug out and put new one in. Some better but still run rough and couldn't speed up engine. Took off air cleaner to check for signs of oil. No problems here. Took plug out again and turned engine over several times. No signs of oil coming out of plug hole. Left air cleaner off and engine seem to run some better but not good enough to mow. Black smoke was coming out of muffler. Could someone please advise what they think it maybe. Mower is still not running smooth at all and I can't mow grass with it. Thanks, JDH.