Why is flames coming out my exhaust on my lawn mower 18 horse kohler?

Your engine is getting too much gas. Either you have a problem in your carburetor or one of the cylinders is not firing and the gas is blowing right out the exhaust and being ignited in the muffler. While the engine is running use a rubber coated pair of pliers to remove one of the spark plug wires. Make sure you keep your hands on the rubber coating or you may receive a good shock. If it changes the sound of the engine and keeps running then both cylinders are firing and the problem is in the carburetor. If it does not change then you have other problems like a bad coil or internal engine problems. If it dies as soon as you remove the wire then put it back on and try the other one. The one that does NOT make it change or die is the problem one.