It's crucial that you know whether your lawn mower has a 2-cycle or a 4-cycle engine. For example, each type of engine requires different fueling techniques. If you try to fuel a 2-cycle engine as you would a 4-cycle engine, it will freeze up. The engine will then likely need to be replaced. Likewise with a 4-cycle engine. Fuel it up improperly and and can foul-up, or even damage, the carburetor.
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Determine how to add fuel to your lawnmower. Both types of mowers will have gas tanks, of course, but look for a crankcase with a filler tube to add oil. The top will screw off to reveal a dipstick. If there is a place to add oil separately, it's a 4-cycle engine. If you mix oil along with the gasoline when you fuel up, then that tells you it's a 2-cycle lawnmower.
Refer to the maintenance section of the owner's manual. If it instructs you to change the oil on a regular basis, as well as before you store it for the winter, then you have a 4-cycle lawnmower.
Try to start the lawnmower during the cold weather. Two-cycle engines start much easier. However, they give off more smoke. Also pay attention to the noise the lawnmower makes when it's running. A machine with a 2-cycle engine is much noisier than a machine with a 4-cycle engine.
Run the lawnmower up a steep incline. A 2-cycle engine won't lose its power like a 4-cycle engine will. It will have approximately twice as much power.