Most push-type lawnmowers are basically the same, regardless of the make or model. You set the throttle to "fast" or "rabbit," pull the rope and start the engine. When the gas runs out, you fill the small tank with fuel and restart the engine. On most push lawnmowers, the fuel tank sits just below the carburetor. The carburetor is what regulates the amount of fuel the enters the combustion chamber. The more fuel that enters the chamber, the more bangs, or explosions, occur, and in turn, the faster the blades revolves to cut the grass. It takes a combination of air and fuel to create that explosion, which is powered by the spark plug. It takes a precise measurement of just the correct combination of fuel, air and spark to make that engine turn the blade to cut the grass.
Pull the Rope and Go
Since most lawnmower carburetors sit just above the gas tank, there must be a way for the fuel to work its way uphill. The carburetor on a small engine takes advantage of a suction that is created by the interior action of the combustion engine. When you pull the starter rope, the cylinder moves in either an up-and-down movement or a back-and-forth action. Regardless of the horizontal or vertical placement of the cylinder, a pressure difference is created by the tightly fitting cylinder. This pressure difference is made possible by two valves called the exhaust and intake. The valves are timed in such a way that the exhaust only opens as the cylinder reaches the top of the stroke to push out the exploded gases and the intake remains closed. As the cylinder moves downward, or away from the top of the cylinder stroke, the exhaust valve closes and the intake opens. This action of the cylinder movement, in concert with the opening and closing of the valves, creates a pressure difference in the cylinder chamber. This pressure difference draws fuel in from the gas tank and through the carburetor.
Carburetor or Mixer
Once the raw gas is in the carburetor, the liquid fuel is passed into a mixing chamber through very small openings, called ports. These ports can be adjusted for the size of the openings by an adjustment screw. The fuel screws help to regulate the amount of liquid fuel that is to be mixed with the air, or vaporized. As the fuel mist, or vapor, is pulled into the cylinder chamber, it must be in this form of a fine mist, otherwise the fuel cannot be ignited by the spark plug. The carburetor performs this task by passing the fuel through a series of openings and injecting the liquid with an inrush of air. The carburetor is self-regulating and it will only pass enough of the fuel and air vapor through the intake valve as needed by the engine. This is due to the suction created by the cylinder cycle. The faster the cylinder cycles, the more fuel and air mixture must be combined by the carburetor.