About Four-Cycle Engines

The four-cycle engine is similar to the two-cycle except that the fuel is drawn directly from the carburetor into the combustion chamber, and out of every four strokes the piston makes (two up and two down), only one of them produces power.

The fuel mixture in a four-cycle engine is moved either by a gravity-feed carburetor or by a fuel pump. However, fuel pumps are rarely found in the type of equipment discussed in this book. Gravity-feed systems are most common; they are used in equipment where the fuel tank is located above the carburetor.

The float carburetor is a typical gravity-feed carburetor found on four-cycle engines. Here, fuel flows into a bowl in the carburetor containing a float mechanism (similar to the kind used in a toilet to maintain the water level). When the fuel level drops as it flows out to the engine, the float also drops and opens a valve that allows more fuel to flow into the bowl. As the level of gasoline increases, the float rises and shuts the valve. This system keeps a constant supply of gasoline ready to be drawn into the carburetor.

When the piston first starts down, by the pull of the starting rope, a valve opens in the combustion chamber above the piston. The downward motion creates a low-pressure area that draws a mixture of gas and air directly from the carburetor through a valve and into the combustion chamber.

The piston then rises, compressing the fuel-air mixture, and the intake valve closes.

Just before the piston is ready to go clown again, the spark plug ignites the compressed fuel-air mixture, which explodes, expands, and drives the piston down. On the fourth and last stroke of the series, the piston rises, the exhaust valve in the combustion chamber opens, and the piston forces out the burned gases.

The Choke

Whether the carburetor functions properly has much to do with the choke. The choke is the thin metal disc that fits in the air horn and controls the air-to-fuel ratio. Closing the choke reduces the amount of air to be mixed with the fuel, thus making the fuel mixture richer. Opening the choke admits more air, thus making the fuel mixture less concentrated, or “leaner.” Close the choke when starting a cold engine—the richer mixture makes it easier to start. Then, when the engine begins to run, open the choke—the leaner fuel mix is required for normal engine performance.

Air Filters

Your engine has one of three types of air filters: an oil-bath air filter, an oil-saturated air filter, or a dry-element air filter. Older engines have oil baths or oil-saturated air filters. The oil bath contains a lubricating oil in its base that the air must pass through before it enters the air horn of the carburetor. The dirt and dust from the air accumulate in the bottom of the oil cup. The oil-saturated air filter forces the air through an oil-saturated foam or wire mesh that traps the dirt.

The dry-element air filter is most common. This dry filter—which can be made of foam, regular paper, or pleated paper—traps dust and debris before the air gets sucked into the carburetor, where it could foul the fuel jets or the engine itself.

Air filters usually are above the carburetor, in a fitted metal or plastic housing.