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How to Repair Small Engines

How to Repair a Small-Engine Fuel System

©2006 Publications International, Ltd. This fuel tank has two filters: one at the opening and one at the entry to the fuel line.

The function of a small engine fuel system is to store and deliver fuel to the combustion chamber. Maintaining a fuel system includes servicing the fuel filter, air cleaner, fuel tank, and fuel lines; adjusting the carburetor; and adjusting the governor. Of course, not all small engines have all of these components.

Servicing Fuel Filters

Some small engines have a fuel strainer in the bottom of the fuel tank. Others have a removable fuel strainer in the fuel line. Still other small engines use disposable in-line fuel filters made of pleated paper. To clean sediment from a tank:


Step 1: Drain or siphon all fuel from the tank.

Step 2: With a flashlight, find the lowest point in the tank: the sediment reservoir. Clean all sediment from the reservoir indentation.

Step 3: Wipe sediment from the end of the filter element.

Step 4: Wipe the inside of the tank with a clean rag.

Step 5: Refill the fuel tank. 

Here is how to clean sediment from a fuel strainer:

Step 1: Find and close the shutoff valve on the fuel line.

Step 2: Loosen the lock nut on the bowl retainer and remove the sediment bowl.

Step 3: Empty and clean the sediment bowl. Clean the filter screen. Refill the sediment bowl with fresh fuel.

Step 4: Reinstall the sediment bowl and bowl retainer, tightening the lock nut.

Step 5: Open the shutoff valve.

Here is how to replace an in-line fuel filter:

Step 1: Find and close the shutoff valve on the fuel line or use a clothespin to pinch the fuel line closed.

Step 2: Disconnect the fuel filter from the fuel line.

Step 3: Replace the in-line fuel filter with an exact replacement part.

Step 4: Open the shutoff valve.

Servicing Air Cleaners

The purpose of an air cleaner on a small engine is to keep large particles in the air from clogging the carburetor. The two types of air cleaners used on small engines are oil bath and dry. Hereis how to service an air cleaner:

Step 1: Remove the cover of the air cleaner, typically by unscrewing a nut on top of the cleaner.

Step 2: If it's an oil cleaner, remove all oil and contaminants from the center channel of the cleaner, wipe it clean, then replace oil to the indicated level. A dry cleaner cannot be cleaned and therefore must be replaced.

Step 3: Replace the air cleaner cover and make sure all fasteners are securely tightened.

Servicing Fuel Tanks and Lines

Fuel systems with pumps use nonpressurized fuel tanks. Outboard engines typically use pressurized tanks. Fuel lines are usually made of neoprene. Here's how to service a fuel tank and line:

Step 1: Remove the cap from the fuel tank.

Step 2: Using a flashlight, check for sediment in the fuel tank. If sediment is found, clean the tank and replace the fuel. Replace the fuel cap when done.

Step 3: Check the fuel line and siphon bulb, if there is one, by squeezing them and inspecting for cracks. If damaged, replace with a line or a bulb of the same inside diameter. Make sure it is approved for use with fuel.

Adjusting Carburetors

A carburetor mixes fuel and air in the correct proportion for use by the engine. The three types of carburetors commonly used in small engines are natural draft, updraft, and downdraft. These names describe the direction that air flows from the inlet to the engine manifold. To maintain your small engine, you will want to make sure the carburetor's speed and mixture are correctly adjusted.

There are many types and models of carburetors used on small gas engines. Some have more available adjustments than others. Depending on the design, some carburetors are set at the factory and don't include adjusting screws. For example, a carburetor may allow adjustment of high speed, idle speed, and idle mixture while another only allows high and idle speed adjustments. Here's how to adjust the typical three-adjustment carburetor:

Step 1: With the engine running, open the throttle wide. Turn the high-speed adjustment needle forward and backward until the highest speed setting is found.

Step 2: Move the throttle to the slowest running speed. Adjust the idle-speed needle until the recommended idle speed is found.

Step 3: Once the idle speed has been set, adjust the idle mixture until the engine runs smoothly.

©2006 Publications International, Ltd. To set the correct idle speed, turn the idle adjustment screw with a screwdriver.

Adjusting Governors

A governor is a device that controls the speed of the engine as the load changes. As the load slows the engine down, the governor opens the throttle to return the engine to a set speed. Governors are commonly used on engine-powered electrical generators where constant speed is important. Two types of governors are installed on small engines: mechanical and air-vane.

Caution: An incorrectly adjusted governor can cause the small engine to operate at excessively high speeds and damage or destroy it. 

©2006 Publications International, Ltd. A mechanical governor responds to the centrifugal force created by the engine's revolution.

Unfortunately, there is no universal adjustment sequence for small engine governors. Much depends on the type of governor, whether the crankshaft is horizontal or vertical, the complexity and pivot points of the linkage, and the intended operating range. Because of these factors, refer to an owner's manual or service manual for your specific engine and application to adjust the governor.

In the next section, learn how the ignition system needs to be routinely serviced so it can deliver a high-voltage spark to help start a small engine.