4-Stroke Small Gas Engine Troubleshooting

The engine does not run when the cord is pulled.

In order for an engine to start running, the carburetor must have a supply of fuel. In many cases with small engines, there will appear to be gas when in actuality none is getting to the engine. Try rocking the engine back and forth to listen for fuel sloshing. If you have a gas canister, try filling the tank up and giving it another shot.

Spark plugs are one of many consumable items in a gasoline engine and have a tendency to die over time and varied use. Once ineffective, the spark plug is unable to generate a spark for the fuel-air mixture to ignite. If a spark plug is dead, the curled end will appear corroded with carbon deposits built up on its tip. For information on how to replace and change out spark plugs, follow our Replacement Guide for Spark Plugs.

If the air filter becomes too clogged with dirt or dust, there may not be sufficient air reaching the cylinder. This can cause the fuel to not burn, and the engine will not start as a result. If upon visual inspection the filter appears dirty or dusty, follow our Air Filter Replacement Guide.

On a ripcord operated engine such as a Briggs & Stratton 125cc motor, the clutch engagement on the crankshaft can wear out over time until the starter will no longer catch its contact points. The easiest symptom of starter wear is the absence of resistance when pulling back on the ripcord. Other signs of starter wear are that the rewind mechanism that returns the cord into the machine is no longer functioning. Replacement parts for a Rewind starter can be found here.

The engine does not run as normal and may stop unexpectedly.

The governor is a spring mechanism that controls the speed and load on the engine. Springs can lose their elasticity and subsequently alter the performance of your small engine. In many cases a small adjustment or re-positioning of the spring will be sufficient enough to keep the engine running smoothly. A guide on Governor Adjustment can be found here.

If the engine is being used to power a device, ensure that the device is not jammed. For example, if the engine is being used to power a lawnmower, ensure that the blades are spinning as they should and are not mechanically jammed or blocked.

The engine creates white or blue smoke while running.

If the engine is tilted excessively to one side or the oil is overfilled, oil may leak out of its chamber and onto the exhaust manifold. If this happens, the oil will burn due to the exhaust heat. Check the oil level to ensure it is within the recommended range. This is not a serious problem, and will resolve itself over time.

The head gasket is a thin sheet that forms a seal between the engine block and the engine head. If it develops a crack, oil may leak from its channel and enter the cylinder. When oil finds its way into the cylinder, it is burned along with fuel. This produces a blue smoke, and is harmful to the engine. Head gaskets are engine specific, and must be found using the engine’s model number.

The engine is noticeably hotter than normal and is not running right.

After mowing for a while, grass and other debris may build up on the exterior of the engine, inhibiting cooling. This is a relatively simple problem to fix; just wait for the engine to cool, then remove any visible debris.

If the air filter becomes too clogged with dirt or dust, there may not be sufficient air reaching the cylinder. This can cause the mixture of air and fuel to not be properly balanced, making the engine run inefficiently and hotter than normal. If upon visual inspection the filter appears dirty or dusty, follow our air filter replacement guide.

The engine occasionally emits a pinging or banging noise.

Through regular use, carbon may build up inside the cylinder from burnt fuel, interfering with proper combustion. To fix this, remove the cylinder head and gently scrape built-up carbon from the inside of the cylinder and the top of the piston using a small scraper or wire brush, being careful to not damage the cylinder or piston.

If the engine begins to get too hot, the air and fuel mixture may ignite prematurely, causing the engine to make a banging noise. See Engine is Overheating above for potential causes and repairs.

Spark plugs are one of many consumable items in a gasoline engine and have a tendency to die over time and varied use. If damaged, the spark plug may not properly ignite the fuel, which can cause a knocking noise. If a spark plug is dead, the curled end will appear corroded with carbon deposits built up on its tip. For information on how to replace and change out spark plugs, follow our Replacement Guide for Spark Plugs.