What causes a lawnmower to backfire? The 6 common causes

Lawnmowers are generally loud machines. The reason is that their exhausts are not very sophisticated. The engine is small and uses a high speed with many revolutions per minute. In addition to this, the blade turning and cutting causes noise and vibration. Sometimes the lawnmowers have a two-stroke engine that inherently makes even more noise. When Lawnmowers get older, you will see more wear and tear, and this can increase the noise. But if your lawnmower starts to make loud bangs, there is something else to blame. It is called a Backfire. It happens as the engine slows down quickly. Events such as turning down the engine too fast or growing up on the seat and triggering a safety switch will cause a backfire. If a power outage follows the mower’s Backfire, the trigger needs further troubleshooting.

What causes a lawnmower to backfire? These are the six most common ones:

• Lowering the engine speed or suddenly shut down
• High blends of alcohol in gasoline
• Higher than normal engine temperature
• Muffler or exhaust construction
• Carburetor adjustment
• Internal passages design

A lawnmower that backfires might be annoying and disturbing, but it is as alarming as it may seem. The loud poofs itself from the exhaust are not dangerous for the mower, but the cause can. It is important to know the cause of the Backfire so you can fix it. A set of signs and needed troubleshooting will be discussed here.

What is a backfire?

The Backfire is an explosion that occurs due to the burning of fuel outside the combustion chamber. The phenomenon of Backfire can be understood easier when you know how the engine works.

• Air Intake: The air enters the intake manifold after getting filtered by the air filter.
• Air-fuel Mix: Air flows into the carburetor where fuel, in a very specific amount, mixes with it. The exact mix is controlled by the carburetor and can be adjusted. If the air in the mixture is too much, this is called a “lean” mix. In the other case, where the fuel is more than the required or normal quantity, the combination is termed “rich.”
• Intake stroke and valve timing: The piston inside the cylinder moves down, creating a negative pressure difference. The valve opens, and the fuel-air mix enters the combustion chamber. There is a specific time and crank angle at which the valves open and close. This is called the valve timing. The timing, together with the valve clearance, can be adjusted. The second is also called the tappet adjustment. If any of these two is out of actual balance, the performance of the lawnmower degrades.
• Compression ratio and efficiency: The valve for the intake closes, and the piston moves up, compressing the mixture that was sucked in earlier. The thermal efficiency of the engine depends on the pressure ratio that is generated.
• Spark and burning: When a specific compression is achieved, the spark occurs from the sparkplug igniting the fuel and imparting a huge momentum on the piston that turns the crankshaft, and power is generated. The moment at which the spark needs to take place is also controlled by the timing.
• Exhaust gases: The piston is pushed down with a jerk, and exhaust gases are left inside the chamber, then pushed out when the piston moves up again due to inertial motion.

There is a certain composition of exhaust gases, temperature, and velocity needed for a backfire to start. The temperature should be high enough, and there should be enough fuel present in the exhaust. In that case, Backfire‘s chance is increased as the remnants of unburnt fuel will catch fire and explode due to elevated temperature. The culprit can be bad valves, poor fuel adjustment, insufficient cooling, or maybe other reasons like the fuel composition.

What Causes a Lawnmower to Backfire:

We will now look at the major causes of a Backfire. Every reason might be different and will lead to the conclusion that some of the essential components and practices of operating lawnmowers need to be fixed or replaced.

•Lowering the engine speed or suddenly shut down

When the engine speed is reduced instantly, or the engine is shut down suddenly. It tries to lower its throttle to bring the timing to the same level. This decreased throttle will cause the formation of a lean air-fuel mix, i.e., excess air is present together with a lower fuel content than normal. Incomplete combustion occurs due to excess air, and this lean mixture is pushed to the exhaust. There it combusts at a late stage, causing the popping sound.

Fix: The underlying reason for this situation might be weaker tappet adjustment that valve opening and closing area of valves do not adjust very quickly to the dropping RPM. There is nothing to worry about in such a case.

•High blends of alcohol in gasoline

Any fuel uses alcohol or ethanol as part of the mixture. Riding mowers may not be calibrated to correctly burn this type of gas, resulting in intermittent backfires and power loss.

Fix: If appropriate, turn to a pure gasoline commodity before deciding whether further troubleshooting is required. Water contaminating the fuel is another potential source of both backfire and power failure.

•Higher than normal engine temperature

The engine of a lawnmower is usually air-cooled. At elevated temperatures, the exhaust gas particles are more likely to burn after expelled from the engine. The reason for high temperature is generally clogged air in-ports which might be due to dirt or grass stuck in the mesh present at the front.

Elevated temperatures are dangerous and will lead to other serious problems as knocking as well. 

Fix: Check and clean the air filter and air intake.

•Muffler or exhaust construction

Sometimes an additional accessory is fitted over the exhaust pipe, leading to choking of the exhaust. This also leads to a backfire. In such a scenario, it is important to use accessories that are well suited and help reduce the noise rather than causing 

Fix: Check the exhaust of the lawnmower. Remove the accessory if applicable.

•Carburetor adjustment

Lean or rich fuel mixture due to poor carburetor adjustment is a major cause of Backfire.

Fix: Check and clean the carburetor. After cleaning, adjust it properly.

•Internal passages design

Some engines have a bit of design constraint that it is inevitable for them to backfire.

Fix: There is not something you can do about this. Check the internet to make sure this a problem for your lawnmower.

General fixes to reduce the change for a Backfire

• Lower engine speed slowly: If the engine backfires, if RPMs are dropped instantly, it is important to adapt your practice accordingly to avoid any booms.
• Recommended fuel usage: Follow small engine fuel recommendations and switch to brands with low or no alcohol.
• Adjust carburetor for optimum performance: The fuel quantity required to run the engine is mentioned in the manual. Also, a proper adjustment is achieved by the hit and trial method and turning the adjustment screw.
• Enhance cooling of the engine: Inquire with equipment manufacturer about increasing air volume to decrease engine temperature

Comparison of Backfire and after-fire

After-fire is defined as the blow or explosion that occurs through exhaust after the engine has been shut down completely.

Backfire and after-fire are not very different from each other. The causes are mostly the same. The biggest difference is the time when both of the phenomena occur.

Causes of after-fire

• Instant drop in engine speed: Switch off the engine at high RPM, allowing the fuel to flow into the machine for ignition.
• The high Alcohol content in fuel: Gasoline containing alcohol can burn more quickly and spark after-fire.
• Smaller engine design: Tiny type of engine muffler and manufacture
• Bad fuel adjustment: Carburetor modification could not be properly set for proper engine output
• Anti-after-fire solenoid could not function properly

Potential remedies of after-fire

• Be patient in engine shutdown: Enable the engine to cool down by idling the engine at proper rpm (15-30 seconds)
• Use recommended fuel only: Change to a particular type of non-alcohol or alcohol fuel
• Adjustment of carburetor: Ensure proper adjustment of the carburetor for maximum engine output
• Change muffle or exhaust attachments: Ask the equipment manufacturer for up-to-date prototypes for air control baffling, mufflers, etc.
• Check for proper operation of the anti-after fire solenoid.

Related Questions

1. Can a dirty lawnmower carburetor be a cause of a Backfire?

A dirty carburetor may cause a lawnmower to backfire. Due to a clogged carburetor, the fuel-air ratio can alter, causing the engine to backfire.

2. What is the remedy to a lawnmower engine knock?

Your engine relies on two main components to fire and produces the force that pushes you forward: oxygen from the air and the tank’s gas. If the gas is exposed to a spark, it burns using the oxygen, causing an explosion. However, an inappropriate blend can produce a smaller, quieter blast. This can be heard as a “knocking” sound. To fix this problem, you can

• Replace your timing belt
• Change the fuel

3. Is a backfire the same as an engine knock?

No, engine knock is a pop or fuel blast that occurs inside the chamber due to bad combustion timing. It causes damage to the piston crown. But backfire is mostly a harmless boom produced outside the combustion chamber.

Final Remarks

Engine backfires in themselves are generally a harmless occurrence. It is, however, an indication for required adjustments of fuel, air, or another cause. An irritating boom that is produced tends to shock the operator and may also be annoying for the people in the neighborhood. Appropriate recommendations and remedies should be followed to reduce the chance of backfires happening.