The Main Components of Your Lawn Mower: And Their Common Issues, Explained

In order for you to describe symptoms, locations of components, or other pertinent information about your lawn mower, it is imperative to grasp the main components that make up its anatomy. Understanding these parts will aid in troubleshooting, diagnosing issues, and communicating necessary information for fixing your mower.

Understanding Lawn Mower Components

Let's begin with the engine. The engine on a lawn mower contains a vertical crankshaft that has a blade bolted to the bottom of it. The blade is mounted to the crankshaft via a blade adapter.

In this example, two studs protrude from the adapter and mate with two holes on the blade itself, ensuring the blade is secure. The design of both blades and adapters varies. You must ensure that the blade adapter used matches the blade it is supposed to be mounted to. There are universal blade adapter options available. The blade adapter will have a keyway that mates with the keyway slot on the crankshaft. This ensures that the blade remains in constant contact with the spinning crankshaft.

Pro Tip: If your mower is exhibiting excessive vibration, this may indicate that the blade has come loose and is not correctly fastened to the blade adapter. Alternatively, the blade may have struck a hard foreign object and become bent, or the crankshaft may have become bent. For more information on how to avoid such scenarios, visit this post How to Use a Lawn Mower: Sticks and Stones May Break Your Blade!.

Underside of a lawn mower deck exposing blade; adapter is behind the blade

The engine is covered by an engine cover or shroud. This cover protects the engine, while the shroud, which is a metal cover, helps direct airflow created by the spinning flywheel over the cooling fins designed into the engine's cylinder head. If the engine cover bolts or screws become loose, vibration may be misdiagnosed as something more serious. Ensure that these fasteners are securely fastened. The flywheel also has cooling fins that aid in this airflow. Dirt and grass buildup on these cooling fins can prevent airflow, resulting in the engine running hotter than acceptable and speeding up its wear.

Plastic engine cover protecting the metal engine shroud and recoil assembly underneath

Cooling fins are designed into the flywheel

The Recoil Assembly

Underneath the engine cover is a recoil assembly. This assembly is attached to the starter rope, which engages the flywheel. The flywheel must be spun manually by the operator pulling the start rope (if no electric start) to start the engine. Ensure that it retracts smoothly and is not bound.

The engine is mounted to the lawn mower body or deck via engine mounting bolts. Ensure that none of these mounting bolts are broken or that the deck material has become cracked. The deck is the steel or sometimes aluminum housing in which the engine is bolted onto, and where the cutting blade resides. Obviously, the deck's structural integrity prevents potential operator injury and supports the extremely fast-moving blade and vibrating engine.

To preserve your deck, review this post How to Properly Clean Your Lawn Mower Without Causing Damage. It outlines the importance of keeping your deck's undercarriage clean and free of grass buildup. Failure to do so encourages corrosion and rot, which can damage your lawn mower's deck.

Essential Components for Engine Operation

The components attached to the engine but necessary for it to run include the carburetor, engine air filter, fuel tank, fuel line, spark plug, throttle linkage, the choke and its linkage, and the governor.

The carburetor is responsible for converting liquid fuel into a vaporized mixture of air and gas that the engine can effectively burn. Ethanol within gasoline absorbs water and causes it to separate in the gas tank, forming layers of water that can obstruct passages and ports necessary for the lawn mower carburetor to function. A varnish forms, blocking these jets and passageways, preventing your equipment from starting or running properly. Corrosion can also form in metal gas tanks.

Air filter housing

Air filter cover removed exposing foam-style filter; arrow points to fuel line

carburetor behind air filter

The engine air filter traps abrasives from entering the combustion chamber of your engine. If allowed to enter, these particulates can damage critical internal engine surfaces by scratching once smooth-finished parts. It is imperative to inspect and service your air filter periodically. The service interval may be affected by operating conditions, such as frequent use in dusty areas or cutting excessively tall grass. Check out this post How's Your Engine Been Breathing Lately? Air Filters and Engine Maintenance Tips for Lawnmowers to Live By! to learn how to service your engine's air filter.

The fuel tank houses the gasoline and may be equipped with a filter screen to catch debris in the gas tank from entering the carburetor. Periodic maintenance of these screens is necessary to prevent total blockage of fuel from entering the carburetor. The fuel travels from the fuel tank, through the filter screen, and into the carburetor via fuel lines. These rubber fuel lines are specially designed to handle gasoline; however, they can break down internally and collapse, preventing or slowing the flow of fuel.

The Spark Plug and Its Importance

The spark plug provides the spark to ignite the air-fuel mixture within the engine's cylinder. If not properly maintained, such as setting the electrode gap or periodic servicing, this electrode can become fouled due to excessive carbon buildup from the by-products of combustion. Refer to this post on how to assess your spark plugs: Your Spark Plug is Telling You Something...and it Never Lies: Read Your Spark Plugs Like A Pro.

The carburetor's air-fuel mixture is modified via linkages from a governor, which increases or decreases engine speed depending on feedback from either a mechanical device or pneumatically controlled air vane. The governor adjusts engine speed based on the operating demands placed on it. If these linkages become bound, disconnected, or broken, this can affect the running speed of the engine. The engine can overspeed beyond its safe operating limits, leading to catastrophic failure.

The choke is a plate that aids in starting the engine. Located within the carburetor, the choke closes on starting and opens when the engine is set to run. It can be thermostatically controlled or manually controlled. If the choke linkage is bound, disconnected, or broken, failure to start or engine running issues may result. The carburetor may also be equipped with a primer bulb. This bulb helps increase the amount of fuel in the engine at cold startup, thus "priming" the engine for starting. Ensure there are no tears in your bulb.

Depiction of primer bulb

Primer bulb attached to carburetor

Operator's viewpoint looking at blade engagement bail

Engaging and Disengaging the Engine

The engine is engaged and disengaged by the operator via a blade brake or blade engagement bail. As outlined in another post Lawn Mower Maintenance Tips: The Ones Often Overlooked, proper lubrication of these cables prevents rust and corrosion from causing them to seize, rendering your machine inoperable!

Failure to service this area may make your mower inoperable!

By referring to this breakdown of the main components of your lawn mower and learning the common issues related to each of these components, you are well on your way to preventing potential issues. You’ll be able to properly diagnose or communicate to those who can what is going on with your lawn mower.

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Never the Norm!