Lawn Mower Maintenance: No Smoking Allowed

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With the weather getting warmer, you’ve decided it’s time to dust off that lawn mower and give it a tune-up so the mower will be ready for some serious grass-cutting over the next few months. However, after firing up the engine, you’ve discovered your lawn mower has developed a bad smoking habit. What’s causing that black smoke to billow out of the engine? There are three likely reasons and they are all related to how a gas-powered lawn mower engine operates.

How a gas-powered lawn mower engine operates

Most gas-powered lawn mowers utilize a four-cycle engine. Similar to an automobile engine, a four-cycle engine draws gasoline from a fuel tank and has a separate sump for oil.

A flywheel helps create a magnetic field to induce electricity

When the lawn mower start button is pressed, the ignition key turned, or the starter rope is pulled, a starter cup on the engine’s flywheel will engage to rotate a crankshaft. The rotating crankshaft connects to the piston which moves up and down within the cylinder and the ignition process begins. The flywheel has permanent magnets built into it, and as it rotates past the ignition coil, a magnetic field is created. The magnetic field induces electricity, allowing the ignition coil to send voltage to the spark plug.

A four-cycle engine has four “strokes”

As the flywheel rotates to create that magnetic field, the piston will travel down the cylinder creating a vacuum that draws fuel and air through the engine’s carburetor where the two elements mix before entering the cylinder through an intake port. This action is called the “intake stroke”. Next, the “compression stroke” takes place as the piston travels back up to the top of the cylinder and the intake valve closes. The spark plug now fires and ignites the fuel and air mixture which forces the piston down (known as the “power stroke”). The momentum of the spinning flywheel provides enough force to push the piston back up. The exhaust valve will then open, and the combustion gasses will exit through the muffler in what is the fourth and final cycle – the “exhaust stroke”. The engine will continue to run and repeat these four cycles until it is turned off.

A choke and primer bulb assist with starting the engine

A lawn mower engine will have a choke, a primer bulb, or both to assist with starting, especially when the engine is cold. The choke will temporarily restrict air flow through the carburetor so more fuel can enter the cylinder. Likewise, a primer bulb draws additional fuel through the carburetor before the engine is started.

The engine exhausts the combustion gasses through a muffler and spark arrestor

The exhaust from the engine will travel through the muffler and, often, a spark arrestor. The muffler reduces engine noise while the spark arrestor prevents burning carbon deposits from exiting the engine and, potentially, starting a fire.

3 reasons why your lawn mower’s engine is smoking

While it’s normal for a small amount of white or gray smoke to be exhausted when a lawn mower engine is first started, a consistent stream of dark smoke indicates a problem with the engine. Here are the three most likely reasons why your lawn mower is smoking:

  1. Malfunctioning carburetor – The carburetor may be getting too much fuel. This can happen if the carburetor float is stuck in the open position or the carburetor choke plate is stuck closed, not allowing the engine to draw in enough air to create the proper air/fuel mixture needed for clean combustion. You should try cleaning the carburetor with carburetor cleaner to ensure the float or choke plate can move freely. If necessary, you can purchase a carburetor repair kit to replace damaged or corroded components or you can install a new carburetor altogether.
  2. Worn piston rings  –  The engine piston rings could be worn and allow oil to seep past and be burned in the combustion chamber. You can replace the old piston rings with a new set to resolve the issue.
  3. Adding too much oil – The engine crankcase could also be filled with too much oil. Adding more engine oil than is necessary can create extra pressure in the crankcase, causing the seals to blow out and allowing oil to migrate into the cylinder head where it will produce excessive exhaust smoke. You should always follow the manufacturer’s recommendations for the proper amount of oil to add to the lawn mower engine.
Lawn Mower Smoking?

DIY lawn mower repair help from Repair Clinic

Repair Clinic is all about helping do-it-yourselfers and landscape professionals maintain and repair all outdoor power equipment. You can start by entering the equipment’s full model number in the “Videos & Articles” search bar to find hundreds of procedural videos, diagrams, and troubleshooting tips. Want to know how to install a new piston ring set on a Briggs & Stratton four-cycle engine (Model 15T2020915FB)? Repair Clinic has a video for that. You can also watch a video showing you how to install a new carburetor float on a Honda lawn mower engine (Model GCV160LAOMY1R280) and review nine important maintenance tips you should follow to keep your lawn mower in peak condition.

Keep your lawn mower engine maintained with genuine OEM parts

As your repair partner, Repair Clinic strongly recommends that you only use original equipment manufacturer (OEM) parts when repairing your lawn mower engine. Whether you need to install new carburetor kit components or replace an ignition coil, flywheel, valve, or muffler, using a genuine OEM part specifically designed for the mower engine will keep the engine running at its best. Repair Clinic stocks thousands of genuine manufacturer parts from such top industry names as Briggs & Stratton, Craftsman, Honda, Husqvarna, MTD, Murray, Snapper, Troy-bilt, and Toro. To find the part you need, simply enter the full model number of the lawn mower’s engine in the Repair Clinic search bar to see a complete list of all compatible parts. You can then use the “Part Category” and “Part Title” navigation filters to narrow that list down to identify the exact replacement part you’re looking for.

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