Gas-powered mowers and small engines of all kinds are truly simple machines. However, they do require maintenance to keep them running properly and to prevent damage. When host Kevin O’Connor had some questions about lawn mower maintenance, he turned to landscape contractor Lee Gilliam for a rundown.
Gas mowers are prone to issues
Gas mowers live a tough life. They run hard all summer but get stored in hot, humid sheds to cold and freezing, depending on location. And, when the grass goes dormant, they won’t run for months at a time. This often leads to fuel issues, gunk, and other challenges, but if you know how to maintain a gas engine, your gas-powered machines can run for years.
Fuel, Air, and Spark
All internal combustion gasoline engines require three basic things to start and run: fuel, air, and ignition by a spark. If any one of these is missing or out of balance, the engine won’t run properly.
Air
The engine needed clean air for oxygen to mix with fuel for combustion to occur. Air is drawn in through an air filter and into the carburetor where it is mixed with fuel.
Check your air filters at least once a season. The air filter prevents debris, clippings, and other items from getting sucked into the cylinder where they can cause damage. At a minimum, it’s important to clean a filter with compressed air at least once per season. If it looks too far gone, it may be better to replace it.
Fuel
Fuel is the energy source for an engine. Most homeowner lawn mower engines require gasoline as fuel. When left in storage, ethanol gas attracts water and leads to water separation, ruining the gas’s flammability and causing damage. If you plan on storing a gas mower for more than a month, add a fuel stabilizer to keep the ethanol gasoline fresh for longer.
Or use an ethanol-free fuel, which is gasoline without the ethanol additive. It can be stored for longer and is less corrosive for the engine. While it is more expensive than ethanol gasoline, it could extend your engine’s life and make for less maintenance.
Under the filter is the carburetor, where fuel is drawn into and mixed with air. Its job is to regulate how much fuel and air are entering the engine cylinder, where the mixture will be compressed and ignited. If there’s an issue, there is typically a bolt underneath the carburetor where fuel can be drained. But fuel with high moisture content can cause gunking, in which case it’s a good idea to remove the carburetor and clean it with a carburetor cleaner and pipe cleaners.
Spark
A spark is needed to ignite the air-fuel mixture inside the cylinder of gasoline engines. A spark plug is a critical component for engine startup.
The spark plug has a small arm (also known as a strap) and an electrode. When energized, a small electric arc jumps between the two, creating the spark required to ignite the fuel. If the gap between the strap and electrode is too big or too small, it can affect the spark. If the strap is missing, it’s best to take the entire mower to a small engine shop where a technician can ensure it’s not trapped inside the engine.
Spark plugs can become fouled with carbon buildup; look for black gunk or soot covering the tip. If you remove the spark plug and the fouling is minimal, the spark plug can usually be cleaned. If it’s too fouled, you’ll want to replace it.
And Don’t Forget About Oil
While fuel, air, and spark get the engine running, it won’t run long without oil. Oil coats the inside of the engine, preventing friction and excessive heat buildup, both of which spell disaster for an engine. The engine oil should be changed about once per year, along with the filter that captures contaminants. Check the owner’s manual for specific oils, but SAE 5W-30 is typically best for warmer temperatures, while SAE 10W-30 has a larger temperature range.
One note: When replacing the oil filter, dip your finger in fresh oil and run a small bead around the rubber seal to ensure the filter can be removed easily in the future.
General Maintenance Tips
Empty the fuel tank for long-term storage (longer than seasonal) or if you are storing a lawn mower inside a house, like a basement, for fire safety.
Clean the mower deck after the season. Be sure to allow it to dry completely before storage.
Store the mower in a dry location. Choose a covered area like a shed or garage, and elevate the mower off the ground if possible.
Sharpen or replace the blade at the start of each mowing season. If you hit a rock, inspect the blade and sharpen as needed. Dull blades tear and rip grass blades, causing the tip of the grass blades to turn brown.
Lubricate moving parts at least once per year. Apply a light coating of spray lubricant to each of the wheel axles, as well as the control cables and any other moving parts (reference your user manual).
Troubleshooting Common Gas Mower Issues
Mower Won’t Start:
• Check that the spark plug is securely connected
• Confirm that the oil level is correct
• Ensure there is fresh fuel in the tank
• Verify that the air filter isn’t cloggedUneven Cutting
• Check that all the wheels are the same height
• Ensure the mower deck is clean and free of debris
• Inspect the blade for damage or dullnessExcessive Vibration
• Examine the blade for balance issues or damage
• Check for loose bolts or parts
• Inspect the crankshaft for bendingEngine Overheating
• Clean the air filter to maintain airflow
• Ensure proper ventilation by cleaning the engine cooling fins
• Verify the oil level and typeResources
Lawn mower shown: DeWalt 21” Gas Self-Propelled Walk Behind Lawn Mower
Fuel Maintenance
- Fuel stabilizer: STA-BIL Storage Fuel Stabilizer—keeps gasoline fresh while engine is in storage.
- Fuel treatment: STA-BIL Fast Fix Small Engine Treatment—fast acting treatment to use if the engine is having rough or no starts because of issues from ethanol blended gasoline.
- Ethanol-free fuel: VP Racing Fuels 4-Cycle Small Engine Fuel—ethanol-blended gas causes varnish buildup, gumming, and corrosion in small engines. Ethanol-free fuel can extend the engine’s life, reduce maintenance, and make for smoother starting.
Carburetor Maintenance
- Carburetor cleaning kit for lawn mower and small engines—to clean out jets and passages.
- Carburetor cleaner: Gumout Carb/Choke Carburetor Cleaner Spray
Spark Plug Maintenance
- Spark plug replacement: based on the make and model of your lawn mower.
Oil Maintenance
Check owner’s manual for oil recommendations. Oil recommendations vary based on temperature ranges of your climate. The most common oils for lawn mowers are:
- SAE 30: best for warmer temperatures. Most common type of oil for lawn mowers.
- SAE 10W-30: larger temperature range. Better for colder weather starts below 40°F. May increase oil consumption.
