Small Engine Maintenance: Keep Your Outdoor Power Tools Running

Small engines on lawn mowers, trimmers, blowers, and chainsaws are mechanically simple, and most maintenance tasks take 15 to 30 minutes with basic hand tools. The biggest cause of small engine problems is stale fuel, followed by neglected air filters. Both are easy and inexpensive to prevent, and staying on top of these two items alone eliminates the majority of no-start complaints that send equipment to the repair shop every spring.

Oil Changes

Four-stroke engines (most lawn mowers, some trimmers and blowers) need regular oil changes. Check your owner's manual for the interval, but 25 to 50 hours of use or once per season is typical for residential equipment. You need a drain pan, the correct weight oil (usually SAE 30 for warm-weather use or 10W-30 for variable temperatures), a funnel, and rags. A quart of small engine oil costs about $5 to $8 at any hardware store. Briggs & Stratton, Honda, and Kohler all publish oil specifications for their engines in the owner's manual.

Run the engine for a few minutes to warm the oil before draining. Warm oil flows out more completely and carries suspended contaminants with it. Remove the drain plug or use the oil drain tube if your mower has one (Honda and Toro include drain tubes on several models). Tilt the mower with the carburetor side up if draining through the fill tube to keep oil out of the air filter housing. Fill with the specified amount of new oil and check the level with the dipstick. Overfilling causes as many problems as underfilling: excess oil can blow past seals, foul the spark plug, and create excessive smoke.

Two-stroke engines (many chainsaws, some string trimmers, and older leaf blowers) mix oil with fuel and do not need separate oil changes. The oil-to-fuel ratio varies by manufacturer. Most modern two-stroke equipment uses a 50:1 ratio (2.6 ounces of oil per gallon of gas). Stihl, Husqvarna, and Echo all spec 50:1 for their current two-stroke lines. Use two-stroke oil rated for air-cooled engines. Do not use automotive motor oil in the fuel mix.

Air Filters

A dirty air filter starves the engine of air, causing poor performance, hard starting, and increased fuel consumption. Check it every 25 hours of use or at least once per season. Paper filters get replaced. Do not try to clean a paper filter by blowing it out with compressed air. This can push dirt through the filter media and damage the engine. Replacement paper filters for common mower engines run $5 to $12 depending on the model.

Foam filters can be washed in warm soapy water, wrung out thoroughly, and re-oiled with a light coat of clean engine oil. Squeeze the oil through the foam until it is evenly distributed, then squeeze out the excess. The oil helps trap fine dust particles that the foam alone would miss. Let the filter dry completely before reinstalling it. A dirty foam filter with no oil is worse than a clean dry foam filter, but a properly oiled foam filter outperforms both.

The air filter housing is usually held on by a single screw, wing nut, or clip on top of or beside the engine. Open it, inspect the filter, and replace or clean as needed. This takes less than five minutes and is probably the single highest-impact maintenance task for reliability. Keep a spare filter on hand so you are not making a parts-store run on mowing day. Briggs & Stratton, Kohler, and Honda all sell multi-packs of OEM filters, and aftermarket filters from Oregon and Stens are widely available at lower cost.

Spark Plugs

Replace the spark plug once per season or every 100 hours of operation. You need a spark plug socket (usually 13/16 inch or 3/4 inch depending on the engine), a ratchet, and the correct replacement plug. Check your manual for the plug number and gap specification. The most common small engine spark plugs are the Champion RJ19LM, NGK BPR6ES, and Autolite 3924. A spark plug gap tool (feeler gauge type or coin-style) sets the gap. Most small engines spec 0.030 inches.

Before removing the old plug, blow debris away from the plug area with compressed air or a brush so dirt does not fall into the cylinder. Unthread the old plug by hand for the first few turns, then use the socket. Inspect the old plug: a light tan or gray electrode means the engine is running normally. Black, oily deposits indicate an overly rich fuel mixture or oil consumption. White or blistered electrodes indicate an overly lean mixture or overheating. These conditions point to other maintenance issues that need attention.

Hand-thread the new plug to avoid cross-threading the aluminum cylinder head, then snug it with the socket. Do not overtighten. Manufacturer specs typically call for a quarter turn past finger-tight for a new plug with a crush washer, or about 12 to 15 foot-pounds of torque. A worn or fouled spark plug causes hard starting, misfires, and poor fuel economy. If the old plug is oil-fouled or the electrode is heavily worn, it was overdue for replacement.

Fuel System

Stale fuel is the number one cause of small engine problems. Gasoline starts degrading after about 30 days, forming varnish and gum deposits that clog carburetor jets and fuel lines. Ethanol-blended fuel (E10 and higher) is worse because ethanol absorbs water from the air through a process called phase separation. That water settles to the bottom of the tank and gets drawn into the carburetor, causing corrosion and starting problems. Use fuel stabilizer in every fill-up if your equipment sits between uses for more than a couple of weeks. STA-BIL and Sea Foam are two widely available stabilizers that cost about $8 to $12 per bottle and treat 40 to 80 gallons of fuel.

At the end of the season, either run the engine dry (let it run until it stalls from lack of fuel) or fill the tank completely with stabilized fuel. A half-empty tank with untreated fuel over winter is the recipe for a carburetor rebuild in spring. Running the engine dry is the simpler approach for most homeowners. If your engine has a fuel shutoff valve, close it and let the engine run until the carburetor is empty. This prevents fuel from sitting in the carburetor jets over the storage period.

If your engine has a fuel filter, replace it annually. Inline fuel filters are cheap ($3 to $6) and a clogged one causes fuel starvation symptoms that mimic carburetor problems: surging, loss of power under load, and stalling on inclines. The filter is usually in the fuel line between the tank and the carburetor. Pinch the fuel line on either side with small clamps, remove the old filter, and install the new one with the flow arrow pointing toward the carburetor. For more on outdoor power tool selection, see our Lawn Care Tools guide.

Seasonal Storage

Before putting equipment away for winter, do all the maintenance at once: change oil, replace or clean the air filter, install a new spark plug, treat the fuel, and clean the exterior. Scrape grass buildup from the underside of the mower deck with a putty knife or a dedicated deck scraper. Packed grass holds moisture against the deck and promotes rust. Some mowers have a deck wash port that connects to a garden hose for easier cleaning.

Inspect the blade while the deck is clean. A blade with deep nicks, a bent profile, or less than half its original edge thickness should be replaced. New mower blades run $15 to $30 depending on the deck size and brand. If the blade is in good shape, sharpen it with a bench grinder, a mill file, or a drill-mounted blade sharpener. Manufacturer specs on most mower blades recommend maintaining a 30-degree cutting angle. Remove the blade with a socket wrench (typically 15/16 inch or 5/8 inch bolt) and clamp it in a vise for sharpening.

Store equipment in a dry space, off the ground if possible. Remove the battery from battery-powered equipment and store it at room temperature at about a 50 percent charge. Lithium-ion batteries degrade faster when stored fully charged or fully depleted. Disconnect the spark plug wire on gas equipment to prevent accidental starting. Hang string trimmers and blowers vertically on wall hooks to save floor space. A few hours of maintenance in the fall prevents frustration and repair bills in the spring.

Frequently Asked Questions

Can I Use Ethanol-Free Gas in My Lawn Mower?

Yes, and manufacturer recommendations from Briggs & Stratton, Honda, and Kohler all support or prefer ethanol-free fuel. Ethanol-free gas does not absorb water, does not degrade as quickly, and will not damage rubber fuel system components the way ethanol-blended fuel can over time. It is especially worth using in equipment that sits between uses, like a chainsaw you only run a few times a year. Ethanol-free fuel typically costs 50 cents to a dollar more per gallon and is available at many stations or as canned fuel (TruFuel, VP Racing) at hardware stores for about $8 per quart.

How Often Should I Sharpen My Lawn Mower Blade?

Every 20 to 25 hours of mowing, or about every two to three weeks during peak season. A sharp blade cuts cleanly, producing a healthy lawn with clean-cut grass tips that heal quickly. A dull blade tears grass, leaving ragged brown tips that are susceptible to disease. You can sharpen the blade with a bench grinder, a 10-inch mill file, or a drill-mounted blade sharpener like the Oregon 42-100. Remove the blade first. Never try to sharpen it while mounted on the mower. Check the blade balance after sharpening by hanging it on a nail through the center hole. If one side drops, file more metal from that side until it hangs level.

My Mower Will Not Start After Winter Storage. What Should I Check First?

In order: stale fuel (drain and refill with fresh gas), spark plug (remove, inspect, replace if fouled or worn), air filter (replace if dirty or oil-soaked), and battery charge (on electric-start models). Stale fuel is the cause about 80 percent of the time. If those four things do not fix it, the carburetor probably needs cleaning. A carburetor rebuild kit costs about $10 to $15 depending on the engine and the job takes about an hour with a screwdriver and basic socket set. User reviews report that a can of carburetor cleaner spray ($5 to $8) resolves most carb issues without a full rebuild.

Related Reading

• Lawn Care Tools
• Seasonal Tool Maintenance
• Generator Guide