Lawn mower carburetor cleaning: what to know first
Why a carburetor clogs, the signs, the fuel and emissions rules, and when a service shop is the next step.
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A gas mower that sat for a season and now will not start, or starts and then runs poorly, may have old fuel in its carburetor. Extension services name stale fuel as one cause. This guide covers what the carburetor does, why it clogs, the signs, what the sources say to check first, the emissions rules, and when to use a service shop. It gives no cleaning steps. Your mower's manual, or a shop, sets those.
What does a carburetor do?
The carburetor is where a gas engine mixes fuel with air before it burns. University of Georgia Extension (UGA) says a small engine needs three things to run: "fuel, fire, and air." It says air reaches the engine through the breather on the carburetor, where a filter keeps dust and dirt out of the fuel system.
A battery or corded electric mower has a motor, not a gas engine, so it has no carburetor.
Why does a carburetor clog?
LSU AgCenter says that running the fuel system dry before storage prevents stale gasoline in the tank and "varnish" buildup in the carburetor from evaporating gasoline. UGA says "Most fuels today contain ethanol." It says ethanol "clogs small jets in carburetors if it sits for a long period of time." It also says ethanol fuel left untreated in a tank for a long time can cause the carburetor to clog and the fuel lines to crack.
EPA says E15, a blend of 15 percent ethanol and 85 percent gasoline, is not for "Engines in nonroad equipment, such as lawnmowers and chain saws". Your manual lists the fuel your engine takes.
What are the signs?
The sources describe symptoms, not a test. LSU says old gasoline can leave an engine hard or impossible to start in the spring. UGA says that when a small engine will not crank or runs poorly, the cause is normally one of three things: fuel, fire, or air.
A symptom does not prove the carburetor is at fault. UGA says a clogged air filter can make an engine run poorly and stall, and a worn or fouled spark plug can keep it from starting. Our maintenance basics guide covers the filter and the plug.
What do the sources say to check first?
Start with the operator's manual. Ohio State University Extension calls it "the best source of information regarding equipment maintenance." EPA says makers explain in the owner's manual what emission controls each model has, and it may list maintenance the owner is meant to do.
If fuel is the suspect, UGA says to drain the old fuel into an approved container and refill with fresh fuel that is either free of ethanol or treated with a stabilizer. Do this outdoors with the engine off and cool. The American Academy of Pediatrics (AAP) says to "Stop the engine and allow it to cool before refueling" and to refuel outdoors rather than inside a garage or shed.
What has to happen before anyone opens the engine?
Ohio State says to "shut off the engine and disconnect the spark plug wire before performing maintenance", and it says mower blades can keep spinning after the engine stops. The American Academy of Orthopaedic Surgeons (AAOS) says hands and feet should never touch the blade, even with the engine off.
Beyond that point, we give no steps, and we found no neutral source that gives carburetor cleaning steps for owners. Carburetors differ from engine to engine. The maker's manual says whether any of this work is meant for owners and how it is done.
Is the carburetor an emissions part?
The federal list of emission-related components in 40 CFR Part 1068, Appendix A, covers engine parts related to the "Air-induction system" and the "Fuel system." The list does not name the carburetor. Because a carburetor mixes fuel with air, we read it as related to both; that is our reading, not the rule's words. EPA says owners "may not disable any emission controls installed on certified engines". For routine service, EPA says "you still need to avoid making changes that might increase emissions". These are EPA's words, not legal advice. Follow the manual for any adjustment or replacement part.
When should the mower go to a shop?
Take it to a service shop when the manual says the job is for a dealer, when you are not sure what the problem is, or when fresh fuel does not fix it. UGA says that if an engine still will not start or keeps stalling, the carburetor and fuel lines may need to be replaced, and that "A small-equipment repair shop can do this". UGA adds that an owner can attempt it; we give no steps for that, and the manual says whether the job is meant for owners. AAOS says to have a mower serviced the first time you use it each season. Our repair or replace guide lists questions to ask a shop.
Sources: University of Georgia Extension, Equipment Maintenance and Troubleshooting for the Landscape and Garden (Circular 1247); LSU AgCenter, Preparing Small Engines for Storage; Ohio State University Extension, Power Lawn Mowers; EPA, Frequently Asked Questions from Owners and Operators of Nonroad Engines, Vehicles, and Equipment Certified to EPA Standards (PDF); EPA, How to Maintain or Rebuild Engines Certified to EPA Standards (PDF); eCFR, 40 CFR Part 1068, Appendix A, Emission-Related Components; EPA, E15 Fuel Registration; American Academy of Pediatrics, HealthyChildren.org, Lawn Mower Safety: Information for Families; AAOS OrthoInfo, Lawn Mower Safety.
FAQ
General information only. Read the operator's manual before you mow. Keep children and pets indoors or well away from the area you mow, and never let a child ride on a mower as a passenger. The American Academy of Pediatrics says that, in general, children should be at least 12 years old to operate a walk-behind power mower safely and at least 16 to operate a riding mower safely. Before you clear, adjust, or service the blade area, turn the mower off, wait for the blade to stop, and disconnect the spark plug wire; on a battery or corded electric mower, follow the manual to disconnect its power. Refuel outdoors, only with the engine off and cool. Wear sturdy closed-toe shoes and eye and hearing protection. In an emergency, call 911.
Next: fuel and storage
LSU and Ohio State on storing a gas mower.
