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A lawn mower won’t start most often because of old gas left in the tank since last season, the top cause listed by Briggs & Stratton. The air filter, the spark plug and the carburetor come next, in that order. Allow half an hour, with no need to take the engine apart.
Common symptoms and causes
- You pull the starter cord (the handle you tug) normally, but the engine does not start.
- The engine runs for a few seconds, then stalls.
- The mower won’t start after winter storage.
- It won’t start even with a new spark plug.
The causes manufacturers list: old gas, a fouled or worn spark plug, a clogged air filter, a dirty carburetor, a closed fuel shutoff valve, the oil level, a safety control or kill switch, the ignition (spark plug wire, flywheel key) and engine compression.
Before you start
Warning: stop the engine, remove the key on models that have one, and wait until the blade has stopped completely before doing anything. Then pull off the spark plug boot (the rubber cap on the spark plug): the engine cannot start by accident while your hands are near the blade.
Warning: gasoline is flammable. Work with a cold engine, outdoors, away from any flame or spark, and do not smoke.
Warning: none of the official sources consulted says which way to tip the mower. If you have to tilt it, follow the engine manual, which tells you which side to keep facing up.
Warning: removing the carburetor or opening the engine can void the warranty. Check the warranty end date before Solution 5.
Get a spark plug socket, a gas can approved for fuel, some rags and the engine manual: it gives the spark plug part number and its gap (the distance between the two electrodes).
Solution 1: check the blade brake control, the choke and the oil level
- Make sure the spark plug boot is pushed firmly onto the spark plug and that its wire is not cut.
- Check the oil level and condition with the engine cold, following the procedure in the manual.
- On a riding mower, set the parking brake and raise the cutting deck: a safety interlock blocks starting while the blade is engaged.
- On a walk-behind mower, hold the blade brake bar (the safety handle) against the handlebar while you pull the starter cord.
- For a cold start, set the choke control to the position given in the manual, if your mower has one.
No official source consulted confirms steps 4 and 5, and they vary by model: follow your mower’s manual. If the oil level is low, the manual tells you which oil to top up with. The engine should start if the problem came from these controls.
Good to know: Briggs & Stratton recommends changing the oil every 25 hours or every season.
Solution 2: replace the old gas
According to Briggs & Stratton, gas starts to break down after about a month.
- Open the fuel shutoff valve if it is closed, and check that it is clean.
- Drain the tank into a gas can, with the engine cold and outdoors.
- Drain the carburetor bowl (the small reservoir under the carburetor) using the method in the engine manual.
- Fill the tank with fresh gas.
- Add a fuel stabilizer to the fresh gas.
- Reconnect the spark plug boot, then pull the starter cord.
The engine should start after a few pulls.
To keep the gas from going stale again, here is where to find a fuel stabilizer.
STA-BIL fuel stabilizer for petrol engines
Additive to pour into fresh petrol to protect it from ageing while the machine is stored. It helps the engine start easily again in spring.
- AmazonSee at Amazon
Tip: pour the drained gas into an approved gas can, never into a bottle, and take it to a hazardous waste collection site.
Solution 3: clean or replace the air filter
According to Husqvarna, a clogged air filter restricts the air reaching the engine.
- Remove the air filter cover.
- Take out the filter element.
- Clean it according to the filter type given in the engine manual: unofficial repair guides suggest compressed air, or rinsing followed by complete drying if the manual allows rinsing.
- Replace it if cleaning is not enough or if the manual calls for it.
- Refit the filter and its cover.
According to repair guides, an engine that starts with the filter removed confirms the filter was too dirty. For this test, follow the engine manual. With a clean filter, the engine should start.
Solution 4: check and replace the spark plug
Briggs & Stratton recommends checking the spark plug once a season, every 25 hours, or whenever the engine won’t start.
- Pull the boot off the spark plug.
- Clear the grass and dirt around the spark plug so that no debris falls into the cylinder.
- Unscrew the spark plug with the spark plug socket.
- Look at the tip: a cracked porcelain, burned electrodes or stubborn deposits mean you need a new spark plug.
- If the deposits are light, clean them with a wire brush or a spark plug cleaner, never by sandblasting or with an abrasive.
- Set the gap of the new spark plug to the value in the engine manual.
- Screw the spark plug in, then tighten it to 15 ft-lbs at most according to Briggs & Stratton (about 20 N·m; check your engine manual).
- Refit the boot and pull the starter cord.
The engine should start. If it runs briefly and then stalls, the spark is probably there, and Briggs & Stratton then points to the gas, the carburetor or the valves.
If you don’t have a suitable socket, here is where to find a spark plug socket.
AEG 3-socket spark plug wrench, 14-16-21 mm
Spark plug wrench with three sockets (14, 16 and 21 mm) to remove and refit the mower's spark plug. It avoids damaging the plug or straining the engine.
- AmazonSee at Amazon
Good to know: the engine manual gives the exact spark plug part number. Buy that one, not a similar one.
Solution 5: dirty carburetor (simple cleaning or repair shop)
A dirty carburetor prevents starting. Briggs & Stratton then calls for cleaning or rebuilding it, which means taking it apart.
- Check that gas reaches the carburetor, with the shutoff valve open.
- Inspect the fuel line (the flexible hose between the tank and the carburetor): it must not be cracked, pinched or blocked.
- If the carburetor bowl contains water or debris, clean it and refill with fresh gas.
- Take the carburetor to a repair shop if it has to be removed or rebuilt.
With a free flow of gas and a clean bowl, the engine should start. If it doesn’t, cleaning inside the carburetor is a job for a repair shop.
If nothing works
Stop if the mower still won’t start with a new spark plug, fresh gas and a clean filter. The remaining causes are for an authorized dealer: a carburetor that has to come off, a damaged flywheel key or doubtful compression. The dealer checks compression with an engine leak-down test.
Have this ready before you call:
- the make and model of the mower;
- a list of what you have already done: gas changed, spark plug replaced, filter cleaned.
Take better care of your lawn mower: our toolbox
Fuel that suits storage, a clean fuel can, a little upkeep and a few protections help prevent the next starting problem.
- Kettlitz-Medialub SK-4T 4-stroke alkylate fuel, 5 L canReady-to-use alkylate petrol for lawn mowers and other 4-stroke engines. It replaces pump petrol in the tank when the machine is stored for a long time.AmazonSee at Amazon
- Valpro 5 L steel petrol jerry canFive-litre metal can with a safety pin, suitable for all fuels. Use it to drain the old petrol from the mower and to store the fresh petrol.AmazonSee at Amazon
- Liqui Moly 5100 MTX carburettor cleaner, 300 mlAdditive poured into the tank to clean the carburettor and valves of 4-stroke petrol engines. It helps limit the build-up that makes starting difficult.AmazonSee at Amazon
- Briggs & Stratton SAE 30 4-stroke lawn mower engine oil, 0.6 LOriginal mineral oil for air-cooled 4-stroke lawn mower engines. Use it to top up the oil level you check in the first solution.AmazonSee at Amazon
- Mechanix Wear Original Covert work glovesClose-fitting synthetic leather gloves, machine washable, that keep the dexterity needed to handle a spark plug wrench. They protect your hands from petrol and from the sharp edges under the mower deck.AmazonSee at Amazon
- 3M Command 2890SC safety gogglesPolycarbonate goggles with indirect ventilation and an anti-scratch coating. They protect your eyes from petrol splashes and debris when you clear the area around the spark plug.AmazonSee at Amazon
Frequently asked questions
Why won’t my lawn mower start after winter?
The most common cause is gas left in the tank, which starts to break down after about a month. Drain the tank and the carburetor bowl, then fill up with fresh gas. After that, check the spark plug and the air filter.
How do I know if my lawn mower’s spark plug is dead?
Replace a spark plug if its porcelain is cracked, its electrodes are burned or its deposits won’t come off. Briggs & Stratton also advises changing it once a season or every 25 hours. If the engine doesn’t turn over at all, try a new spark plug first before suspecting the ignition.
Should I drain the gas from a mower before winter?
Drain it or add a fuel stabilizer before putting the mower away, as the manual directs. Briggs & Stratton puts the start of its breakdown at about a month. In spring, drain any old gas before the first start.
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