Your mower worked last fall and now it cranks and coughs or does nothing at all. Before you haul it to a shop, run this list. Most no-start problems on a gas push mower are stale fuel, a fouled spark plug, or a dirty air filter, and all three are cheap to fix at home.
1. Check the Fuel
Gas left in the tank over winter breaks down and can gum up the carburetor. If the mower has sat for months, drain the old fuel into an approved container and refill with fresh gasoline. Make sure the tank actually has fuel and that the fuel valve, if your model has one, is open.
2. Prime the Engine and Check the Choke
Many mowers use a primer bulb. Press it the number of times your manual specifies, set the choke for a cold start, and pull firmly. If the engine floods, move the choke to run, hold the throttle open, and pull several times to clear the excess fuel.
3. Inspect and Replace the Spark Plug
Disconnect the spark plug wire first. Pull the plug and look at the tip. A black, wet, or crusty plug is fouled and a new one is inexpensive. Confirm the replacement matches the plug listed in your manual and that the gap is set to the manual's spec before you install it.
4. Clean or Replace the Air Filter
A clogged air filter starves the engine of air. Remove the cover and check the filter. A paper filter that is dirty or oily should be replaced, and a foam filter can be washed, dried, and lightly oiled if your manual allows it.
5. Clear the Deck
Packed wet grass under the deck can lock the blade and make the engine hard to pull or stall right after starting. Disconnect the spark plug wire, tip the mower with the air filter side up so fuel and oil do not run into it, and scrape the deck clean.
6. Check the Safety Controls
Walk-behind mowers will not run unless the safety bar is held against the handle. Check that the control cable is not loose or stretched and that the bar fully engages. On models with an electric start, check the battery connection as well.
7. Look at the Oil Level and Condition
Some engines have a low-oil shutdown, so an engine that will not stay running may simply be low. Check the dipstick on level ground and top up to the mark. Old, dark oil is worth changing while you have the mower tipped or drained for service.
When to Stop and Call Someone
If the fuel is fresh, the plug is new, the filter is clean, and the safety bar works but the mower still will not start or runs rough, the carburetor probably needs a deeper cleaning or rebuild, or there is an internal problem such as a sheared flywheel key. At that point a small-engine shop is worth the cost, especially on an older mower where a rebuild may approach the price of a new one.
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FAQ
Why does my lawn mower crank but not start?
The usual causes are old fuel, a fouled spark plug, a dirty air filter, or a flooded engine. Try fresh gas, a new spark plug, and a clean filter, in that order.
How long can gas sit in a lawn mower?
Gasoline begins to degrade within a few months, and stale fuel is one of the most common reasons a mower will not start in spring. Drain it and refill with fresh gas if the mower has sat all winter.
Is it safe to tip a mower to clean the deck?
Disconnect the spark plug wire first, and tip the mower with the air filter and carburetor side facing up so oil and fuel do not flow into the air filter. Follow your owner's manual for your model.
Is it worth fixing an old lawn mower or should I replace it?
If the fix is fuel, a plug, a filter, or deck cleaning, repair it since the parts are cheap. If it needs a full carburetor rebuild or has internal engine damage, compare the repair quote with the cost of a new mower.