Few things are more frustrating than finally getting your lawnmower to start—only for it to sputter and stall a few seconds later. This is a common headache for homeowners and pros alike, and it can strike both push and riding mowers, as well as gas and even some battery models. The good news? Most causes are simple to fix, and you can get your mower running smoothly again with just a little know-how. This guide explains every major reason a lawnmower won’t stay running and how to solve it, step by step.
Quick-Check: Top Reasons Your Mower Starts, Then Stalls
- Dirty or clogged carburetor
- Old, stale, or contaminated gasoline
- Blocked or dirty fuel filter
- Cracked, kinked, or clogged fuel line
- Faulty or dirty spark plug
- Clogged air filter
- Bad ignition coil or faulty safety switch
- Vapor lock or fuel cap vent blocked
- Water or debris in the gas tank
- Overheated engine or low oil (for some models with low-oil shutoff)
- Stuck choke or improper choke/throttle setting
- Mechanical issues: sticking valves, internal engine wear
1. Start with the Basics: Fresh Fuel, Clean Filter, and Good Spark
- Fresh gasoline: Old fuel quickly goes bad—especially with ethanol blends. Drain and replace with fresh gas if yours is older than 30 days.
- Fuel filter: Replace if dirty or slow to flow. A blocked filter lets the engine start, but it quickly starves for fuel and stalls.
- Spark plug: Remove, inspect, and clean or replace if fouled, rusty, or worn. Set the correct gap as per your manual (usually 0.028–0.030 inches).
- Air filter: A clogged filter chokes the engine and causes stalling. Replace if dirty or darkened.
2. Carburetor Issues: The Most Common Culprit
- Modern fuels often gum up carburetor jets and floats after just a few weeks—especially if the mower sits unused.
- If your mower starts but dies after a few seconds, or only runs with the choke on, the carburetor is likely dirty or clogged.
- Try spraying carb cleaner into the carburetor throat with the engine off. Then start the mower. If it runs longer but still stalls, clean or rebuild the carburetor.
- Remove and clean the carburetor’s bowl, float, needle, and jets. For stubborn varnish, soak parts in carburetor cleaner.
- Replace all gaskets during reassembly for a proper seal.
3. Fuel Line Problems
- Inspect all fuel lines for kinks, cracks, or collapses—especially on older mowers or after winter storage.
- Blow through the line (with it removed from the carb and tank) to check for blockages. Replace soft, brittle, or blocked lines.
- Check that fuel flows freely from the tank to the carb when you disconnect the line (place the end in a clear cup to catch spills).
4. Fuel Cap and Vapor Lock
- If the fuel cap vent is clogged, a vacuum forms in the tank, starving the carburetor after a minute or two.
- Try running the mower with the cap loose. If it stays running, clean or replace the cap.
- This is especially common in hot weather or after mowing tall/thick grass.
5. Air Filter & Intake Issues
- A clean air filter is vital for steady engine running. Replace foam filters yearly and paper filters as soon as they look dirty or gray.
- Remove the filter and briefly run the mower (just as a test). If it runs smoothly, replace the filter before using again.
- Check for insect or rodent nests in the air intake if the mower has been sitting.
6. Spark Plug & Ignition Problems
- A weak or intermittent spark can cause stalling. Replace the plug yearly, and ensure the spark plug wire is tight.
- Check the ignition coil if the spark looks weak or orange, or if you have repeated stalling even after plug replacement.
- Some mowers have a safety kill wire (deadman switch) that grounds the ignition if a handle or lever isn’t held. Ensure all controls are engaged and wires are undamaged.
7. Choke & Throttle Settings
- If your mower only runs with the choke engaged, the carb is almost always gummed up or has a blocked jet.
- Ensure the choke and throttle cables move freely and the choke plate opens as the engine warms up.
- Adjust or lubricate cables if needed, and repair sticky or binding levers.
8. Engine Oil and Overheating
- Low oil or very old oil can trigger shutoff in some mowers with a low-oil sensor. Top up or change oil as needed.
- Overheating from clogged cooling fins or heavy grass can also cause stalling. Clean cooling fins and engine shroud regularly.
9. Mechanical or Internal Issues
- Sticking intake or exhaust valves, worn rings, or low compression can also cause an engine to stall, though this is less common than fuel issues.
- If you’ve checked all fuel, air, and spark items, consider a compression test or consult a professional mechanic.
10. Special Note for Battery/Electric Mowers
- If your cordless mower starts and stops quickly, check the battery charge, seating, and cleanliness of contacts.
- Remove the blade and check for jams or overload. Let the mower cool before restarting.
- For corded models, check for a faulty extension cord or tripped outlet breaker.
Frequently Asked Questions (FAQ)
Why does my mower start and then die after a few seconds?
Most often, this is a carburetor issue—dirty jets or stuck float. It can also be a clogged fuel filter, dirty air filter, or fuel cap vent problem.
What if my mower only runs with the choke on?
This means the carburetor is partially blocked, usually from stale fuel. Clean or rebuild the carb for best results.
Can old gas make my mower stall?
Absolutely. Old, varnished gasoline gums up jets and filters, leading to poor running and stalling.
Should I replace my spark plug or just clean it?
It’s best to replace annually for reliable starts and steady running. Cleaning helps for minor fouling, but new plugs are inexpensive.
How do I keep my mower running smoothly all season?
Use fresh gas, replace filters and spark plugs every spring, clean the carburetor, and keep the air intake clear. Store your mower properly in the off-season.
Conclusion: Keep Your Mower Running Strong
If your lawnmower won’t stay on, the fix is usually simple—clean out the carburetor, replace old fuel and filters, and check the spark plug. With these easy troubleshooting steps, most mowers are running like new in minutes. For recurring issues or suspected engine problems, don’t hesitate to call a local repair shop. A little routine care and quick repairs keep your mower running smoothly all season long—so you can enjoy a great-looking yard with less hassle.
Ready to finish the job? With these fixes, you’ll keep your mower running and your lawn looking its best!