Lawn Mower Won’t Start: Top Causes and Fixes

Why Your Lawn Mower Won’t Start: A Pro’s Guide to Diagnosing a Stubborn Engine

There’s nothing more frustrating than dragging your mower out on a sunny day, only to discover the lawn mower won’t start. I have lots of experience with small engines, and I can tell you that no-start problems after coming out of storage are almost always caused by stale fuel, carburetor corrosion, a stuck needle/seat in the carburetor, or clogged main or idle jets. In other words, most no-start problems after storage are fuel-related, not ignition-related. Whether you’re dealing with a push mower, riding mower, or string trimmer, the root causes are surprisingly similar. When a small engine won’t start, there are only a few systems you need to check: fuel and spark

Let me walk you through the most common reasons your lawn mower won’t start and how to troubleshoot them like a pro.

1. Lawn Mower Won’t Start Due to Old or Contaminated Fuel

Hands down, the number one reason a small engine won’t start is old gasoline. Gas starts to degrade within 30 days. Fuel goes stale due to evaporation of its most volatile components, oxidation, and moisture accumulation. Loss of the most volatile components makes it much harder to fire up. Oxidation causes the fuel to form gums and varnish that clog the jets and small passages in your lawn mower’s carburetor. Moisture accumulation, especially in ethanol fuel, causes phase separation, where the ethanol and water separate from the gas and fall to the bottom of the tank.

What to do if you suspect stale fuel

If your lawn mower won’t start after sitting all winter, drain the old gas and replace it with fresh fuel. If it still won’t start, spray a shot of carb cleaner into the air intake and try again. If that gets it to fire up, you’ve got a fuel delivery issue—probably a gummed-up carburetor that requires rebuilding or replacement.

2. Clogged Carburetor

If fresh gas doesn’t solve the issue, the carburetor is likely gunked up. When a small engine won’t start and you’ve already verified that the fuel is good, the next step is cleaning, rebuilding, or replacing the carburetor.

Waterlogged fuel causes corrosion. Once that occurs, the carburetor must be replaced. Oxidized fuel forms tar-like deposits that clog the main and idle jets and prevent proper operation of the fuel inlet needle valve.

Evaluate the Pros and Cons of Rebuilding Versus Replacing Your Carburetor

Carburetor rebuilding kits are fairly cheap. However, if the passages are really plugged, a rebuld can’t always remove all the buildup. And, rebuilding isn’t always the cheapest option. In the image below, we see that a typical rebuilding kit runs $17-$35. Plus, you’ll need a gallon of carburetor cleaner for around $40. You’ll be able to reuse the cleaner, but there’s still the upfront cost.

Compare that to the price of a genuine OE carburetor shown on the right. A genuine OE carburetor for a Briggs and Stratton engine runs about $59, while an Amazon aftermarket unit costs only $17. You’ll be mighty tempted to buy the aftermarket unit. Don’t. They’re garbage. Either rebuild yours or buy an OE carburetor.

Rebuilding your carburetor isn’t always cheaper than replacing it.

Steps to Replace or Rebuild Your Carburetor

1) Clamp off the fuel line to the carburetor using a fuel line clamp or a small C-clamp. Do NOT use a vice grip.
2) Remove the nut at the bottom of the carburetor to drain the fuel from the bowl.
3) Use you cell phone camera to shoot pictures of the linkage and spring positions prior to removing them.
4) Remove the fuel line from the carb, and then the two bolts holding it to the intake manifold.
5) Remove any remaining gasket material from the intake manifold
6) Install new gaskets and then the new carburetor.
7) Torque the bolts to specification, reinstall the linkage and springs. Then reconnect the fuel line and remove the clamp.

Test The Ignition System for a Worn or Damaged Spark Plug or a Failed Ignition Module

You wouldn’t believe how many times I’ve seen a lawn mower won’t start situation caused by a simple spark plug problem. Spark plugs wear out, get fouled with carbon, or crack over time.

Remove the plug and check its condition

If it looks black or wet, it’s probably fouled. Replace it with a fresh one. Next, check the condition of the center electrode. It should have a sharp edge. If the edge is rounded, it’s worn out. Next, make sure the gap is set correctly. You can also ground the plug against the engine block and pull the starter rope—if you don’t see a strong blue spark, you’ve got an ignition problem.

Check the Condition of the Ignition Module Using a Spark Tester
No spark from the plug? Your ignition coil could be bad if the plug is new and correctly gapped but still has no spark. The ignition system is the next place to look when a small engine won’t start and there’s no spark.

Many DIYers ground the side electrode of the spark plug and crank the engine, looking for spark. That test proves the ignition coils output enough voltage to fire the plug in open air. But it’s not a valid test for when the spark plug is installed in the engine. Firing a spark plug requires a much higher voltage when it’s under compression than in open air. That’s why you must use an in-line spark tester to test an ignition coil accurately.

Coils fail from heat and age. Use a multimeter to check resistance, or just swap in a known good coil if you have one. Make sure the kill switch wire isn’t grounding out, which can also prevent spark.

Check For A Clogged Air Filter

Engines need air to run, and if the air filter is clogged with dust or grass, your mower may not start or may start and stall. If your lawn mower won’t start and the fuel and spark systems are in order, pull the air filter and inspect it.

If it’s filthy, replace it. On some mowers, you can run it briefly without the filter to see if it starts—just don’t mow the lawn like that or you’ll suck dirt into the engine.

©, 2025 Rick Muscoplat