Why Is My Lawn Mower Turning Over But Not Starting

You pull the cord or turn the key, and your lawn mower engine turns over but doesn’t start. It’s a common and frustrating problem. This guide will help you figure out why your lawn mower is turning over but not starting and how to fix it yourself.

When an engine “turns over,” it means the mechanical parts are spinning. But for it to actually run, three critical things must happen: it needs fuel, air, and spark. If any one of these is missing, you’ll get exactly the symptom you’re experiencing. We’ll check each system, starting with the simplest and most common fixes first.

The Usual Suspects: Quick Checks First

Before we get into the more involved troubleshooting, always start here. These quick checks solve a huge percentage of non-starting mowers.

Fuel Valve: Is it in the “On” position? It seems obvious, but it’s often overlooked.
Fresh Gasoline: Old gas is the #1 culprit. Gasoline starts to degrade in as little as 30 days. If it’s been sitting over winter, you likely have bad fuel.
Choke/Throttle Setting: For a cold start, the choke should be closed. If it’s open, the engine may not get the rich fuel mixture it needs to start.
Spark Plug Wire: Make sure it’s firmly attached to the plug. It can vibrate loose during operation.

Fuel System Problems

If the quick checks don’t work, the fuel system is your next stop. A clog or blockage here prevents gas from reaching the engine.

Step 1: Drain and Replace the Fuel

Use a siphon pump to remove all the old gas from the tank.
Dispose of it safely at a recycling center.
Fill the tank with fresh, clean gasoline. For ethanol-free gas if possible, as it’s better for small engines.

Step 2: Check the Fuel Line and Filter

Follow the fuel line from the tank to the carburetor. Look for cracks or kinks. If your mower has an inline fuel filter, check if it’s clogged or dirty. A clogged filter will starve the engine of fuel.

Step 3: Inspect the Carburetor

The carburetor mixes air and fuel. Over time, tiny passages inside can get gummed up with old gas residue. You might need to clean it.

Remove the air filter cover and the filter itself.
Spray a generous amount of carburetor cleaner into the intake.
Let it sit for a few minutes, then try starting. If it starts briefly, you’ve confirmed a carb issue.

A full carb clean or rebuild might be necessary, which involves taking it apart and cleaning each jet and passage.

Ignition System Issues

No spark means no combustion. This system is simple to test and fix.

Checking and Replacing the Spark Plug

Pull off the spark plug wire boot.
Use a spark plug socket to remove the plug.
Inspect it. Is it fouled with black carbon? Is the electrode gap huge or the porcelain cracked? Any of these mean it’s bad.
To check for spark, reconnect the wire to the plug, hold the metal body against the engine block (away from the spark plug hole), and pull the starter cord. You should see a bright blue spark jump the gap. If not, you have an ignition problem.
If the plug is bad, replace it with a new one gapped to your mower’s specifications.

If there’s still no spark with a new plug, the issue could be a faulty ignition coil or a broken stop switch wire.

Air Supply Blockages

An engine needs clean air to run. A severe blockage can prevent starting.

Air Filter: Remove the air filter cover and take out the filter. Is it caked with dirt and oil? Try starting the mower without the filter (just for a quick test). If it starts, you need a new air filter. Never run it without one for long, as dirt will ruin the engine.
Air Intake: Look for any obstructions, like a mouse nest or packed grass clippings, around the air intake area.

Compression and Other Mechanical Faults

If fuel, spark, and air are all good, the problem may be mechanical. Low compression means the engine can’t build the pressure needed for combustion.

Common causes include:

A stuck or broken valve.
A worn piston ring.
A blown head gasket (you might see oil leaking from the cylinder head area).

Checking compression requires a special gauge. If you suspect this, it might be time to consult a small engine repair pro.

Safety Switch Interference

Modern mowers have several safety switches. They stop the engine if you let go of the handle or if the blade control is not engaged. A faulty switch can interupt the spark circuit.

Listen for a clicking sound when you pull the starter with the blade control engaged. Sometimes, wiggling the handle or control lever can make a bad connection work temporarily. Diagnosing a specific bad switch can be tricky and may require a wiring diagram.

Step-by-Step Troubleshooting Summary

Quick Checks: Fuel on? Choke on? Fresh gas? Plug wire connected?
Fuel: Drain old gas. Check fuel line and filter. Clean carburetor.
Spark: Remove and inspect spark plug. Test for spark. Replace if needed.
Air: Remove and inspect air filter. Check for intake blockages.
Safety: Check that all control levers are fully engaged. Consider safety switches.
Compression: If all else fails, consider a mechanical compression issue.

FAQ: Lawn Mower Won’t Start

Q: Why won’t my mower start after winter?
A: This is almost always due to old, degraded gasoline left in the system. It turns to varnish and clogs the carburetor. Draining the fuel before storage or using a fuel stabilizer prevents this.

Q: My mower has spark and fuel, but still no start. What gives?
A> Double-check that you have enough fuel reaching the cylinder. A weak spark or incorrect spark timing can also cause this. Also, ensure the air filter isn’t completely blocked.

Q: How do I know if my carburetor is bad?
A> If cleaning it doesn’t help, or if you see visible damage like a cracked float, it may need to be replaced. Often, a rebuild kit with new gaskets and needles is cheaper than a whole new carb.

Q: Can a dirty air filter really prevent starting?
A: Yes, absolutely. If the filter is severely clogged, it restricts the air flow so much that the fuel-air mixture becomes too rich to ignite.

Q: I hear a clicking when I try to start. What is that?
A: That’s often the sound of a safety switch activating. Ensure you are fully depressing the blade control lever and that you are holding the handlebar safety lever (if equipped). The click could also indicate a weak battery on an electric-start model.

By following this logical process, you can usually identify and fix the reason your lawn mower is turning over but not starting. Remember to always disconnect the spark plug wire before doing any work for safety. With a little patience, you’ll have your mower running smoothly again and your lawn looking tidy in no time.