Why Is My Lawn Mower Turning Over But Not Starting

You pull the cord or turn the key, and your lawn mower engine is turning over but not starting. It’s a common and frustrating problem that can have several causes. This guide will help you figure out why your mower won’t fire up and get you back to cutting grass quickly.

We’ll walk through the most common issues step-by-step. You don’t need to be a mechanic. With a few basic tools and some logical troubleshooting, you can often solve the problem yourself. Let’s start with the simplest checks first.

This specific problem means the engine has enough compression to spin when you engage the starter, but one of the other critical elements for combustion is missing. An engine needs three things to run: fuel, spark, and air. If any one of these is missing or compromised, you’ll get exactly this situation. We’ll check each system methodically.

The Very First Thing to Check

Before you dive into complex diagnostics, always check these two simple things. They are the cause of more non-starting mowers than anything else.

The Fuel Valve: Many mowers have a small fuel shut-off valve on the gas line. Make sure it’s in the “On” position.
The Blades/Drive Lever: Your mower’s safety system requires you to be holding down the blade engagement lever (or drive lever) for it to start. Ensure you are fully engaging it.

Issue 1: Old or Contaminated Fuel

Bad gasoline is the #1 culprit for small engine problems. Gasoline begins to degrade in as little as 30 days. It can form varnish and gum that clog the tiny passages in your carburetor.

The Sniff Test: Drain a little fuel into a clear container. If it smells sour or looks darker than fresh gas, it’s bad.
Water Contamination: Water is heavier than gas and will settle at the bottom of your tank and carburetor bowl. This prevents fuel from reaching the engine.

How to Fix a Fuel Problem

Drain all the old fuel from the tank and the carburetor bowl (there’s usually a small nut on the bottom of the carburetor for this).
If the fuel was very old, you may need to add a fuel system cleaner to the new gas.
Always use fresh, ethanol-free gasoline if possible, or add a fuel stabilizer if you plan to store the mower.

Issue 2: A Clogged Carburetor

The carburetor’s job is to mix air and fuel in the perfect ratio. If its jets or float bowl are clogged with old fuel residue, no fuel gets to the engine.

Signs of a clogged carb include the engine turning over but not starting, or perhaps starting only when you spray starter fluid into the air intake (a test we’ll cover later).

Cleaning the Carburetor

Disconnect the spark plug wire for safety.
Locate the carburetor (usually attached to the air filter housing).
Remove the bowl and the main jet (a small brass piece with a hole in it).
Spray all parts thoroughly with carburetor cleaner, using a thin wire to clear the tiny jet holes.
Reassemble and try starting.

Issue 3: A Dirty or Clogged Air Filter

A severely dirty air filter chokes the engine, preventing it from getting the air it needs for combustion. This is an easy fix.

Paper Filters: Tap it gently to remove loose dirt. If it’s very dark or oily, replace it.
Foam Filters: Wash these in warm, soapy water, rinse, and let them dry completely. Then apply a few drops of clean engine oil and squeeze it to distribute evenly.

Issue 4: No Spark

If fuel and air are present, the next suspect is the ignition system. A weak or absent spark won’t ignite the fuel mixture. The main components here are the spark plug and the ignition coil.

Checking and Replacing the Spark Plug

Pull the spark plug wire off the plug.
Use a spark plug socket to remove the plug.
Inspect the plug. It should be a light tan color. If it’s black and sooty, wet with gas, or has a cracked porcelain insulator, it needs replacement.
Check the electrode gap with a feeler gauge and adjust it to the spec in your manual (usually around .030 inches).
To test for spark, reconnect the wire to the plug, hold the metal base of the plug against the engine block (so it’s grounded), and pull the starter cord. You should see a bright blue spark jump the gap. CAUTION: Do not hold the plug with your bare hands.

If there’s no spark, the ignition coil may be faulty. This is a more advanced repair, but replacement coils are readily available.

Issue 5: Engine Flooding

Sometimes, you can have to much of a good thing. If you’ve pulled the starter cord repeatedly without the engine starting, you may have flooded it with to much fuel. This washes away the oil on the cylinder walls and prevents compression.

The fix is simple. Remove the spark plug. Pull the starter cord several times to clear excess fuel from the cylinder. Let the mower sit for 15-20 minutes to allow fuel to evaporate. Then reinstall the (dry) plug and try starting normally.

Issue 6: Valve Problems

This is a less common but serious issue, especially on older mowers. The intake and exhaust valves must open and close at precise times. If they are stuck, burned, or have too much or too little clearance (gap), the engine loses compression and won’t start.

Signs include a very easy pull cord (low compression) or a clicking/tapping sound from the engine. Adjusting valve clearance requires specific tools and knowledge of your engine’s specs. It’s often best left to a professional if you’re not comfortable.

A Systematic Troubleshooting Plan

Follow this order to find the problem efficiently without missing anything.

Step 1: The Starter Fluid Test

This quick test tells you if your problem is fuel-related.

Remove the air filter cover and filter.
Spray a small (1-2 second) burst of engine starter fluid directly into the air intake.
Quickly try to start the engine.

Result: If the engine starts for a few seconds and then dies, you have a fuel delivery problem (bad gas, clogged carb, stuck needle valve). If it doesn’t start at all, you likely have a spark or compression issue.

Step 2: Verify Spark

Perform the spark plug test described above. A strong blue spark is good. A weak yellow spark or no spark means you need to focus on the ignition system.

Step 3: Check Compression (Basic Test)

While a compression gauge is the proper tool, you can do a simple test. Carefully place your thumb over the spark plug hole and pull the starter cord. You should feel a strong suction and pressure pushing your thumb away. If you feel very little, compression is low, indicating worn piston rings, cylinder damage, or valve issues.

Less Common Issues to Consider

If you’ve checked all the above and your mower is still turning over but not starting, think about these possibilities.

Safety Switch Failure

Modern mowers have several safety switches (seat, blade engagement, brake). If one fails, it can interrupt the spark or starter circuit. Diagnosing these requires a multimeter and wiring diagram.

Sheared Flywheel Key

The flywheel key is a small metal piece that aligns the flywheel with the crankshaft. If you hit a solid object, the key can shear, throwing the ignition timing off. The engine will spin but will not start. Replacing it is straightforward but requires a flywheel holder and socket.

Blocked Fuel Cap Vent

The gas cap has a small vent hole to allow air into the tank. If this gets clogged, a vacuum lock forms, preventing fuel from flowing to the carburetor. Try starting the mower with the gas cap slightly loose to test this.

Prevention is Key

A little seasonal maintenance prevents most of these problems. Here’s a quick end-of-season routine:

Add fuel stabilizer to a full tank and run the mower for 5 minutes to get it into the carburetor.
Change the oil.
Clean or replace the air filter.
Remove the spark plug, add a teaspoon of oil to the cylinder, and pull the cord slowly to coat the walls. Reinstall the plug.
Store in a dry place.

FAQ Section

Why does my lawn mower crank but not start after winter?

This is almost always due to old, degraded gasoline clogging the carburetor. Follow the steps above to drain the old fuel and clean the carburetor jets.

What would cause a mower to have no spark?

A faulty spark plug, a disconnected or damaged spark plug wire, a failed ignition coil, or a triggered safety switch can all cause a loss of spark. Start by checking and replacing the plug.

My mower starts with starter fluid but then dies. What’s wrong?

This confirms the engine has spark and compression. The problem is fuel delivery. You have a clogged carburetor, a stuck fuel valve, a dirty fuel filter, or bad gas in the tank.

How do I know if my lawn mower carburetor is bad?

Signs include the engine not starting, starting only with starter fluid, running very rough, or leaking fuel from the bowl. Often, a thorough cleaning is all it needs before considering replacement.

Can a dirty air filter prevent a mower from starting?

Yes, a extremely clogged air filter can restrict air flow so much that the fuel-air mixture becomes too rich to ignite. Always check the air filter first; it’s one of the easiest things to fix.

Figuring out why your lawn mower is turning over but not starting requires patience. Start with the simplest solutions—fresh fuel, a clean air filter, and a good spark plug. These three items account for the vast majority of problems. By working through this checklist, you’ll likely identify the issue and can get back to maintaining your lawn without the cost of a repair shop.