Graham FisherBy Graham Fisher

Updated November 2026

A lawn mower that won’t start after hitting something, like a rock, a stick, a tree root or a curb, usually has one of a handful of problems: a bent or cracked blade, a damaged blade bolt, a knocked-loose or damaged spark plug wire, a cracked flywheel key, a bent crankshaft or governor linkage, or fuel and air problems caused by the impact. This guide walks you through the checks in the order that keeps you safe and saves the most time, so you can find the cause without guesswork.

Safety First Before You Touch the Mower

Every step below involves the blade, so start by making the mower safe to work on. Disconnect the spark plug wire and tuck it away from the plug so it cannot spring back onto the terminal. For a riding mower or a battery-start model, disconnect the battery as well. Tip the mower onto its side only if the manufacturer allows it, and when you do, keep the air filter and carburetor pointed up to avoid oil and fuel flooding them. Wear heavy gloves when handling the blade, since a blade can rotate even when it looks still.

Shopping for one? See our guide: Lawn Mower Won't Start After Running Out of Gas: 3 Best Starting Aids Compared.

Step 1: Inspect the Blade and Blade Bolt

The blade takes the hardest hit in most impacts, so it is the first place to look. Remove the spark plug wire, then check the blade from both the top and the underside.

What to look for

  • Bends or warping. A blade that is bent even slightly can throw the engine out of balance and cause vibration that loosens other parts.
  • Cracks, especially near the center hole or the cutting tips.
  • Chips or missing chunks of metal, which can make the blade unbalanced.
  • A blade bolt that is loose, stripped, or sheared. Check the torque with a torque wrench if you have one.

Manufacturers generally recommend replacing a bent or cracked blade rather than hammering it straight. Use the blade part number from your owner’s manual. Torque the blade bolt to the specification in the manual, which is commonly in the range of 40 to 60 foot-pounds for walk-behind mowers, but confirm the exact value for your model.

Step 2: Check the Spark Plug and Plug Wire

A sharp impact can crack the plug porcelain, bend the electrode, or pull the plug wire loose from its boot. Remove the plug and inspect it.

  1. Look for cracks in the white insulator. Even a hairline crack can cause misfiring.
  2. Check the electrode gap against the value in your owner’s manual, often around 0.030 inch for small engines.
  3. Inspect the wire for cuts, kinks, or a loose connection at the boot.
  4. Replace the plug if it is cracked or badly fouled. A fresh plug is inexpensive insurance.

If the spark is weak or absent when you test it (with the plug grounded to the engine and the wire attached), the ignition coil or flywheel magnet may be damaged. Those components usually need a professional or a qualified repair technician.

Step 3: Inspect the Flywheel Key and Crankshaft

A flywheel key is a small metal shear pin that locks the flywheel to the crankshaft. Its job is to break first, protecting the crankshaft and the engine when something hard stops the blade. If the mower hit a solid object, the key may have sheared.

Signs of a sheared key

  • The engine turns over when you pull the starter rope, but the flywheel seems to spin freely with no compression.
  • The engine has no ignition because the flywheel is out of timing with the magnet and coil.
  • You hear a rattle or knock when you rock the flywheel back and forth.

Replacing a flywheel key involves removing the flywheel, so consult your service manual. If the crankshaft itself is bent, the engine may need major repair. Mowers that hit a large stump or buried metal can bend the crankshaft, and this is a job for a small-engine shop.

Step 4: Check the Governor Linkage and Throttle

Impacts can bend the throttle cable, the governor arm, or the linkage between them. If the throttle does not move freely, the carburetor may not get the right setting to start. Move the throttle lever through its full range by hand and watch the carburetor throttle plate. It should open and close smoothly. Straighten or replace any bent linkage.

Step 5: Look at the Fuel System

An impact can crack the fuel line, loosen the fuel cap, or knock the carburetor out of position. Check each of these items.

  • Inspect the fuel line for cracks, leaks, or soft spots.
  • Make sure the fuel shutoff valve (if equipped) is open.
  • Look for fuel stains or puddles under the mower.
  • Check the fuel tank cap and vent. A blocked vent can cause fuel starvation.

If the carburetor was knocked loose, check its mounting bolts and the gasket. A loose carburetor can let in unmetered air and cause a lean run or a no-start. If fuel has been sitting for weeks, the carburetor may also be gummed, which is a separate problem. For that, see the related guide on carburetors below.

Step 6: Check the Air Filter and Intake

A cracked air filter cover or a knocked-off intake can let grass and dirt into the engine. Remove the filter, check it for damage, and replace it if the foam or paper element is torn. Clean any debris out of the air intake area.

Step 7: Test the Starting System

On a pull-start mower, check the recoil starter for a broken rope, a worn pawl, or a damaged spring. On an electric-start model, check the battery voltage and the starter solenoid connections, since impacts can crack battery terminals or pull wires loose.

Symptom Likely cause Next step
Engine cranks but won’t fire Spark plug, plug wire, or coil problem Replace plug, inspect wire, test spark
Engine spins with no compression Sheared flywheel key or damaged valve Remove flywheel and inspect key
Vibration after impact Bent blade or loose blade bolt Replace blade and torque bolt
Starts then dies Fuel delivery or air leak Check fuel line and carburetor mounting
Rope won’t pull or spins Recoil starter damage Inspect rope, pawl, and spring

Common Mistakes

  • Starting the mower with the spark plug wire connected while you are still working on the blade.
  • Straightening a bent blade by hammering it. This creates stress cracks that can fail later.
  • Replacing the spark plug and assuming the problem is solved when the real issue is a sheared flywheel key.
  • Overtightening the blade bolt, which can stretch or snap it.
  • Pulling the starter rope repeatedly on an engine with a damaged crankshaft or key, which can cause more damage.

Tips for Getting the Mower Back Running

  • Keep a spare blade and blade bolt on hand, so a single impact doesn’t leave you without a working mower for the season.
  • After any impact, check the blade and spark plug before attempting to start the engine.
  • Mow with a clear path and walk the lawn for debris, rocks, and sticks before each session.
  • If the engine has a decompression valve, check that it still operates freely after an impact.
  • A torque wrench, so you can set the blade bolt to the manufacturer’s specification rather than guessing. View on Amazon
  • A spark plug socket with a rubber insert, which holds the plug steady while you remove it. View on Amazon
  • A spark plug gap tool, for checking the electrode gap against your manual’s spec. View on Amazon
  • Heavy work gloves for blade handling and cuts. View on Amazon

FAQ

Can I straighten a bent mower blade?

Some people do, but manufacturers generally recommend replacing a bent blade. Straightening can create hidden stress cracks that may fail at speed, and a slightly bent blade can still unbalance the engine.

Will hitting a rock break a lawn mower?

It can. Rocks commonly chip or bend the blade, crack the spark plug, or shear the flywheel key. Always inspect the blade and plug after an impact, even if the damage is not obvious.

How do I know if the flywheel key is sheared?

A sheared key often causes the flywheel to spin without the engine firing, or the flywheel to be out of timing. Remove the flywheel to inspect the key, and follow the service manual for safe removal.

Is it safe to start the mower after hitting something?

Do not start it until you have inspected the blade, blade bolt, and spark plug wire. Starting with a damaged blade can throw debris or cause severe injury.

Why does my mower start and then die after hitting something?

This usually points to a fuel or air leak caused by a cracked line, a loose carburetor, or a damaged air filter housing. Check those components first.

Should I take my mower to a repair shop?

If you find a bent crankshaft, a damaged coil or flywheel magnet, or you are not comfortable removing the flywheel, a qualified small-engine technician is the safer choice. Consult a professional before doing internal engine work.

How much does a blade replacement take?

Replacing a blade is usually a quick job with basic hand tools, often taking about 15 to 30 minutes for a first-time owner, depending on how stuck the bolt is.

What if the mower still won’t start after these checks?

Go back through the fuel system and ignition, then check the compression. If the engine still will not run, a professional diagnosis can save you time and prevent further damage.

Conclusion

When a lawn mower won’t start after hitting something, begin with safety and the blade, then move to the spark plug, flywheel key, throttle linkage, fuel system, and starter. Most impact damage shows up in one of these places, and checking them in order keeps the work manageable. If you find internal engine damage or are unsure about any step, a qualified small-engine technician can help you get the mower running again safely.

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Graham Fisher

About Graham Fisher

Graham Fisher is part of our editorial team and reviews and edits our lawn mowers guides, checking product details against manufacturer specifications and owner feedback.