How do I troubleshoot & repair common small engine problems?

Having engine problems? Use this comprehensive guide when troubleshooting issues with your lawn mower, snow blower or other outdoor power products - whether your product won’t start, is stalling, surging or running rough.

Lawn Mower / Equipment Engine Won’t Start
Small Engine Runs Poorly
Maintenance & Repair Schedule

WARNING: Always read the engine and equipment manual(s) before starting, operating, or servicing your engine or equipment to avoid personal injury or property damage. See an authorized dealer or contact Briggs & Stratton if you are unsure of any procedure or have additional questions. Find all Engine Safety Warnings

Lawn Mower / Equipment Engine Won’t Start

Engine starting problems have a few common causes, especially if you’re using your lawn mower or outdoor power product after it’s been stored for the winter or off-season.

Safety Tip: ALWAYS remove / disconnect the spark plug before performing engine repair.

Step 1: Check Your Fuel

Troubleshoot Small Engine Problems

Do you have fresh, clean fuel in the tank? If it’s empty, fill it up and go! If it’s full, check that the fuel shut-off valve is open and that it is clean.

Stale fuel, dirt and debris are the most common cause of outdoor power equipment not starting properly. If you store equipment with untreated gas in the tank, it can lead to engine damage.

Prevent this problem in advance through proper lawn mower & outdoor power equipment storage. Using fuel stabilizer will keep your gas from creating small engine problems.

Step 2: Clean the Carburetor

Troubleshoot Small Engine Problems

Carburetor Problems Cause Mower Engines Not To StartThe carburetor regulates how air and fuel move through the engine to power your equipment. If it is dirty, that can cause poor engine performance or starting problems.

>> Follow these steps for cleaning carburetors

>> Follow these steps for carburetor rebuild

Step 3: Check the Spark Plug & Possible Ignition Problems

Troubleshoot Small Engine Problems

Disconnected, dirty or fouled spark plugs are common causes for engines that won’t start. Spark plugs typically need to be replaced every season or 25 hours of use. You should also check that the spark plug gap is set properly.

>> Follow this guide on checking and replacing spark plugs

If your spark plugs look good, problems with your ignition system can also preventing a spark. These can range from a faulty spark plug lead, shorted kill switch or flywheel key damage.

To test and fix, check out our Small Engine Ignition Systems FAQ or reach out to your local Briggs & Stratton dealer for advanced repair help.

Step 4: Check the Valves & Compression System

Troubleshoot Small Engine Problems

Air-fuel compression is crucial to engines powering lawn mowers and outdoor power equipment. The compressions system is made up of valves, piston, cylinders and rings that control how the air and fuel vapors move through the engine. Valves let air in (the intake valve) and out (the exhaust valve), pistons move back and forth pushing the air-fuel mixture to the ignition system and the piston rings keep it all sealed up tight.

If there isn’t proper valve clearance or there’s a leak, this can cause compression problems that keep an engine from starting. You can perform a compression system with a leak down tester. If you don’t have one of these, we recommend visiting your Briggs & Stratton repair dealer to perform the test and check that these components are in good shape.

Small Engine Runs Poorly

Use this simple Q&A format list to troubleshoot small engine problems you may experience with your lawn mower, snow blower or other power equipment.

Engine Stalling, Surging or Running Rough

When your engine starts and stalls, it is often due to standard maintenance problems. Prevent stalling problems before they start with a tune-up kit. See “Why My Engine Won’t Start” section above or use the Q&A guide below.

Is your fuel fresh?
Stale, untreated gas begins to break down after about a month. Drain the gas from your lawn mower our outdoor power equipment, then replace with fresh gas and fuel stabilizer to extend the life of your fuel.

Is the air filter dirty / plugged?
Clean or replace the air filter.

Is the spark plug fouled?
Clean spark plugs where applicable (Follow instructions for checking and changing spark plugs).

Is your oil fresh?
Check your oil level and appearance every time you use your engine while it is cold. Your oil needs to be changed every 25 hours of use or season. (See Checking and Changing Mower Oil)

Engines Smoking

Troubleshoot Small Engine Problems

Is the engine emitting white / blue smoke?
This means your engine burning oil. (see Engine emits white/blue smoke while running).

Is the engine emitting black smoke?
You need to perform a Carburetor Adjustment.

Is the air filter dirty / plugged?
Clean or replace the air filter.

Engine Overheating

Troubleshoot Small Engine Problems

Is the engine dirty?
Follow instructions for safely cleaning & removing debris from small engines.

Is the oil level low?
Add oil to the engine. NOTE: Never add oil to the gasoline for a four-stroke engine (see Checking & Changing Oil).

Are any shrouds or cooling fins missing or broken?
Install new parts as needed. Reference your repair manual, available for purchase.

Is the fuel mixture too lean?
You can determine if your air-fuel mixture is off by checking the state of your carburetor. Use this Carburetor Adjustment FAQ as a guide.

Is there a leaky gasket?
This is another common carburetor problem associated with overheating engines. Learn how to check and replace the gasket (see Lawn Mower Carburetor Rebuild FAQ).

Is the fuel tank vent or fuel tank screen plugged?
Clean the fuel tank vent and screen.

Engine Knocking

Troubleshoot Small Engine Problems

Does the combustion chamber contain excess carbon?
Carbon deposits can form on your combustion chamber. If you see any, simply clean carbon from the piston and head (see Removing Engine Carbon Deposits).

Is the flywheel loose?
Inspect the flywheel and key; replace as needed (see Inspecting the Flywheel & Key).

Is the spark plug lead faulty?
Test the lead with a spark tester, then test the engine (see Servicing Spark Plugs).

Spark Plug Fails When In Use

Troubleshoot Small Engine Problems

Is the spark plug fouled?
Clean spark plugs where applicable (Follow instructions for checking & changing spark plugs).

Is the spark plug faulty or gap incorrect?
Replace the spark plug or adjust the spark plug gap (see Spark Plug Gap FAQ).

Are the breaker points faulty?
Check the ignition systems’ breaker points. If they are faulty, install a solid-state ignition (see Troubleshooting Ignition Problems).

Are your carburetor settings correct?
Check your small engine’s carburetor settings. Find the proper carburetor settings and step-by-step directions for Carburetor Adjustment.

Is the valve spring weak?

Troubleshoot Small Engine Problems

Check that the valve spring still has tension.

To access this, you’ll have to remove the muffler, crankcase and other components blocking the valve chamber. Then, remove the cylinder head bolts (label for easy re-installation)
Adjust the jaws of the valve spring compressor until they touch the top and bottom of the valve chamber
Push the tool in to compress the spring and tighten the jaws
Remove the retainers and lift out the valves , compressors and springs
If you find weak springs, replace them based on your small engine’s specifications. These can be found in your operator’s manual.

Is the valve clearance set incorrectly?
If your valve spring is still working well, check the clearance. Valves that stick or have leaks can cause an engine to stop running. Adjust the valve clearance to recommended settings (see your operator’s manual).

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