A lawn mower that cranks but refuses to start is a frustrating problem for any homeowner. You turn the key, or pull the cord, and the engine whirs and spins, but it just won’t catch and roar to life. What could be wrong? The good news is that this is a very common issue, and with a little bit of troubleshooting, you can often diagnose and fix the problem yourself, saving time and money. This comprehensive guide will walk you through the most likely causes, step-by-step, so you can get your mower running again.
Fuel System Issues: The Most Common Culprit
Often, the reason a lawn mower cranks but doesn’t start lies within the fuel system. From old gas to a clogged carburetor, several factors can prevent fuel from reaching the engine and causing it to stall.
Old or Stale Fuel
Gasoline degrades over time, especially when exposed to air. As it ages, it can form gums and varnishes that clog small passages in the fuel system. This is why using fresh fuel is crucial. If your mower has been sitting idle for a few months, particularly over the winter, old fuel is the prime suspect.
The solution is simple: drain the old fuel and replace it with fresh gasoline. You can use a fuel stabilizer to help prevent fuel from going stale in the future. Fuel stabilizers work by slowing down the oxidation process and preventing the formation of gum and varnish. Add fuel stabilizer to your gas can whenever you purchase gasoline, especially if you don’t use it frequently.
Clogged Fuel Filter
The fuel filter’s job is to prevent debris from entering the carburetor. Over time, this filter can become clogged with dirt, rust, and other contaminants. A clogged fuel filter restricts the flow of fuel to the engine, preventing it from starting.
To check the fuel filter, locate it along the fuel line, usually between the fuel tank and the carburetor. If it’s a clear filter, you can visually inspect it for debris. If it’s opaque, you’ll need to disconnect it and check for flow. If the filter is clogged, replace it with a new one. Fuel filters are inexpensive and easy to replace.
Blocked Fuel Line
Similar to a clogged fuel filter, a blocked fuel line can also restrict fuel flow. Debris, kinks, or cracks in the fuel line can all cause problems.
Inspect the fuel line for any visible damage or obstructions. Disconnect the fuel line at both ends and blow compressed air through it to clear any blockages. If the fuel line is cracked or damaged, replace it immediately.
Carburetor Problems
The carburetor mixes air and fuel in the correct ratio for combustion. It contains very small passages that are easily clogged by stale fuel or debris. A clogged carburetor is a very common reason why a lawn mower cranks but won’t start.
Symptoms of a clogged carburetor include difficulty starting, rough idling, and stalling. Cleaning or rebuilding the carburetor is often necessary to resolve these issues.
Cleaning the Carburetor
You can try cleaning the carburetor yourself using carburetor cleaner. Remove the air filter and spray the cleaner directly into the carburetor intake while attempting to start the engine. This might dislodge some of the debris. However, for a thorough cleaning, you may need to remove the carburetor and disassemble it.
Carefully disassemble the carburetor, taking note of how all the parts fit together. Soak the parts in carburetor cleaner and use small wires or carburetor cleaning tools to clear any blocked passages. Reassemble the carburetor and reinstall it on the engine. If you are not comfortable disassembling the carburetor, you may want to take it to a professional.
Replacing the Carburetor
If cleaning doesn’t solve the problem, or if the carburetor is severely damaged, you may need to replace it. Carburetors are relatively inexpensive, and replacing one can be a straightforward process. Make sure to purchase the correct carburetor for your specific lawn mower model.
Ignition System Issues: Spark is Essential
Even with fuel, your lawn mower won’t start without a spark to ignite the air-fuel mixture. Problems with the ignition system can prevent the spark plug from firing.
Faulty Spark Plug
The spark plug provides the spark that ignites the air-fuel mixture. Over time, spark plugs can become fouled with carbon deposits, cracked, or otherwise damaged. A bad spark plug will prevent the engine from starting.
Remove the spark plug and inspect it. Look for signs of damage, such as cracks or chips in the porcelain insulator. Also, check the electrode for excessive wear or carbon buildup.
Testing the Spark Plug
You can test the spark plug by grounding it against the engine block and pulling the starter cord. If the spark plug is working, you should see a bright blue spark jump across the electrode gap. If there is no spark, or if the spark is weak and yellow, the spark plug needs to be replaced.
Replacing the Spark Plug
Spark plugs are relatively inexpensive and easy to replace. Make sure to use the correct spark plug for your lawn mower model. When installing the new spark plug, tighten it to the manufacturer’s specified torque.
Defective Ignition Coil
The ignition coil provides the high voltage needed to create a spark at the spark plug. If the ignition coil is defective, it won’t produce enough voltage to fire the spark plug.
Testing the ignition coil requires a multimeter and some knowledge of electrical testing procedures. If you’re not comfortable performing electrical tests, it’s best to take your lawn mower to a qualified technician.
However, a simple check you can do is visually inspect the ignition coil for any signs of damage, such as cracks or burns. If you suspect the ignition coil is defective, replace it with a new one.
Kill Switch Problems
Most lawn mowers have a kill switch that prevents the engine from starting if the operator handle is not engaged. A faulty kill switch can prevent the engine from starting, even if everything else is working properly.
Check the kill switch to make sure it is functioning correctly. Make sure the operator handle is fully engaged when you are trying to start the mower. If the kill switch is faulty, it may need to be replaced.
Compression Issues: A Less Frequent Cause
While less common than fuel or ignition problems, low compression can also prevent a lawn mower from starting. Compression is the squeezing of the air-fuel mixture inside the cylinder, which is necessary for proper combustion.
Worn Piston Rings
Piston rings seal the gap between the piston and the cylinder wall. Over time, these rings can wear down, allowing combustion gases to leak past the piston. This reduces compression and makes it difficult for the engine to start.
Checking compression requires a compression tester. Insert the tester into the spark plug hole and pull the starter cord several times. The gauge will indicate the compression pressure. Consult your lawn mower’s service manual to determine the correct compression pressure for your model. Low compression indicates worn piston rings or other internal engine damage.
Repairing worn piston rings typically requires disassembling the engine, which is a complex and time-consuming job. In some cases, it may be more cost-effective to replace the entire engine.
Leaking Valves
Valves control the flow of air and exhaust gases into and out of the cylinder. If the valves are not sealing properly, compression will be lost.
Valve problems can be caused by carbon buildup, worn valve seats, or damaged valves. Checking for leaking valves requires specialized tools and knowledge. It’s best to take your lawn mower to a qualified technician if you suspect valve problems.
Other Potential Issues
While fuel, ignition, and compression are the most common culprits, other less frequent problems can also prevent a lawn mower from starting.
Sheared Flywheel Key
The flywheel key is a small key that connects the flywheel to the crankshaft. If the engine experiences a sudden stop, such as hitting a rock, the flywheel key can shear. This throws off the engine timing and prevents it from starting.
To check the flywheel key, you’ll need to remove the flywheel cover. Once the cover is removed, inspect the flywheel key for any signs of damage. A sheared flywheel key will need to be replaced.
Low Oil Level
Some lawn mowers have a low oil sensor that prevents the engine from starting if the oil level is too low. This is to protect the engine from damage.
Check the oil level and add oil if necessary. Make sure to use the correct type of oil for your lawn mower.
Clogged Air Filter
A clogged air filter restricts airflow to the engine, which can make it difficult to start.
Remove the air filter and inspect it. If it’s dirty, clean it or replace it with a new one.
A Step-by-Step Troubleshooting Guide
Here’s a step-by-step guide to help you troubleshoot why your lawn mower cranks but won’t start:
- Check the Fuel: Drain old fuel and replace it with fresh gasoline. Add fuel stabilizer.
- Inspect the Fuel Filter: Replace if clogged.
- Examine the Fuel Line: Look for damage or blockages.
- Clean the Carburetor: Try spraying carburetor cleaner or disassemble and clean thoroughly.
- Check the Spark Plug: Inspect for damage and test for spark. Replace if necessary.
- Inspect the Ignition Coil: Look for signs of damage. Replace if necessary.
- Verify the Kill Switch: Ensure it’s functioning correctly.
- Check Compression: Test compression pressure.
- Inspect the Flywheel Key: Look for signs of shearing.
- Check Oil Level: Add oil if necessary.
- Inspect the Air Filter: Clean or replace if clogged.
By systematically working through these steps, you should be able to identify the cause of the problem and get your lawn mower running again. Remember to consult your lawn mower’s service manual for specific instructions and torque specifications. Safety is paramount; always disconnect the spark plug wire before working on the fuel system or ignition system. If you are uncomfortable performing any of these repairs, take your lawn mower to a qualified technician. With patience and the right tools, you can often fix a lawn mower that cranks but won’t start and enjoy a well-manicured lawn once again.
Why is my lawn mower cranking but not starting?
If your lawn mower is cranking, it means the starter motor is working and attempting to turn the engine over. However, the engine isn’t firing up because it’s missing one or more of the essential ingredients for combustion: fuel, air, or spark. A lack of fuel could be due to a blocked fuel line, a faulty fuel pump (if equipped), or stale gasoline. A lack of spark could be caused by a bad spark plug, a faulty ignition coil, or a disconnected kill switch wire.
Another common reason is a compression issue, though this is less frequent. Compression problems can be caused by worn piston rings or valves, but these problems usually manifest over time. A good place to start is to check the spark plug, ensure the fuel is fresh, and verify there are no obstructions in the air filter. By systematically eliminating these potential causes, you can narrow down the problem and get your mower running again.
How do I check if my lawn mower is getting fuel?
The simplest way to check for fuel delivery is to start by inspecting the fuel tank. Ensure there’s enough fuel and that it isn’t stale or contaminated. Stale fuel can break down and leave deposits that clog fuel lines and carburetors. Drain the old fuel and replace it with fresh gasoline, preferably with a fuel stabilizer added.
Next, disconnect the fuel line where it enters the carburetor. Carefully direct the disconnected fuel line into a suitable container and check if fuel flows freely when the fuel valve is open. If no fuel or a weak stream appears, the fuel line, fuel filter (if equipped), or the fuel tank itself may be blocked. Clean or replace these components as necessary. If fuel flows freely, the problem may lie within the carburetor itself, such as a blocked jet.
What if my lawn mower has a clean air filter but still won’t start?
While a clogged air filter is a common culprit for starting problems, a clean air filter doesn’t guarantee a properly running engine. Even with a clean filter, the air mixture could still be incorrect. This might happen if the carburetor is incorrectly adjusted or if there is an air leak somewhere in the intake system.
Consider checking the carburetor for any signs of damage or wear, especially around the throttle and choke linkages. Look for loose screws or gaskets that might be allowing unmetered air into the engine. Additionally, check the intake manifold (if applicable) for cracks or loose connections. If you suspect an air leak, try spraying a small amount of starting fluid around the suspected areas while cranking the engine. If the engine briefly sputters or starts, that indicates an air leak in that area.
How can I test the spark plug on my lawn mower?
To test the spark plug, first disconnect the spark plug wire. Remove the spark plug using a spark plug wrench. Visually inspect the spark plug for any signs of damage, such as cracks in the porcelain insulator or excessive carbon buildup on the electrode. If the spark plug looks fouled, clean it with a wire brush or replace it.
Next, reattach the spark plug wire to the spark plug. Hold the spark plug against a metal part of the engine, ensuring good ground contact. Have someone crank the engine, or if the mower has an electric starter, crank it yourself while observing the spark plug gap. You should see a strong, consistent blue spark jumping across the gap. If the spark is weak, intermittent, or nonexistent, the spark plug is likely faulty and needs to be replaced. If a new spark plug also fails to produce a good spark, the problem could be with the ignition coil or the wiring.
What do I do if the carburetor is suspected to be the problem?
If you suspect the carburetor is the issue, start by checking for any external signs of damage or leaks. Look for fuel dripping from the carburetor or any obvious cracks. Next, try tapping the carburetor gently with a screwdriver handle; sometimes, this can dislodge small particles blocking the jets.
If these steps don’t work, consider cleaning the carburetor. Carefully remove the carburetor from the engine, taking note of the linkage and fuel line connections. Disassemble the carburetor and clean each part thoroughly with carburetor cleaner, paying close attention to the jets and passages. Use compressed air to blow out any remaining debris. Reassemble the carburetor and reinstall it on the engine, ensuring all connections are secure. It’s also crucial to make sure the choke is operating properly, as a stuck or improperly adjusted choke can cause starting problems.
What if my lawn mower has a kill switch problem?
A faulty kill switch can prevent your lawn mower from starting, even if it’s cranking, because it interrupts the ignition circuit. The kill switch is designed to ground the ignition system when activated, effectively stopping the engine. However, a damaged or corroded kill switch can inadvertently ground the ignition even when it’s in the “run” position.
To troubleshoot a kill switch problem, locate the kill switch wiring, typically found near the handlebar or engine housing. Disconnect the kill switch wire from the ignition coil. Then, try starting the lawn mower. If the mower starts with the kill switch disconnected, the kill switch itself is likely the problem and should be replaced. If the mower still doesn’t start, the problem lies elsewhere in the ignition system.
Could old gas be the reason my lawn mower cranks but won’t start?
Absolutely. Gasoline degrades over time, especially when exposed to air and temperature fluctuations. As gasoline ages, it loses its volatile components, making it harder for the engine to ignite the fuel-air mixture. This can lead to a situation where the engine cranks but fails to start because the fuel is simply too stale to burn properly.
Furthermore, old gasoline can leave behind varnish and deposits that clog fuel lines, carburetors, and fuel filters. These clogs restrict fuel flow and prevent the engine from receiving the fuel it needs to start. Therefore, the first step in troubleshooting a cranking-but-not-starting lawn mower should always be to drain the old fuel and replace it with fresh gasoline, ideally with a fuel stabilizer added to prevent future degradation.