How to Troubleshoot Your No Start Lawn Mower

To troubleshoot a lawn mower that won't start, follow a logical process by focusing on the three essential elements an engine requires to function: fuel, spark, and air. Start with the simplest and most apparent factors, and systematically eliminate variables and possible causes related to each element that might be contributing to the starting problem.

Typically, multiple issues are contributing to a no-start condition, and several of the essential components required for a healthy engine to operate are being impacted.

This shows how the three ingredients are interrelated; rarely is a no start caused with an ingredient totally in isolation

First, it's important to rule out a no-start issue caused by incorrectly set operator controls. Check this guide: How to Start Your Lawn Mower-For the Beginner: A Step-By-Step Guide. If you've previously used your lawn mower, you can likely skip that post and move on to the troubleshooting guide provided next. You can also consult our no-start fixer walkthrough guide to get your mower running if the operator's manual isn't available, by visiting: https://app.jotform.com/251301165938253

We suggest, if feasible, to first drain all fluids (gas and oil) from the lawn mower. Follow the guidance provided in the post How to Properly Tilt Your Lawn Mower On Its Side: You're Probably Doing It Wrong! for detailed instructions on how to properly tilt your mower before doing so.

If it is not feasible to do this, and you need to get mowing sooner rather than later we suggest that we begin with the obvious of each element described above:

Fuel: Is the tank filled with gasoline? Do you know how old the gas is? If possible, check inside the tank to see if the gas looks clean and clear, or if there are any visible debris like rust flakes or dirt.

The significance of gas quality cannot be emphasized enough! As detailed in this post Why Won't My Push Mower Start? Solution: Mind your Gas!

ethanol in gas absorbs water, leading to phase separation, which results in a water layer separating from the gasoline. This water can severely damage the carburetor and engine. If the gas appears questionable and is older than a month, it is advisable to empty and clean the gas tank initially.

To accomplish this, you can either remove the tank from the unit or tilt it to drain the old gas. Removing the tank is preferable for a more thorough cleaning. Gas tanks are typically plastic or metal containers connected to the carburetor via a fuel line. Sometimes, the carburetor is positioned directly on top of the gas tank. Identify any fasteners or clips securing the gas tank. Use suitable pliers to remove pinch clips, twist any stubborn fuel lines, and, if needed, use a small pick to detach the fuel line from any fittings.

After removing the fuel tank, it is important to pour the old gasoline into a suitable container designed for gasoline storage, as it is highly flammable. Check for any possible blockages that might prevent fuel from flowing from the tank, through the fuel line, and into the carburetor. If the gasoline is old and there is debris in the tank, there is a high likelihood that the fuel line may be blocked or partially blocked. If you removed the fuel tank with the fuel line still attached, clean both the exterior and interior of the tank using a clean rag. Dispose of gas-soaked rags in proper containers since they are highly combustible. Once cleaned, add a small amount of fresh gasoline, shake the tank to circulate it, and then dispose of this gasoline used for cleaning.

This is an ideal moment to inspect the fuel line. Does it look cracked? Occasionally, the internal diameter of the gas line may be collapsed, obstructing the full passage of gas. To check this, observe the gas flow through the line. Is it flowing freely, and is the flow size approximately equal to the inner diameter of the fuel line? If yes, then there are no obstructions.

Pro Tip: The fuel screen inside the fuel tank (if present) can also cause gas obstruction. Make sure this screen or filter is clean. It is located at the bottom of the fuel tank.

Congratulations! Having cleaned your gas tank and ensured there are no blockages in the fuel line to the carburetor, you're on track to preventing future starting issues, as long as the gas you add is relatively fresh and used within two weeks.

The next step is to clean your machine's carburetor. Notably, about two-thirds of service calls to small engine repair shops involve fuel-delivery problems, such as dirty carburetors. To begin, follow the general guidelines in this post: Learn How to Clean Your Carburetor and Get Your Mower Running Again! If the gas was found to be old and debris was present in the gas tank, it's likely the carburetor will also need cleaning to function properly. An important tip from the linked post is to take a photo of the carburetor's linkages and assembly, which greatly simplifies reassembly.

With your gasoline tank now cleaned and free of obstructions, allowing gas to flow to the carburetor, and the carburetor itself cleaned and reassembled, it is probable that you've addressed the main cause of the starting issue. However, before adding gas and attempting to start it, let's consider the next essential component for a well-functioning lawn mower: air.

Air: If you have cleaned the gas tank and cleaned the carburetor, it is required that you remove the air filter housing and air filter, before doing so. With these removed lets observe the housing and the filter itself. If there is debris in the housing and on the filter, all this proves is that the unit was being used and the filter was doing its job in preventing debris from entering the engine. Lets determine if the filter needs cleaning and or it needs replacing. Refer to this post How's Your Engine Been Breathing Lately? Air Filters and Engine Maintenance Tips for Lawnmowers to Live By! to determine whether you can clean yours or it requires replacement. Symptoms of dirty air filters are no starts and fouled spark plugs. Which brings us to our next investigation: spark.

Spark: Many people assume that a non-running lawn mower simply needs a new spark plug, however this is rarely the case. By following the points listed in the article here: Spark Plug Basics: The Following Insights will Surely Spark Your Interest and this post describing how to effectively "read" your plugs: Your Spark Plug is Telling You Something...and it Never Lies: Read Your Spark Plugs Like A Pro you can get to the bottom of any spark related issues.

A tan or grey color on the porcelain part of your spark plug indicates a well-functioning engine. If the plug is dirty but the electrode remains relatively square and undamaged by combustion, it can be cleaned. Use a wire brush to remove carbon buildup from the electrode. Ensure the plug is clean before re-installing it to avoid introducing particles into the engine's cylinder. Manufacturers advise replacing fouled spark plugs with new ones when possible.

It's crucial to understand how a dirty air filter, which restricts the full flow of air into the intake, can lead the carburetor to consume more fuel and consequently foul the spark plug electrode, weakening the spark. To effectively troubleshoot a lawn mower that won't start, both the individual components and their interconnections should be examined.

Regardless of how fresh and clean the fuel is or how spotless the carburetor may be, without a spark, the air-fuel mixture won't ignite for combustion. To quickly check for a spark, ensure the spark plug is removed and its wire is connected. Place it on a solid ground like the cylinder head and pull the start rope as if starting the engine. If you observe a spark jumping the gap, then you have a spark. This rules out the ignition coil, spark plug wire, and kill switch wires as reasons for a lack of spark. However, this doesn't confirm that the spark remains consistent while the machine is running. If your machine starts but then stops after a short period, such as five minutes or slightly longer, the spark might be weakening due to heat or compression. The only way to accurately diagnose these conditions is by using a spark tester.

Attach the spark tester to the spark plug wire and the spark plug, then pull the rope to check for a spark. If there is no spark, bypass the plug and connect the spark tester to the plug wire and a solid ground on the machine, like the cylinder head. If a spark appears, the plug is faulty. If there is still no spark in either scenario, further investigation of the ignition system is necessary.

After resolving the typical problems associated with fuel, air, and spark that can cause a lawn mower not to start, you can proceed to add fresh fuel to the gas tank to operate the machine. Check the carburetor for any leaks. Confirm that the oil level is correct before starting and ensure the spark plug is securely attached (first thread it by hand, then use the appropriate spark plug socket to tighten it, but avoid over-tightening).

Pro Tip: If your lawn mower starts and then stops shortly, as if it's not getting enough fuel, try removing the gas cap. Do you hear a whoosh of air from the tank? If the mower runs again and you take off the cap before it stalls, causing it to suddenly run smoothly, the gas cap vent may need cleaning or the cap might need replacing.

By adopting this structured method for troubleshooting your non-starting lawn mower, you have gained insight into the three essential components required for a lawn mower engine to operate. More importantly, you have understood how a problem in one component can influence or lead to an issue in another. This knowledge will enhance your ability to troubleshoot your lawn mower effectively.

Never the Norm!