Lawn mower weak spark. 9 Fixes For When Your Lawn Mower Won’t Start

How To Tell If A Lawn Mower Spark Plug Is Bad

It can ruin your day when you try to mow your lawn, but your mower is difficult or won’t start at all. But there is frequently a simple solution if you know the cause of the issue. Bad spark plugs are a likely reason your engine is having trouble starting.

A center electrode and a side-firing electrode are the two electrodes on a spark plug. The air-fuel mixture in the cylinder is ignited by a spark created to flow through the tiny gap between the two electrodes.

Spark plugs are required for a gasoline-riding lawnmower, chainsaws, or other gasoline-powered devices or vehicles to ignite fuel in the combustion chamber to function properly.

Our guide will FOCUS on a spark plug on a lawn mower. By the end, you’ll know all the reasons why your spark plugs are bad and whether you need to change spark plugs, or it just needs cleaning, or you need a spark plug gauge to test the spark plug gap. (Learn How Long Does Ethanol Free Gas Last)

What A Spark Plug Does

A spark plug is an essential component of a gasoline-powered engine despite its diminutive size. The spark plug in a lawnmower’s engine does the same job as in a car engine.

A spark plug is an electrical device that creates a spark as the magneto or the ignition coil delivers a high voltage to the central electrode.

The spark ignites the fuel and air mixture inside the cylinder head. There will be no ignition or improper ignition if the spark plug cannot perform its job properly, which can cause a variety of issues. The worst-case scenario is that your engine might not even turn on.

How to Tell If a Lawnmower Spark Plug is Bad

You know the major symptoms now; let’s get into their details so that judging becomes more comfortable for you.

Engine won’t start easily:

This is the most common symptom of a bad spark plug. A damaged spark plug prevents a bad engine start. It will take several pulls on the starter rope when using a push mower before the engine turns on. Similarly, you won’t be able to start a lawn tractor’s engine with just one turn of the key.

Knowing what a spark plug does helps to make the explanation of this symptom fairly simple. The spark produced by a malfunctioning spark plug does not ignite the air-fuel mixture from the carburetor via the fuel tank.

The mixture floods the engine as it enters the cylinder, and only a small portion ignites.

Poor Performance:

Even if the engine starts with a bad spark plug, it can take many attempts; it won’t be able to continue running for long. The engine will shut off or stop mowing because the spark plug is not functioning correctly, and you’ll find the engine sputtering before it dies.

The difficulty of starting the engine increases after each attempt as the engine warms up. This is because heat expands metals, which widens the space between the two electrodes and reduces the intensity of the ignition spark.

The problem can be worse on a riding mower as you go about lawn care, you may be stranded, and a lawn tractor can’t be pushed. Another reason you can have a lawn mower bad spark plug isn’t the gap is incorrect; it is the carbon buildup on the electrodes.

You get this from unburned oil and fuel pumped into the combustion chamber or seeps past the piston rings, and the electrodes can’t create sparks. (Read Spark Plug Socket Size Guide)

High Fuel Consumption:

A bad spark plug could cause your lawnmower to use more fuel than usual to complete the same job. A bad spark plug causes inefficient ignition and improper fuel combustion. This makes the mower less fuel-efficient, raising the fuel used.

The odor of your mower is another indicator that it is not burning and consuming fuel efficiently. When the mower runs, raw gasoline is produced because a bad spark plug prevents the fuel from being burned properly.

How To Diagnose a Spark Plug Problem?

You won’t have to wait for symptoms to appear because a bad spark plug will give you a sufficient warning. Periodically inspecting the spark plug will keep you informed about the condition of the spark plug in your mower.

You can assess the condition of your spark plug based on several factors. You can fit a new spark plug or clean your spark plug and adjust the gap from there.

Understanding Spark Plug Problems:

Know the engine problems a bad spark plug can cause. These problems are explained in the manual or online.

• Check the spark plug if the engine won’t start.
• Check the spark plug to see if the engine cuts out.
• Check the spark plug if you keep adding fuel to lawn mowers gas tank.

Spark Plug Gap:

The spark plug gap is between the electrodes. The spark plug gap is crucial. Deviations from the specified width can cause problems. As the engine heats, the gap widens. Cold engine contraction reduces the gap.

Over time, electrodes develop residual stresses and strain, distorting the gap, and an incorrect gap weakens ignition. Spark plug gaps can change during shipping, and handling, if the plug falls or the anode thins, so before installing a spark plug, check the gap. You often need feeler gauges to do this, although a quick way is using the cardboard from a box of matches.

The gap should be the same thickness as this, so you shouldn’t need your user manual unless you are using gauges and need to know the proper gap.

Cleaning your Spark Plug:

If the spark plug is badly corroded or has thick burnt deposits, you must replace it. If a spark plug is wet from gas or oil, it can be cleaned using a wire brush. The central electrode should be flat on top. If its top is damaged, replace the spark plug. If you see cracks in the spark plugs porcelain sheath, this needs to be changed immediately.

Only a wet gasoline or oil plug with no visible damage can be cleaned. Oil-soaked plugs mean your engine is leaking oil, and most often, this is from the engine cylinder head and pistons.

Replacing a Spark Plug:

The lawnmower spark plug is under the black spark plug wire. Remove the wire and plug to inspect. You need the right-sized socket and a ratcheting socket driver to remove the plug.

Follow these steps to replace a spark plug in your lawn mower’s engine:

• Get a Spark Plug: Ensure the replacement plug is the same size and has the same specs.
• Disconnect the Spark Plug: Remove the lead and clean around the spark plug wire.
• Remove the Spark Plug lead: Take the faulty spark plug using your wrench and spark plug socket, and check the electrode for signs of damage. Too wet could signal a blocked air filter.
• Put the new Spark Plug in: Put the new plugin by first turning it in by hand to ensure that you don’t cross-thread. When it can’t be turned by hand, use the wrench to tighten the new plug, and don’t over-tighten.
• Reconnect the Spark Plug wires: Connect the lawnmower’s spark plug lead.

Depending on your mower manufacturer, they may sell tune-up kits for end-of-season storage.

Briggs Stratton offers a small engine kit that includes:

Where does a spark plug get power from?

A magneto or ignition coil’s high voltage output is connected to the spark plug. As the electrons exit the ignition coil, a voltage forms between the center and side electrodes.

Since the fuel and air in the gap are insulators, no current can flow there. However, as the voltage increases, the structure of the gases between the electrodes starts to change.

Gases become ionized when the voltage rises above their dielectric limit. The ionized gas transforms into a conductor, enabling the flow of electrons across the gap. Small Engine spark plugs typically need a voltage of between 5 and 15,000 volts. (Read Lawn Mower Won’t Stay Running)

Other Signs That You Have A Bad Lawn Mower Spark Plug

The job of the lawn mower spark plug was briefly mentioned, and knowing this will help you understand why you have some of the common symptoms of a faulty spark plug.

Our lawnmower may have appeared to be having engine issues, which could have cost you hundreds of dollars or caused purchasing a new mower, but it may only result from a cracked or defective spark plug.

By the way, the information on recognizing a bad spark plug also applies to examining the spark plug condition in any gas-powered weed eater or brush cutter.

Listen to the spark plug socket.

As we previously mentioned, once the lawnmower engine runs, it may shut off suddenly and seemingly for no reason. It might be simpler to comprehend what is happening now that we know a spark plug’s job.

The spark produced by the spark plug frequently does not ignite the fuel that mixes with the air in the carburetor, which is the major cause of engine failure. The fuel not being burned off properly can frequently cause the engine to flood as more fuel is added to the cylinder.

Lengthy periods of engine operation cause the spark plug to heat, which can widen the gap between the firing electrodes and the spark plug center, weakening the ignition spark even more.

If you’ve been trying to start the lawnmower for a while, but it keeps cutting out, this may be the reason you need to let it cool down.

Excessive Gas Consumption

Excessive fuel consumption is another red flag that indicates a bad spark plug in your lawn mower. The need to fill the engine with gas more frequently than usual may have become apparent. Faulty spark plugs are frequently to blame for this. (Read Too Much Oil In Lawn Mower)

Your fuel consumption will rise, and your fuel efficiency will fall if the gasoline is not burned completely or efficiently. The air may smell like fuel, frequently like raw gasoline, as the gasoline isn’t being burned off as it should be. To clean your engine, use a soft wire brush.

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There are a number of reasons, mechanical and otherwise, why a mower won’t run. The good news is that fixing most all of the issues is easy enough for a DIYer to handle.

By Tony Carrick and Manasa Reddigari | Updated Aug 8, 2022 4:03 PM

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Lawn care can be tedious, but once the grass starts growing in the spring, mowing becomes a fact of life in most neighborhoods. When you finally muster the strength to tackle that first cut of the season, there are few sounds as disheartening as that of a lawn mower engine that turns over but doesn’t start.

Before you drag the mower in for repairs or invest in costly replacement parts, first make sure that a clogged air filter, soiled spark plug, damaged safety cable, clogged mowing deck, or contaminated gas isn’t to blame. Work through the following steps, and you may be able to get your puttering grass guzzler up and running again in no time.

A lawn mower repair professional can help. Get free, no-commitment repair estimates from pros near you.

Change the lawn mower carburetor filter.

Your lawn mower’s air filter guards the carburetor and engine from debris like grass clippings and dirt. When the air filter becomes clogged or too dirty, it can prevent the engine from starting. To keep this from happening, replace paper filters—or clean or replace foam filters—after every 25 hours of engine use.

The process for removing the filter depends on whether you are operating a riding or walk-behind lawn mower. For a riding mower, turn off the engine and engage the parking brake; for a walk-behind mower, pull the spark plug wire from the plug. Then, lift the filter from its housing.

The only choice for paper filters is replacement. If you’re cleaning a foam filter, wash it in a solution of hot water and detergent to loosen grime. Allow it to dry completely, and then wipe fresh motor oil over the filter, replace it in its housing, and power up the mower—this time to the pleasant whirring of an engine in tip-top condition.

Check the spark plug.

Is your lawn mower still being stubborn? The culprit may be the spark plug, which is responsible for creating the spark that ignites the fuel in the engine. If it’s loosened, disconnected, or coated in water or carbon residue, the spark plug may be the cause of your machine’s malfunction.

Locate the spark plug, often found on the front of the mower, and disconnect the spark plug wire, revealing the plug beneath. Use a socket wrench to unscrew the spark plug and remove it.

Check the electrode and insulator. If you see buildup, spray brake cleaner onto the plug, and let it soak for several minutes before wiping it with a clean cloth. Reinstall the spark plug, first by hand, and then with a socket wrench for a final tightening. If the problem persists, consider changing the spark plug.

Clear the mower deck of debris.

The mower’s deck prevents grass clippings from showering into the air like confetti, but it also creates a place for them to collect. Grass clippings can clog the mower deck, especially while mowing a wet lawn, preventing the blade from turning.

If the starter rope seems stuck or is difficult to pull, then it’s probably due to a clogged deck. With the mower safely turned off, tip it over onto its side and examine the underbelly. If there are large clumps of cut grass caught between the blade and deck, use a trowel to scrape these clippings free. When the deck is clean again, set the mower back on its feet and start it up.

Clear the vent in the lawn mower fuel cap.

The mower started just fine, you’ve made the first few passes, then all of a sudden the mower quits. You pull the cord a few times, but the engine just sputters and dies. What’s happening? It could have something to do with the fuel cap. Most mowers have a vented fuel cap. This vent is intended to release pressure, allowing fuel to flow from the tank to the carburetor. Without the vent, the gas fumes inside the tank begin to build up, creating a vacuum that eventually becomes so strong that it stops the flow of fuel.

To find out if this is the problem, remove the gas cap to break the vacuum, then reattach it. The mower should start right up. But if the lawn mower won’t stay running and cuts off again after 10 minutes or so, you’ll need to get a new gas cap.

Clean and refill the lawn mower fuel tank.

An obvious—and often overlooked—reason your mower may not be starting is that the tank is empty or contains gas that is either old or contaminated with excess moisture and dirt. If your gas is more than a month old, use an oil siphon pump to drain it from the tank.

(It’s important to be careful as spilled oil can cause smoking, but there are other reasons this might happen. Read more about what to do when your lawn mower is smoking.)

Add fuel stabilizer to the tank.

Fill the tank with fresh fuel and a fuel stabilizer to extend the life of the gas and prevent future buildup. A clogged fuel filter is another possible reason for a lawn mower not to start. When the filter is clogged, the engine can’t access the gas that makes the system go. If your mower has a fuel filter (not all do), check to make sure it’s functioning properly.

First, remove the fuel line at the carburetor. Gas should flow out. If it doesn’t, confirm that the fuel shutoff valve isn’t accidentally closed. Then remove the fuel line that’s ahead of the fuel filter inlet. If gas runs out freely, there’s a problem with the fuel filter. Consult your owner’s manual for instructions on replacing the filter and reassembling the mower.

Inspect the safety release mechanism cable.

Your lawn mower’s reluctance to start may have nothing to do with the engine at all but rather with one of the mower’s safety features: the dead man’s control. This colorfully named safety bar must be held in place by the operator for the engine to start or run. When the bar is released, the engine stops. While this mechanism cuts down on the likelihood of horrific lawn mower accidents, it also can be the reason the mower won’t start.

The safety bar of a dead man’s control is attached to a metal cable that connects to the engine’s ignition coil, which is responsible for sending current to the spark plug. If your lawn mower’s engine won’t start, check to see if that cable is damaged or broken. If it is, you’ll need to replace it before the mower will start.

Fortunately, replacing a broken control cable is an easy job. You may, however, have to wait a few days to get the part. Jot down the serial number of your lawn mower, then head to the manufacturer’s website to order a new cable.

Check to see if the flywheel brake is fully engaged.

The flywheel helps to make the engine work smoothly through inertia. When it isn’t working properly, it will prevent the mower’s engine from working.

If it is fully engaged, it can make a mower’s pull cord hard to pull. Check the brake pad to see if it makes full contact with the flywheel and that there isn’t anything jamming the blade so the control lever can move freely.

If the flywheel brake’s key sheared, the mower may have run over something that got tangled in the blade. It is possible to replace a flywheel key, but it does require taking apart the mower.

Look out for signs that the mower needs professional repairs.

While repairing lawn mowers can be a DIY job, there are times when it can be best to ask a professional to help repair a lawn mower. If you’ve done all of the proper mower maintenance that is recommended by the manufacturer, and gone through all of the possible ways to fix the mower from the steps above, then it may be best to call a pro. Here are a few signs that indicate when a pro’s help is a good idea.

• You see black smoke. The engine will benefit from a technician’s evaluation, as it could be cracked or something else might be worn out.
• Excessive oil or gas usage. If you’ve changed the spark plugs, and done all of the other maintenance tasks, and the mower is consuming more than its usual amount of oil or gas, consult a professional for an evaluation.
• The lawn mower is making a knocking sound. When a lawn mower starts making a knocking sound, something could be bent or out of alignment. It may be tough to figure this out on your own, so a pro could help.
• A vibrating or shaking lawn mower can be a sign of a problem beyond a DIY fix. Usually something is loose or not aligning properly.

Briggs Stratton lawnmower type small engine Won’t Start or Run — No Spark Problem — Check. Fix. Repair

Briggs Stratton small engines can be found powering many pedestrian-operated walk-behind vehicles and portable machines; your lawnmower most likely uses one of their engines. Other popular applications include; portable generators, pumps, soil tillers, and cement mixers, all of which may have a small petrol / gasoline engine to drive the machine, made by Briggs Stratton. Of course, Briggs Stratton are not the only folk making small engines, other manufacturers include Honda, and Kawasaki, for example.

Every spring and summer for the last 25 years I’ve used my little lawnmower powered by a Briggs Stratton engine — it never misses a beat and always starts first time. and with that reliability benefit in mind, I really do wish I’d purchased a Briggs Stratton-engined chainsaw! Am I a Briggs Stratton fan? Yes Sir, I am! But today, my FOCUS is on my (normally!) super-easy-to-start 10 year-old Briggs Stratton 4-stroke petrol-powered portable generator which generates household AC current because that’s the machine that I need fixing without delay. I need that portable generator up and running to use power tools in places where extension cables are not practical. For example, I put the genny in the trailer of my tractor and I can use lights and electrical power tools that I need to fix or build something elsewhere. It’s well handy, hence the name, Handy Gen 2500a. The label on the top says it’s powered by a Briggs Stratton Quantum LSQ 55 Silent System.

But what do I do if it won’t start or start easily?

Other than actually generating a spark at the spark plug, the two other things that I need to get my small engine running again are;

But this article is about troubleshooting why there is no spark at the spark-plug and the assumption is that the two things above (carburation ignition timing) are correct and ok as-is, which they are.

My troubleshooting guide checklist for a small engine no spark scenario such as a, it’s turning over, but not starting issue:

Watch the YouTube video about me fitting a brand-new ignition module to my Briggs Stratton DIY Briggs Stratton lawnmower type small engine Won’t Start/Run—No Spark Problem—Check|Fix|Repair

You can see in the above photo that my old armature / ignition coil ( part number 802574) is marked CYL SIDE meaning cylinder side, so any new replacement ignition coil must be fitted the right way up.

Finding the correct ignition coil and other engine parts for your particular Briggs Stratton engine; an Engine number guide:

The photo below is of my engine cowling on the muffler / exhaust side of the engine and shows a row of letters and numbers.

In the picture above, the characters 12X812 0119 B1 04050356 represent my particular model, engine, and code.

To find this information on your Briggs Stratton small engine, use their on-line website guide here Locate Your Model Number

To find a supplier for the spare part that you need, all you do is search on-line using the Model Number and the Engine Number like this for example 12X812 0119 B1 and you should find web sites on-line selling the bits you need.

On many of those web sites you’ll discover an exploded-view parts diagram shown interactively or as a.pdf where you can visually search for the part and get its official Briggs Stratton part number. With a Briggs Stratton part number it is possible to search for a genuine OEM (original equipment manufacturer) part or replacement spare part.

To find out more about choosing and gapping the correct spark plugs for your Briggs Stratton small engine, here is their on-line page about spark plugs and spark plug gaps. https://www.briggsandstratton.com/eu/en_gb/support/faqs/browse/identifying-the-correct-spark-plug-and-gap.html

Near the end of the video I mentioned the repairs kit I bought which comprised of an ignition coil, a replacement air filter and a spark plug. I have read that this spark plug is apparently compatible with my particular Briggs Stratton engine but I’ve found no official direct conversion charts to convert the number into comparative NGK or Champion plugs for example. Apparently, its a Chinese brand called Torch.

Finally, here’s a page about Briggs Stratton petrol generators if you’re looking to buy a generator soon Briggs Stratton Portable Petrol Generators

I’m no expert on fixing engines. All what I’ve learned is self-taught through necessity and experimentation since my teenage years using old cars and motorbikes as my daily drivers. So, please don’t rate my DIY repairs as a definitive example of how it should all be done. it is not a professional tutorial !

My son is working with me in the video to fix the motor and I teach and involve him in just about everything I do and pass-on know-how about many, many things which hopefully he’ll put to good use after I’ve popped my clogs. It will help him save some money, and should help him avoid any needless stress and anxiety whilst navigating life’s many twists, turns, and cul-de-sacs!

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DISCLAIMER: How To Bloke is NOT a professional or qualified engine mechanic or an automotive engineer. Use of any information found in the video above or in the text of this article or linked to it or from it is at your own risk.

SAFETY NOTICE: If you choose to work on any engine, always remember, Safety First!

This post “DIY Briggs Stratton lawnmower type small engine Won’t Start or Run — No Spark Problem — Check|Fix|Repair” appeared first at HowToBloke.com on 22nd January 2022.

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How To Tell If a Lawn Mower Spark Plug Is Bad

A spark plug provides source of ignition for small engines, like the engine that powers your lawn mower.

It consists of a threaded shank to install it into an engine block, a center electrode which is encased in a porcelain sheath, and a firing electrode, which is a metal tab that makes an arch above a center electrode. [1]

How can you tell when it’s time to change a lawnmower’s spark plug?

Spark plugs should be checked and replaced annually [2], or sooner if you discover that your lawn mower won’t start.

Here are a few tips that will show you how you can check for a damaged spark plug:

If your lawn mower spark plug didn’t pass the test, it’s time to replace it.

I recommend using a small-engine tune up kit that contains spark plugs, air filter, oil filter, and fluid that’s engine specific (fuel stabilizer, lawnmower oil, etc.).

Here are the next steps:

Test your mower and you should be good to go!

If the engine is still having issues, you may want to take it to a dealer or local small engine repair shop for further diagnosis.

My mower won’t start

Starting problems with your lawn mower fall into 2 basic causes: fuel problems and ignition problems. The following is a simple checklist you can follow to help isolate a possible cause. This is not an exhaustive guide and certainly not a repair manual. However, there a few steps you can take before taking it to the repair shop for a thorough fix.

Fuel

This is the first and most obvious thing to check, especially if you weren’t having startup problems last year. Be sure that the machine has an ample supply of fresh fuel. It’s amazing how often people will get their mower out of cold storage the first warm day in spring and find their mower just won’t start. Forgetting that they left the fuel in the tank from the previous year. Today’s fuels don’t age well. In fact today’s gasoline is much like milk and starts to go bad quickly, often in as short a period as 60 days.

Ignition

Make sure that the ignition switch is on, that all attachments are disengaged and that the transmission is in neutral. Most machines have safety mechanisms that will not allow them to start otherwise. Consult your owner’s manual for the starting procedure for your machine.

Make sure the spark plug wire is firmly attached to the spark plug.

If it still doesn’t start, check for a spark by removing the spark plug and grounding the hex part of the spark plug to a bare metal part of the engine. Remove any spilled gasoline that is nearby first, then spin the engine by pulling the rope or turning the key. You should see a blue spark jump across the plug gap. If you have a good hot spark, skip to the carburetor section.

lf the spark is yellow and weak, or there is no spark then the first thing to do is try a new spark plug. Don’t try cleaning the old one. Set the new plug gap to the proper spec (usually 0.030) and check for spark again. If you do see a spark, install the new plug and try to start the machine.

If there’s still no spark, try cleaning and setting the points on older machines, or replacing the electronic ignition module on newer machines. Before replacing the old module test it by disconnecting all the wires from the module terminal that connect to the kill switches and recheck for spark, if you do see a spark then a wire or switch is probably defective. Also check the flywheel key. That affects the spark timing, usually the key will need replacing if you strike something while mowing and the engine stops.

Checking the carburetor and fuel system

If the fuel is fresh, you have a good spark and still won’t start, you must make sure that fresh gasoline is getting to the carburetor. Make sure that the fuel shutoff valve (if present) is opened, and that the fuel line is not plugged or kinked.

Also be sure the fuel cap vent is open and any screens in the tank are clear of debris. If fuel can flow to the carburetor, carefully place 1 teaspoon of gasoline down the spark plug hole (or a add shot of starting fluid into the carburetor throat). Re-tighten the plug and try to start the machine. If it runs for a second or two, then quits, chances are the carburetor needs to be serviced (disassembled and cleaned inspected, then rebuilt) or you have an air leak somewhere.

Make sure all mounting screws or bolts are snug and gaskets are not missing sections. If the carburetor is a bowl-type check for water in the bottom of the bowl (but do not let the float swing all the way down or the float needle will fall out!).

Hopefully, this will give you a little insight in what to look for if your mower won’t start. Most starting problems are fuel related because the previous year’s fuel wasn’t removed. You may have gone years without removing the fuel at the end of the year without having any starting problems. There is a chemical process called varnishing that puts a coating on the inside workings of your fuel system. Over time this process continually worsens until your system becomes inoperable.

Precautions

Gasoline can ignite very easily. Work only in well-ventilated areas and away from sources of heat, sparks, and flames. Always wear safety glasses, and be careful of sharp blades. Keep a fire extinguisher nearby.