9 Fixes For When Your Lawn Mower Won’t Start. Ride on mower engine

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.

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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.

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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.

How to un seize a riding lawnmower engine

One may face many technical problems from time to time if he owns a riding lawnmower. A seized lawnmower engine is, however, one of the rarest of problems. If you do not want to take your Lawnmower to the mechanic because you fear that he might charge more, don’t worry. This blog will help you distinguish the damaged parts, replace them, and test them, all at a home scale.

A lawnmower engine is usually a sturdy component. A seized engine is usually due to the piston and rings sticking in the bore. With the help of some freeing fluid or cleaner, this problem can be solved. However, there might be other contributing factors to this too. Here we take you through them all.

How to un seize a riding lawnmower engine:

  • Step 1: Remove the spark plug
  • Step 2: Clean the combustion chamber with some cleaner
  • Step 3: Open the head and gently tap the piston cylinders
  • Step 4:Check the motor oil
  • Step 5: Test the blades and close all the components

We shall provide in-depth details of the steps mentioned above, like what sort of tools you need and any complexities you might face. Please stick with us as we go through all the potential issues and troubleshoot each of them.

Steps to Un Seize a Riding Lawn Mower Engine:

A riding lawnmower engine may get seized after you de-winterize it due to such distant usage. The seized engine may be due to the parts that are stuck or due to the machine’s absence of motor oil. The procedure below explains all the necessary steps to deal with the situation.

Step 1: Remove the spark plug:

Whenever a defect occurs related to the internal parts of a lawnmower, removing the spark plug is the first step. The spark plug is removed before cleaning the rest of the parts for two purposes. One is so that no accident occurs when one is pulling and cleaning the cord. The second is to clean the spark plug itself.

To do this, tilt the lawnmower body to a side so that its side now faces the roof. With the help of a plier, unscrew the spark plug because it may be hard to remove it by hand (seized up chamber). Now, using a feeler gauge, you may check the gap between the spark plug terminals. Compare it with manual reading. Now, either clean the plug or replace it.

Step 2: Clean the combustion chamber with some cleaner:

Through the removed spark plug hole, you can access the piston cylinders and the combustion chamber. You can quickly get aerosol sprays like “Blaster PB” or “WD-40” from a local hardware store. When you buy them, do get yourself a pair of safety gloves as these might be toxic to spray, hence injurious to health. Now, through the hole, flood in an available amount of the cleaner spray and let it rest there for a couple of hours at the least.

The cleaner spray acts as an anti-rust and lubricating agent. It removes the clog built up in the chamber. After a few hours, let the cleaner fluid drain out from the spark plug hole. Now, try to move the blades manually. Make sure that while rotating them, they should be rotated in their natural directions. You will feel lesser resistance to the movement now.

An additional step can be to pull the cord and see the blade’s rotation. This may further give some idea about the extent of seizing in the engine. Pulling the card will produce little motion in the lawnmower blades, a sign for a seized up engine.

Step 3: Open the head and gently tap the piston cylinders:

Sometimes, only using a cleaner doesn’t solve all of the problems as the riding Lawnmower may have been idle for a longer time. In this case, remove the seat to expose the head of the engine. Opening the head with a screwdriver or wrench will allow you actually to see the piston cylinders. The engine may be blocked and may require an internal push start.

So, hit the piston head preferably with a wooden hammer. Tap it gently as it moves down. This pretty much solves the problem. Besides, check the piston rings, lubricate them. Check for any damaged seals, take no risk, and replace them. Clean the cylinders and remove any dirt or debris that has hardened up and is stuck. Throw in some cleaner spray to moisten the dirt and clogged up the oil. Clean it and let it dry for a few hours.

Step 4: Check the motor oil:

We have already taken you through the basics. These were easy to solve. Now, a more complex issue that may require more of your FOCUS is here. In all types of seized activities, the one that is caused by running the engine on an insufficient motor oil spray is the most serious.

Through the removed piston head, disintegrate the crankshaft. Same as before, using a wooden hammer, give the cylinders a strike to thump them out. Also, rotate the crank. This helps remove the crude oil that got stuck up in the moving parts. Use a cleaner to purify them thoroughly.

Remove the connecting rods and seals. Clean them too, as crude oil may still be stuck there. Give the insides a thorough washing. You can use petrol to wash it too. Now, let the engine bask in the sun and dry up naturally. Add lubrication to each component as you close up the casing and side by side, give the lawnmower blades a gentle thrust to make them move.

Step 5: Close all the components:

After all this day long hustle, most of your technical work is done. However, closing all the components is the essential key for prolonged safety from damage. After all the parts have been lubricated and dried up, conceal them as they were taken out. Ensure the seals are tight, the crankshaft is fixed, and piston cylinders are’ t free. Close the head and fix the seat back in its place. Add new motor oil to your Lawnmower. Remove the old crude fuel. Add new fresh fuel, preferably with some anti-clogging agents mixed in. For a week on, use the mower gently and regularly so that engine gets back to its full potential.

Even if, after all these steps, the blades aren’t softening up as much as they should, it is time to call in an expert.

Related Questions:

1) Why should I use a wooden hammer or wood to strike the piston cylinders?

The combustion chambers are quite sensitive parts in terms of dimensions and sensitivities of pressure, are, and volume. A regular hammer, if used to thump the cylinder, may cause the cylinder to disorientate. At the very least, it can cause a dent in the cylinder or piston due to its rigid nature. Deformities can cause a change in the critical pressure or volume of rotating machines, which can be very dangerous.

A wooden hammer, on the other hand, is rigid and flexible at the same time. You may have observed that all denters use a wooden hammer to remove the dents too. This allows them to give a considerable power strike keeping the flexibility and geometry intact.

2) How can you tell if your lawnmower engine is seized?

To tell if a lawnmower engine is seized, you have to follow through a few steps. Start by removing out the spark plug to expose the channel to the combustion chamber. If a lawnmower with a removed spark plug still jump-starts, its valves may be worn out and hence, need replacement. However, if the engine doesn’t run and the blades are rigid and don’t turn over, it is seized. A seized lawnmower engine has locked cylinders, blades, and pistons.

Mower will not start? How to diagnose and fix EVERYTHING electrical on a riding mower or zero turn.

3) Why removing the spark plug is necessary?

When operating on a riding lawnmower with sharp blades, precaution is a must. While cleaning, one has to clean the combustion chambers, piston cylinders, and valves. Sometimes, even the cord needs maintenance, so it has to be pulled. So, as a safety measure, it is advisable to remove the spark plug before any Lawnmower action to keep away from any harm.

Final Remarks:

For those who love their hobby of gardening, a lawnmower is the best buddy one must-have. However, after long gaps or after de-winterization, its use may pose some problems in engine activity seizing up. A seized engine doesn’t mean the machine’s death, but it sure affects its life if not addressed. Following the steps mentioned above in definite order, you can efficiently deal with this issue. It may seem huge at first glance, but following the guide will help you step by step. Even after all these steps, your Lawnmower fails to work correctly, and it is better to take it to a mechanic. Happy lawnmowing!

Riding Lawn Mower Won’t Start – Mechanics troubleshooting list

Riding mowers aren’t complex, and most problems can easily be fixed by the owner. I’ve covered all the most common faults here in this guide; you’ll be mowing in no time!

So why won’t your riding mower start? The most common reason for a riding mower cranking over but not starting is bad gas, but other possible reasons include:

Important – If your mower is not cranking over, you need to check out the repair guide – “Lawn Tractor Won’t Start No Click”.

What’s Cranking?

This is the turning of the engine by battery and starter power as you turn the key. It can be seen at the top of most mower engines as the flywheel spins around while attempting to start the engine.

The riding mowers used here for demo purposes may be different from yours, but no matter, the testing will be identical or very similar. At various points along the way, you will be directed to a solution for your problem, he said confidently.

I know this guide is long but don’t be put off; most of it will not be relevant to you.

Check The Basics

Doing some basic checks on your lawn tractor will sometimes solve the problem or at least point you in the right direction. The basics include oil level check; fuel level check; fuel tap on; air filter check; plug wire on; choke applied, and following the correct starting procedure.

Oil Level

Some tractor-mowers just won’t start if the oil level is low; it’s designed that way. It protects your engine from that Oooo moment.

It’s good practice to check the oil level every time you fill the gas tank. Check out “Does my mower need oil?”.


Is there gas in the mower? Sometimes the obvious is the solution. Was the gas fresh?

At my shop, I have found many strange concoctions – diesel, water, white spirits, vinegar, and of course, last year’s gas makes a regular appearance. Hey, we’ve all done it! Check out “Carburetor troubleshooting”.

Gas Tap

Some lawn tractors will have a gas valve, is it turned on? Gas valves stop the flow of gas to the carburetor and, if fitted, are usually turned off when the mowers are not in use. Check out “Lawn mower gas tap”.

Battery Charged

A strong healthy battery is critical to starting any electric key start lawnmower. A typical mower engine will only create enough energy for the spark plug to fire if the engine cranks over fast enough, around 350rpm min.

If your engine sounds like it isn’t cranking over at the usual speed, try jump-starting. This will rule out a weak battery. Check out “Tractor mower jump starting”.

Air Filter

A blocked air filter will prevent the mower from starting. The air filter needs to be kept clean; check it every 25 hours of use. Most mower filters are easy to access. Try starting your engine without the filter, but replace it as soon as possible if you find it’s the root cause.

Plug Wire

It’s easy for the plug wire to come loose, happens all the time. The wire usually lives right at the front of the engine, it’s a push-on fit, and as mowers vibrate a lot, the metal plug cap connection widens and becomes loose.


Most mowers will have a manual choke, and more modern mowers may be auto-choke. If you have a manual choke, you’ll need to set it to full to start a cold engine. Check out “How to start a lawn mower”.

Lock Out

All tractor-mowers will have lock-out or safety sensors fitted. As you know, they will stop the engine from cranking over or starting unless a set procedure is followed.

If you need one-to-one help, check out the JustAnswer Mower Mechanic link below, where you’ll chat directly with a mower mechanic within minutes. I earn a small commission for recommending JustAnswer mechanics, and I’m happy to do so; these guys are good, I know they’ll save you money.

Mower Cranks, But Won’t Start

Riding mower turns over but won’t start is the most common complaint I hear, and I usually hear it in the spring. Riding lawn mower engines are quite simple; they need three things to start:

Thing 1. Gas/Air MixThing 2. SparkThing 3. Compression

What is Gas/Air Mix?

Fuel is always at the top of the checklist. The reason fuel causes problems is that some fuels are ethanol-blended, fuels like e10. While small engine manufacturers say their engines run on these fuels, they would recommend you use regular gas, and the reason for that – is their engines were not designed to run on ethanol.

What’s Ethanol?

Ethanol is an alcohol fuel that is made from plants such as corn and sugar; this alcohol is then mixed with regular gas. Ethanol e10, which you see at the gas station, is 10% ethanol and 90% gas. E15, as you’ve guessed it, is 15% alcohol and 85% gas. E15 should never be used in small engines.

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Ethanol burns hotter, and it’s thought the higher alcohol content is harmful to the rubber and plastic components of the fuel system. Using e15 fuel will invalidate your small engine manufacturer warranty. Ethanol fuels will turn stale after about one month; regular fuel will turn stale after three months.

The Problem With Ethanol

It attracts moisture; for cars, this isn’t an issue because the gas tanks are sealed, meaning the moisture can’t get in. Lawnmower fuel tanks are not sealed; they need to breathe. Moisture makes its way into the fuel tank, albeit in very small amounts.

This isn’t an issue when the mower is being used on a regular basis. The problem usually arises in the spring because, over the winter months, the alcohol in the carburetor evaporates and leaves moisture behind; this then corrodes and gums up the carburetor – Now it’s a no-start.

Gas Stabilizer

Fuel stabilizers were designed for gas-powered equipment that may sit for long periods between uses. As said earlier, regular gas will go stale in about three months. Ethanol fuels will go stale in one month; using a fuel stabilizer will keep these fuels fresh for up to two years.

If you choose not to use a fuel stabilizer – simply drain the fuel tank and run the mower until the engine stops. This should eliminate corrosion, gumming, and varnish build-up associated with stale fuel, but I prefer to use a stabilizer.

Check out Sta-bil fuel stabilizer on the “Small engine repair tools page”; it’s easy to use and will save you money in the long run.

Air Filter

Clean air is as important as fuel, however, is much less problematic. Filtering the air before it enters the engine is important as it prevents grit from damaging the carburetor and engine components, it also settles the air.

Most engine manufacturers make it easy for owners to quickly check the air filter. Filter covers are usually held on by plastic clips or simple wing nuts.

The filter should be checked and cleaned every 25 hours and, more often in dusty environments, replaced every 100 hours. Common filter types are pleated paper elements, fiber elements, and foam. Some air filters will have a pre-filter, usually foam, wrapped around the main filter; its function is to catch larger debris.

Paper and fiber elements can be cleaned using compressed air or banging them on the groundwork reasonably well. If the paper filters are oil or fuel soaked, they will need to be replaced as this blocks airflow.

Foam elements can be washed in soapy water and refitted when dry. Check out “Lawn tractor maintenance.”

What Is Spark?

When we talk about the spark, we’re talking about the whole ignition system. The spark is more than just the plug; the whole system comprises of, depending on how old the mower is: Battery; Starter; ​​Spark plug; Coil and Plug wire; Flywheel; Points; Ignition switch; Control module.


As you know, your battery must be in great shape; if it isn’t strong enough to turn the engine over fast enough, the flywheel and coil can’t make a good spark. As said earlier, you can rule out this as the issue by jump-starting the mower from your car, truck, or any 12-volt battery.

If you check out Battery Testing you’ll also learn how to test the battery; for this, however, you will need a voltmeter, but you can get one on this page “Small engine repair tools”.

Of course, the engine may be cranking over slowly for other reasons; if the weather is very cold, it causes battery performance to suffer. Using oil that’s too thick or overfilling can cause a slow crank speed.

A binding starter motor or, worse case, internal engine damage can cause a slow crank speed.

Spark Plug

If you have no spark, many times, it’s a failed spark plug. Having a spare plug that you can change out is useful for testing and minimizing downtime.

Removing the plug and checking its condition will usually tell you what’s going on inside the engine.

  • Wet Plug tells you it’s getting fuel, maybe too much (Flooding)
  • Black Oily plug could be a mechanical fault or simply too much oil in the engine
  • Dry Plug could be a choke fault or blocked fuel system

In many cases, you can simply clean the plug, and you’re away mowing.

Using the correct plug is important; spark plugs have different thread lengths and have a particular heat range. Plugs are designed to run hot enough to burn off contaminants but not so hot that it pre-ignite (firing when they shouldn’t).

Fitting the wrong thread length and heat range can damage the engine. A quick check online with your engine manufacturer will give you the correct plug code.


The coil is where the voltage is created; coils will produce thousands of volts. They have a tough job, and they work hard in a hot location, right above the cylinder – it’s no wonder they are the next most common ignition component to fail.

Testing and replacing the coil is a simple job. Flywheels are a basic component and don’t give many issues. Points are fitted to much older mowers, they’re a serviceable item, but I won’t cover it in this guide.

Control Module

These are fitted to most modern mowers; they process the safety sensors signals and start/stop commands. In some cases, they are incorporated into the dash light panel. Most are basic printed circuits with resistors and relays.

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Modules do fail and are vulnerable to moisture. Your mower may not have a control module; if so, the ignition switch does all the work. The advantage of not having a control module means the system is easier to fault find.

Ignition Switch

These are pretty basic and can be tested with a digital voltmeter. The number of pins at the rear of the switch will vary depending on the model. Ignition switches do fail, and terminals tend to corrode. We can test inputs and outputs and visually inspect for damage.

What Is Compression?

In simple terms, compression is the engine’s ability to build pressure in the cylinder without it leaking. As an engine gets older, the compression value reduces as compression begins to slip past worn sealing rings and valves.

To test this in the workshop, I used a compression tester. Modern engines have a compression release mechanism, which makes them easier to pull start. And so, on more modern engines, I use a leak-down test. This test pumps air into the engine and measures its ability to hold the pressure over a given amount of time.

My father was a mechanic too, and when he was serving his apprenticeship, car engines could only cover about eight thousand miles before needing some pretty heavy-duty maintenance. When I served my apprenticeship, car engines could easily do over 100 thousand before needing the same type of semi-major repair.

Some of the latest engines from Briggs and Stratton won’t ever need their oil changed; advancements in the design of engines and materials used are such that major mechanical failure is uncommon. Of course, misuse or lack of maintenance will cause failure.

Measuring Compression

Measuring compression accurately without the proper kit is impossible. However, there is an unscientific DIY test. This crude test will tell you if you have some compression, not an actual value.

If you prefer the correct tool for an accurate value, check out “Small engine tools” and look at the leak-down test kit.

With the plug removed and wearing protective gloves, put your thumb over the plug hole as a helper turns over the engine slowly. If you have compression, air will rush out past your thumb. If you’re looking for something a little more sophisticated, you’ll need a compression tester kit.

If your engine is lacking compression, suspect a sticking open valve. To fix a sticking valve – use a screwdriver to gently lever the spring into the released position. A sticking valve is a common complaint on engines that lay up for long periods.

Check out “Valve lash adjustment” it’s for a walk-behind mower, but the setup is identical.

The Gas Shot Test

To quickly find the problem, we need to narrow down the search area. Most nonstarting mowers are caused by fueling faults, and that’s why we start with the gas shot test. This test is an elimination round if you like. This test bypasses the fueling system, simultaneously testing the fuel system and the ignition system.

To run this test remove the air filter; you’ll need fresh gas. If the gas isn’t fresh, this test won’t work. Fill a bottle cap full of gas and drop it into the carburetor. Attempt to start your mower as normal with a full choke.

Gas shot – Begin with clean, fresh fuel. Remove the air filter cover and air filter (some will be fixed on with screws or wing nuts, others will just pull off)

Pour shot fresh gas into the carburetor, about a cap full. Now for some carburetor setups, this won’t be possible. And so, instead of removing the spark plug and using a funnel, pour gas straight into the cylinder, then refit the plug.

Turn-Key – Now attempt to start the mower in the normal way.


If the mower attempted to start or started – You have a fueling fault; move on to the choke system check below.If your mower made no attempt to start – You have eliminated a fueling fault, and your fault will likely be a lack of spark; you need to move on to the spark system check below.

Lawn Mower Will Not Start?.This is Probably Why! ‘Simple Fixes’

The Choke System Test

OK, so you have identified a fueling system fault. In this next step, we need to be sure the choke system is working and being used correctly. Most riding mower owners already know this, but in my experience, lots of customers have never been shown how to start their riding mower correctly. And since cars don’t have a choke anymore, understandably, many owners aren’t familiar with a choke, what it is, and when they need to use it.

This post covers how and when to use coke – “How to start a tractor mower.”


The function of a choke is to enrich the fuel mixture so a cold engine starts smoothly. The choke does this by restricting the amount of air entering the carburetor.

Gas engines run best when the ratio of air to fuel is 14.7 to 1. Meaning 14.7 parts air to 1 part fuel, also known as an air-fuel ratio (AFR). Using the choke counteracts the lean condition caused by the dense cold air.

Using Choke

Before you start your mower, move the choke lever to full choke, this is generally only needed on a cold engine. However, some engines may require a small amount of choke to start the engine, even when hot.

When the engine warms up, turn off the choke and move to full throttle. Check out “How to start a tractor mower.”

Test Choke Operation

Choke plates are usually operated by cable and will require adjustment from time to time. In this part of the guide, you will check if the choke plate is working correctly.

A choke that isn’t closing all the way will cause a no-start, and a choke that’s sticking “On” will cause poor running and black smoke.

If you can see the choke plate clearly when you remove the air filter, then you do not need to remove the engine cover. However, some engines will require the blower cover removed.

View the choke plate and note its position. Not necessary to remove this intake pipe. You can see into the choke plate to check its operation.

Carburetor was removed to show the choke “On” position.

Choke On – This is the correct position for starting a cold engine.

Adjust – If the choke plate is not moving to the closed position, adjust the cable so that it does.

If you found no issues with your choke system, check out “Carburetor troubleshooting.”

If you found no issues with your choke system, then your issue is most likely a dirty or contaminated carburetor. It’s a very common issue, and that’s why I have dedicated a separate page for it. Whichever carb problem you have, we’ll get it fixed here on the “Carburetor troubleshooting” Page.

The Spark System Test

If you have identified a likely spark system fault, then let’s test all the components of the system, beginning with the most common failures – plug; coil; control module (if fitted); ignition switch.

Have a new plug on hand, a helper, insulated pliers, and a plug removal tool. These tests are simple; however, take care to ground the plug against the engine securely, as the bad ground will lead to an incorrect diagnosis.

Alternatively, make life easy for yourself and buy the Ignition spark tester; you’ll find the type I use here on the “Small engine tools page”.

For this test, you will need a new plug, plug spanner, insulated pliers, and a helper.

Note: The best way to test the spark is with a spark tester tool, as it will load up and stress tests the coil.

Remove the spark plug and check the condition. Refit the plug wire and ground the plug on the engine.

Have a helper turn the key while you check for spark. Now try a new spark plug.


No spark or your spark is poor – you could have a faulty: Coil; Spark plug wire; Plug wire cap; Short circuit of coil control wire; or Faulty control module/Ignition switch.

Coil, also known as armature faults, are very common.

Look for obvious signs of damage. Arcing and corrosion of the plug cap, check it fits snugly and securely.

Chafing of wiring against the engine is common.

your, lawn, mower, start, engine

Remove the engine cover if not already removed. Locate the coil control wire connector. Remove the coil control wire; you may need pliers.

Now check for spark as you did earlier.


If you found no fault with the wiring (try wiggling) – Then go ahead and replace the control module. If your mower doesn’t have a control module fitted – then replace the ignition switch.

Fitting A Coil

Coil (also known as Armature) failure is common as these components work hard. Fitting a new one is a simple job; no special tools are needed. Only the engine covers need to be removed. When fitting the coil, an air gap must be maintained between the flywheel and the coil.

A feeler gauge is normally used to measure this gap. However, a business card also works. Fitting the coil is covered in the guide below.

Check out mower coils on the Amazon link below, which covers the most common types.

Remove the engine cover if not already removed.

Remove coil bolts, plug wire, and control wire connector.

Get quality parts; they are tested. Nobody likes revisiting the same job. Although coils all look pretty similar, they are different. Locate your coil part number and reference it when ordering.

An air gap must be maintained between the coil and the flywheel. A business card is just the right thickness. Tighten the bolts while pushing the coil towards the flywheel. Remove business cards by turning the flywheel.

Nice work, now rebuild in reverse order; you nailed it!

The Shear Key

The Shear key is a small piece of metal that’s designed to break under certain conditions. It lives between the flywheel and the crankshaft. It has two jobs, (1) protect the engine from serious damage and (2) Align the flywheel and crankshaft precisely.

They often break after the blades have impacted something solid like a curb, tree stump, etc.

The symptoms of a broken shear key vary – no start, poor running, backfiring, weak spark. The repair of the shear key isn’t covered here, but you can check out “Lawn mower shear key replacement” it’s for a walk-behind mower, but the setup is close to identical.

Related Questions

Why does my lawnmower sputter and then die? This commonly happens when the carburetor is dirty, or the gas is bad. Removing and cleaning the carburetor gas bowl will usually fix the problem, but you’ll need to be sure your gas is fresh. Gas older than one month is likely stale.

Is it normal for a new lawnmower to smoke? A healthy mower engine shouldn’t smoke. Blue/white smoke is a sign that your mower is burning oil; this sometimes happens if it’s overfilled with oil. Black smoke means it’s getting too much gas, try cleaning or replacing the air filter.

Hey, I’m John, and I’m a Red Seal Qualified Service Technician with over twenty-five years experience.

I’ve worked on all types of mechanical equipment, from cars to grass machinery, and this site is where I share fluff-free hacks, tips, and insider know-how.

What Type of Lawn Mower Oil Should I Use


Your lawn mower needs the right kind of engine oil used in the right way. Read on to learn about different lawn mowers and the oil they need.

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Why Lawn Mower Oil Matters

Like all internal combustion engines, lawn mower engines need oil to run. Even simple engines have many moving parts, often designed to work at extremely high speeds and temperatures. This is why the lubricating and cooling action of oil is essential. Without it, your lawn mower’s engine would quickly overheat, seize and be ruined.

Lawn Mower Oil Types

Motor oil comes in different grades, based on viscosity and how the oil behaves at different temperatures. Most mowers have what are called four-stroke engines. This means they burn straight gasoline as it comes from the service station pump, but they also require motor oil to be added separately to the crankcase of the engine. 10W30 is a common motor oil grade suitable for many lawn mowers. Your owner’s manual will tell you the exact grade required, but in almost all cases 10W30 is the right stuff for a four-stroke engines.

Any brand of oil that’s suitable for cars or trucks will work fine in your mower. All reputable oil includes a service rating in addition to a viscosity rating. Look for oil that’s designated SF, SG, SH, SJ or higher.

  • Single Grade Oil: A single grade level oil typically without additives to change its viscosity and represents only at higher temperatures (100°C).
  • Multi Grade Oil: A multi grade level oil that uses additives to provide better viscosity at a range of temperatures.
  • Synthetic Blend Oil: A mixture of regular and synthetic oil with additives to help perform at colder temperatures without the cost of a full synthetic oil.
  • Full Synthetic Oil: An artificially created lubricant with a wide range of benefits designed for use in high performance and commercial engines

Some lawn mowers have two-stroke engines, and these require oil in a different way than four-stroke engines. All two-stroke engines burn gasoline and oil at the same time. In the case of lawn mowers, two-stroke engine oil is mixed with the gasoline before it goes into the tank. Mixing ratios of gas to oil vary, but usually range from 30:1 (4-1/4-oz. of oil to one gal. of gas) to 50:1 (2-1/2-oz. of oil to one gal. of gas). The owner’s manual for your lawn mower lists the mixing ratio of gas to oil.

Two-strokes are becoming less common because of emissions regulations, but they’re still around. How do you know if you’ve got a two-stroke or four-stroke engine in your lawn mower? Your owner’s manual is the best source of guidance.

How to Choose the Right Lawn Mower Oil

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Some experts say that more expensive “small engine oil” is the only type of oil you should put in your mower with a four-stroke engine, but that’s not true. Standard engine oil made for cars and trucks is the highest quality available today and it works optimally with all four-stroke engines. Got a two-stroke engine? Any two-stroke motor oil made for air-cooled engines, such those in chainsaws, water pumps and weed eaters, will work perfectly in your two-stroke lawn mower engine.

  • SAE 30 Oil: Engine oil best suited for warmer temperatures. Try top rated Pennzoil SAE 30 Motor Oil.
  • SAE 5w-30 Synthetic Oil: Synthetic mower oil good for warm and cold weather use. Try top rated Castrol Edge 5W-30 Full Synthetic Motor Oil.
  • SAE 10w-30 Synthetic Oil: Synthetic oil that can help in colder temperatures. Try top rated Mobil 1 Advanced Full Synthetic 10W-30 Motor Oil.
  • SAE 15w-50 Synthetic Oil: Synthetic oil typically used for high end and commercial engines. Try top rated Mobil 1 Advanced Full Synthetic 15W-50 Motor Oil.

The best way to mix gas and oil for a two-stroke engine is to put the required amount of oil into your empty gas can, then go to the gas station and fill it up. Before using the mixed gas, give the can a shake to so the oil and gas are properly mixed.

What is Synthetic Oil and Should I Use It In My Lawn Mower?

Synthetic oil is superior to lubricants made from crude oil, and your lawn mower engine may last longer if you use synthetic. Essentially, it is a synthetic lubricant made up of chemical compounds designed to give engines the performance and protection that natural oil may not be able to provide. According to Briggs and Stratton, one of the world’s largest manufacturers of small engines, the use of synthetic oil does not alter required oil change intervals. Regular, non-synthetic oil works well, too. I’ve used non-synthetic in some of my small engines for 30 years, and these motors still start and run as if they were new.

How Often to Check and Change Lawn Mower Oil

Only lawn mowers with four-stroke engines have oil that can be checked and changed. Tuning up a lawn mower at least once a season, which includes changing the oil, is essential for maximizing fuel economy and extending the life of the engine. Aside from that:

If your four-stroke engine lawn mower is new, change the oil after the first three to five hours of use. As parts of a new engine wear initially, the internal movement of parts releases tiny metal filings into the oil that will cause excess wear if left there.

  • Walk-Behind Mowers: Change oil in mower at least once a season or every 50 hours of use.
  • Riding Mowers: Change oil in mower at least once a season or every 100 hours of use.

The owner’s manual for your lawn mower lists the amount of oil required, but you’ll do fine following the dip stick or oil level mark that’s part of every four-stroke lawn mower engine.

How to Check Lawn Mower Oil

Before each mowing session, you should check your lawn mower’s oil level and top it off if necessary. To do so:

  • Place your lawn mower on a level surface and let it sit idle for a few minutes so that the engine oil can settle.
  • Remove the oil cap and wipe the dipstick off with a clean cloth. Put it back into the oil tank and tighten the cap.
  • Once again, remove the cap and check the oil level on the dipstick. The level should fall between the “full” and “add” marks. There may be differences in the appearance of these marks depending on the brand of mower you own. Some dipsticks may have only two holes to indicate “full” and “add”, or a cross-hatched pattern. Either way, you want the oil level to be between the two holes or marks. As close to the “full” side as possible without exceeding it.
  • Whenever more oil is needed, add it in small increments and repeat this process between each addition to prevent overfilling the engine.

How to Change Lawn Mower Oil

When looking to change the oil in a lawn mower, follow these steps to check off this simple and easy maintenance check.

How Much Oil Does a Mower Take?

Depending on the make and model of lawn mower, push mowers have an oil capacity ranging between 13-1/2-and 22-ounces and riding mowers between 48-and 64-ounces. A mower’s operator’s manual will always list the proper amount of oil recommended for its engine.

What Does SAE Stand For in Oil?

SAE is the acronym for the Society of Automotive Engineers. They are an organization that sets global standards in a variety of fields related to transportation and aerospace. It is the responsibility of the SAE to ensure that automotive oil is standardized throughout the world.

Can You Use Car Oil in a Lawn Mower?

Yes. As previously stated, engine oil made for cars and trucks is the highest quality oil on the market and it works optimally with nearly all four-stroke engines.

Steve Maxwell is an award-winning content creator who has published more than 5,000 articles, shot countless photos and produced video since 1988. Using his experience as a carpenter, builder, stone mason and cabinetmaker, he has created content for Mother Earth News, Reader’s Digest, Family Handyman, Cottage Life, Canadian Contractor, Canadian Home Workshop, and many more. Steve lives on Manitoulin Island, Canada with his wife and children in a stone house he built himself. His website gets 180,000 views each month, his YouTube channel has 58,000 subscribers and his weekly newsletter is received by 31,000 subscribers each Saturday morning.