Tractor Mower

Riding Lawn Mower Won't Start

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:

 

  • Gas Valve Off
  • No Gas
  • Choke Fault
  • Plug Wet / Faulty
  • Plug Wire Off
  • Bad Battery
  • Air filter Blocked
  • Gummed / Faulty Carburettor
  • Coil Failure
  • Faulty Control Module
  • Flywheel Timing Off (Shear-key)
  • Engine Compression / Valve Fault

Important - If your mower is not cranking over, you need to check out 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 to 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 form that Oooo moment. It's good practice to check the oil level every time you fill the gas tank. Check out "Tractor oil level check".

 

Gas

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 years gas makes a regular appearance. Hey, we've all done it! Check out "Carburettor troubleshooting".

 

Gas Tap

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

 

Battery Charged

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

 

Plug Wire 

Its 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. 

 

Choke

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. Check out "Tractor starting procedure".

That's all the easy stuff checked.

 

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 really, they need three things to start:

 

Thing 1. Gas/Air Mix

Thing 2. Spark

Thing 3. Compression

 

 

Small Engine Repair Tools

This is a link to a list of helpful repair tools you may need.

Repair Tools

What is Gas/Air Mix?

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

 

What's Ethanol

Ethanol is an alcohol fuel which 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, is you've guessed it, 15% alcohol and 85% gas. E15 should never be used in small engines. 

 

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 manufactures 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. Lawn mower fuel tanks are not sealed, they need to breath. 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 carburettor evaporates and leaves the moisture behind, this then corrodes and gums up the carburettor - Now it's a no start.

 

Gas Stabilizer

Fuel stabilisers 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 stabiliser will keep these fuels fresh for up to two years.

 

If you choose not to use a fuel stabiliser - 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 stabiliser.

 

Check out Sta-bil fuel stabiliser on Amazon, 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 carburettor and engine components, it also settles the air.

 

Most engine manufactures 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, replace every 100 hours. Common filter types are pleated paper element, fibre 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 fibre elements can be cleaned using compressed air, or banging them on the ground works reasonable well. If the paper filters are oil or fuel soaked, they will need to be replaced as this blocks air flow. Foam elements can be washed in soapy water and refitted when dry. Check out "Lawn tractor maintenance".

 

What Is Spark?

When we talk about 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.

 

Battery

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 I've 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 volt meter.

 

Of course the engine may be cranking over slowly for other reasons, if the weathers very cold, it causes battery performance to suffer. Using oil that's too thick or over filling 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 to minimise down time. Removing the plug and checking its condition will usually tell you whats going on inside the engine.

 

Wet Plug tells you its 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 your 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-ignites (firing when it shouldn't).

 

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

 

Briggs and Stratton specification

Honda specification

Kohler specification

Kawasaki specification

 

 

Coil

The coil is where the spark is created, coils will produce thousands of volts. They have a though 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 don'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 a basic printed circuit with resisters and relays.

 

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 to 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 volt meter. 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, outputs and visually inspect for damage. 

 

What Is Compression?

In simple terms, compression is the engines 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 use 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 it's 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 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. 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 your 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 set up is identical.

 

 

The Gas Shot Test

To quickly find the problem we need to narrow down the search area. Most non starting mowers are caused by fuelling faults, and that's why we start with the gas shot test. 

 

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 in to the carburettor. Attempt to start your mower as normal with full choke.

 

Mower gas can

Gas

Begin with clean fresh fuel.

Remove

Remove air filter cover and air filter, some will be fixed on with screws or wing nuts, others will just pull off.


Mower air filter
Mower Engine

Spray Shot

I spray starter fluid into the carburettor, but you can use a cap full of gas just the same.

Pour Shot

Fresh gas into the carburettor, about a cap full.


Mower engine repair
mower key

Turn Key

Now attempt to start the mower in the normal way.

Result?

If the mower attempted to start or started - You have a fuelling fault, move on to Choke System Check.

If your mower made no attempt to start - You have eliminated a fuelling fault and your fault will likely be a lack of spark, you need to move on to Spark System Check.


Mower Choke Button

Gas


Mower Gas Can

Begin with clean fresh fuel.

Remove


riding mower air filter

Remove air filter cover and air filter, some will be fixed on with screws or wing nuts, others will just pull off.

Spray Shot


Mower engine repair

I spray starter fluid into the carburettor, but you can use a cap full of gas just the same.

Pour Shot


Mower engine Cover

Fresh gas into the carburettor, about a cap full.

Attempt Start


Mower Key

Attempt to start your mower in the normal way.

Result?


riding mower cables

If the mower attempted to start or started - You have a fuelling fault, move on to Choke System Check.

If your mower made no attempt to start - You have eliminated a fuelling fault and your fault will likely be a lack of spark, you need to move on to Spark System Check.

The Choke System Test

OK, so you have identified a fuelling 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. Check out "How to start a tractor mower".

 

Function

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 carburettor. 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 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.

 

Remove


riding mower engine cover

If you can see the choke plate clearly when you remove the air filter, then you do not need to remove the engine cover.

Remove


riding mower engine cover

Otherwise, go ahead and remove it. This will give you a better view of the carburettor.

Apply Choke


riding mower choke

Apply full choke.

Carburettor


riding mower carb

Not necessary to remove this intake pipe.

You can see into the choke plate to check its operation.

Choke On


riding mower carb

Here I have removed the pipe so you can clearly see the choke in the full on position.

This is the correct position for starting a cold engine.

Adjust


riding mower choke

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

If you found no issues with your choke system, check out "Carburettor Cleaning".

Mower engine cover

Remove

If you can see the choke plate clearly when you remove the air filter, then you do not need to remove the engine cover.

Remove

Otherwise, go ahead and remove it.

This will give you a better view of the carburettor.


Ride-on mower engine cover
Ride-on mower choke

Pull

Apply full choke.

Carburettor

Not necessary to remove this intake pipe.

You can see into the choke plate to check its operation.


Ride-on mower carb
Ride-on mower carb

Choke On

Here I have removed the pipe so you can clearly see the choke in the full on position.

This is the correct position for starting a cold engine.

Adjust

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

If you found no issues with your choke system, check out "Carburettor Cleaning".


Ride-on mower choke adjust

The Spark System Test

If you have identified a likely spark system fault, then lets 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 a bad ground will lead to an incorrect diagnoses. Alternatively make life easy for your self and buy the Ignition spark tester.

 

Tools


mower plug tool

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

Test


mower spark testing

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

Remove the spark plug and check condition.

Refit the plug wire and ground the plug on the engine, really well.

Crank


mower key

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

Result?


mower plugs

At this point you have:

Good spark - Move on and check Compression.

No or weak spark - Move on to next step.

Cap


mower coil cap

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; Faulty control module/Ignition switch.

Check


mower coil wire

Look for obvious signs of damage. Arcing and corrosion of the plug cap (top pic), check it fits snuggly and securely.

Chafing of wiring against the engine is common.

Mower plug tool

Tools

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

Remove


riding mower coil

Remove engine cover if not already removed.

Check

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

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


Mower spark plug test

Ride-onignition key

Crank

Have a helper turn the key while you check for spark.

Now try a new spark plug.

Result?

At this point you have:

Good spark - Move on and check Compression.

No or weak spark - Move on to next step.


Ride-on mower ignition
Ride-on solenoid

Cap

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; Faulty control module/Ignition switch.

Check

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.


Ride-on mower ignition

Ride-on engine coil

Remove

Remove the engine cover, if not already removed.

Connector

Locate coil control wire connector.


Ride-on mower ignition coil
Ride-on coil

Remove

Remove the coil control wire, you may need a Pliers.

Now check for spark as you did earlier.

Result?

Do you have spark now?

No, still no spark - Replace coil it's faulty.

Yes, have spark now - Check the coil control wire from the coil to control module (or ignition switch) for chafing and shorting.

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 ignition switch. 


Ride-on mower ignition wires Ride-on mower ignition module

Locate


riding mower engine

Locate coil control wire connector.

Remove


riding mower coil

Now repeat spark plug check as before.

Result?


riding mower wiring riding mower module

Do you have spark now?

No, still no spark - Replace coil it's faulty.

Yes, have spark now - Check the coil control wire from the coil to control module (or ignition switch) for chafing and shorting.

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 ignition switch. 

Fitting A Coil

Coil 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.

 

The Shear Key

The Shear key is a small piece of metal that is designed to break under certain conditions. It lives between the flywheel and 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 can be varied, such as no start, poor running, back firing, 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 set up is close to identical.

 

Ride-on mower engine

Remove

Remove cover, if not already removed.

Remove


mower engine cover

Remove cover, if not already removed.

Bolts


mower coil

Remove both bolts and set aside.

Bolts

Remove both bolts, and put one side.


Ride-on coil
mower coil wire

Wire

Remove plug wire and push on wire connector.

Replace

If you can, get OEM parts, they are tested.

Nobody likes revisiting the same job.


Ride-on coil

Remove


Riding mower coil

Pull off connector.

Replace


mower carburetor needle

If you can, get OEM parts, they are tested. Nobody likes revisiting the same job. 

Ride-on mower coil

Push On

Fit connector.

Air Gap

An air gap must be maintained between the coil and flywheel. A business card is just the right thickness.

Tighten the bolts while keeping a push on the coil towards the flywheel. Remove business card by turning the flywheel.

Nice work, now rebuild in reverse order.


Ride-on mower carburetor seal

Push On


Riding mower ignition coil

Fit connector - Push on fit.

Air Gap


riding mower coil

An air gap must be maintained between the coil and flywheel. A business card is just the right thickness.

Tighten the bolts while keeping a push on the coil towards the flywheel.

Remove business card by turning the flywheel.

Nice work! Now rebuild in reverse order.

Related Questions

Why does my lawn mower sputter then die? This commonly happens when the carburettor is dirty or the gas is bad. Removing and cleaning the carburettor 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 lawn mower to smoke? No, 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 over filled with oil. Black smoke means it's getting too much gas, try cleaning or replacing the air filter.

 

Auto Technician and Writer at | Website

John Cunningham is an Automotive Technician and writer on Lawnmowerfixed.com. I've been a mechanic for over twenty years, I use my knowledge and experience to write "How to" articles that help fellow gear-heads with all aspects of mechanical repairs, from lawn mowers to classic cars.