Skip to Content

Riding Lawn Mower Won’t Start – Mechanics troubleshooting list

By: Author John Cunningham. Published: 2019/01/07 at 2:33 pm

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 Carburetor
  • 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 the repair guide – “Lawn Tractor Won’t Start No Click.”

Tractor Component Locations

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, it 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.

Check out “How to start a lawn mower.”

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; they need three things to start:
Thing 1. Gas/Air Mix
Thing 2. Spark
Thing 3. Compression


1 What is Gas/Air Mix?


Internal combustion engines, as you know, like air and gas mixed together, and they like a particular ratio.

14.7 parts air to 1 part gas is the sweet spot.

When this ratio falls outside of this range (either too much or too little air), the engine either fails to run or runs poorly, so fuel quality and ratio are always at the top of the checklist when looking at a no-start.

Especially in recent years where ethanol fuels are for many the only fuel type available.

Air to fuel ratio

What’s Ethanol?

The reason fuel causes problems is that some fuels are ethanol-blended, such as fuels such as E10. While small engine manufacturers say their engines run on these fuels, they would prefer we use regular gas, and the reason for that – their older engines are not designed to run on ethanol.

Ethanol is an alcohol fuel 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, is 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 manufacturer warranty and is outlawed by the EPA for use in outdoor powered equipment.

Manufacturers recommend we use at most E10 with a minimum octane rating of 89.


Ethanol fuels will turn stale after about one month; regular fuel will turn stale after three months, so that’s why we need to use a fuel stabilizer.

The Problem With Ethanol

It attracts moisture; this isn’t an issue for cars because the gas tanks are sealed, meaning the moisture can’t get in. Lawnmower fuel tanks are sealed, but not to the same extent; they use a one-way valve to allow air in to displace the fuel leaving the gas tank. Moisture makes its way into the fuel tank, albeit in very small amounts.

This isn’t an issue when the mower is being used regularly as the gas is used up. 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, it 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.”

2 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, and 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, worst 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.

Briggs and Stratton





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 sensor 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.

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 is that 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. 

3 What Is Compression?

Small engine 4 stroke cycle

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 it 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.

Need more info on the fuel system, carburetor components, and how they work, you can check them out here.

Small engine carburetor components

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. 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.

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.

Apply full choke.

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.

Snowblower ignition system overview

You may find the following post helpful – How ignition systems work?

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 bad ground will lead to an incorrect diagnosis.

Good ground

Grounding spark plug

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.


At this point, you have:

  • Good spark – Check Compression.
  • No or weak spark – Move on to the next step.

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.

Remove the engine cover (blower 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.


Do you have spark now?

  • No, still no spark – Replace the coil. It’s faulty. (see fitting coil below)
  • Yes, have a spark now – Check the coil control wire from the coil to the 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 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.

Amazon Lawn Mower Coil Modules

Remove the engine cover if not already removed.

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

Remove the coil control wire.

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.