The most common reason for a mower cranking over but not starting, is bad gas. 

In this guide, I will cover some of the common reasons, and the solutions.

Jump to Test

Important - If your mower is not cranking over, you need to check out repair guide - "Lawn Tractor Won't Start No Click".

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. 

01. Gas Valve Off

02. No/Bad Gas

03. Choke Fault

04. Plug Wet/Faulty

05. Plug Wire Off

06. Bad Battery

07. Air filter Blocked

08. Gummed/Faulty Carburetor

09. Coil Failure

10. Faulty Control Module

11. Flywheel Timing Off (Shearkey)

12. Engine Compression/Valve Fault

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.

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".

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 "Carburetor troubleshooting".

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 not in use. Check out "Lawn mower gas tap".

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".

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.

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. 

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".

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.

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

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.

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.

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 carburetor evaporates and leaves the moisture behind, this then corrodes and gums up the carburetor - Now it's a no start.

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 prefare to use a stabilizer.

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 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, 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 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".

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.

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.

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 minimize 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

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.

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.

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. 

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 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.

To quickly find the problem we need to narrow down the search area. Most non starting mowers are caused by fueling 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 carburetor. Attempt to start your mower as normal with full choke.

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. Check out "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 air fuel ratio (AFR). Using the choke counteracts the lean condition caused by the dense cold air.

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".

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 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.

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 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.