Riding Mower Won't Start When Cold (This is why)

By: Author John Cunningham. Published: 2020/03/11 at 2:19 pm

A riding mower is perfectly capable of starting no matter what the weather or temperature. But cold weather will often show you the weak components in your mower.

So why won’t a riding mower start when cold? The top six reasons your riding mower won’t start when it’s cold.

Choke not set to “On”
Choke not working
Battery discharged
Battery faulty
Ignition fault
Engine oil fault

All of these causes are easily fixed, and in this guide, you’ll learn how to find your problem and how to fix it right now. Strap yourself in.

1 Choke Not Set To “On”

Choke is, as you know, needed to start a cold engine. The choke lever on most mowers is incorporated in the throttle lever. The choke has a distinct symbol, and on later mowers, they use an ice partial symbol. Applying the choke lever creates just the right conditions for a cold engine to start.

A gas engine runs at its optimum when the air-to-fuel ratio (known as AFR) is 14.7:1. That’s 14.7 parts oxygen to 1 part gas. Problems arise in cold weather because cold air is oxygen-rich, as you can imagine, which throws the ratio way off.

Without the choke, the engine gets too much air for the amount of gas the engine receives. We call this a lean condition. A mower just won’t start or start and stalls right away when the AFR is too lean.

The choke, as its name suggests, restricts the volume of air that enters the engine. It does this with the use of a simple flap known as the choke plate. It’s located inside the carburetor. Moving the choke lever controls the choke plate.

Restricting the volume of air brings the ratio closer to 14.7:1, and your engine starts smoothly.

You’ll notice as the engine warms up, it heats the air entering the carburetor until, after a minute or so, the choke flap can open completely. I know you probably know how to use your choke and when you use it, but for those that don’t know, check out this guide “How to start your riding mower”

Set choke to “On”

2 Choke Not Working

If you’re happy with choke use, then let’s make sure the choke is actually working. As said earlier, your choke may be incorporated with the throttle lever or a stand-alone knob. Either way, these guys are operated by cable and are adjusted as part of your regular mower maintenance. We’ll need to check that the choke plate is closing fully when the lever is operated.

On most mowers, we’ll need to remove the air filter intake housing. Usually, this isn’t a big job; on a Briggs & Stratton, it means removing the plastic engine cover first and then the intake manifold.

Removing the covers lets us see the plate operate; we can check that it is closing fully and adjust the cable if needed so that it does. I wrote a complete guide to check your choke, “Riding mower won’t start.”

3 Battery Discharged

This might not seem like it’s your problem because the engine is turning over. But what you may not know is that a mower engine needs to spin over (cranking) at a min of 350 rpm before the armature and flywheel create enough voltage to fire the spark plug. No spark plug firing means no start.

You can easily eliminate this as a possible issue by either testing the battery using a DVOM or a battery tester.

But just before you attempt a jump start, do check that both battery terminals are tight and corrosion-free; both are common and will cause resistance.

The other option is to boost your mower from your car or truck; your mower is a 12-volt battery same as your car or truck. You can check how to jump-start here “How to jump start your mower with a car.”

Test Voltage – Check volts at rest using a voltmeter.

100% charged is 12.7 – 13.2 volts
75% charged is 12.4 volts
50% charged is 12.2 volts
25% charged is 12.0 volts
Discharged (Flat) 0 – 11.9 volts

The battery needs to be in the 75% range to have enough oomph to start a mower engine. Check out lawnmower batteries delivered to your door, fully charged, and ready to go on the Amazon link below.

Amazon Riding Lawnmower Battery

4 Battery Faulty

Like all batteries, your mower battery doesn’t like cold weather. Their Oomph is zapped by the low ambient temperatures. A failing battery will happily work all summer long, right up until the temperatures drop. A mower battery will typically last about 3-4 years, older than 4 years and you can expect failures. If you suspect your battery is failing, check out this link “Riding mower won’t start just clicks.”

Battery Crank Test – With a voltmeter set to volts DC, red to red, and black to black. Have a helper crank over the engine; if the meter drops below 8 volts, the battery is faulty.

Jump-starting

Start by connecting the positive red (+) of the mower to the red (+) of the car. Now connect the negative black (-) on the car to a ground (GRD) source on the mower. (Any bare metal will work)

Put the jumpers on in sequences 1, 2, 3, and 4, and start your mower. With the mower running, remove the jumpers in reverse order: 4, 3, 2, and 1.

5 Ignition System Fault

A mower ignition system can cause intermittent starting problems. The main components of an ignition system are:

Battery
Ignition switch
Control module
Armature
Spark plug

A plug and armature can give trouble hot or cold; both these components are common failures. I wrote a complete guide on ignition system testing “Riding mower won’t start”, or check out the video here.

6 Engine Oil Too Full

This one gets overlooked all the time. Most mowers I work on either have too little or too much engine oil. Both are bad for the health of the motor. Too much oil can cause engine backpressure. It’s like the engine components have no room to move inside the engine because of the excessive oil.

The net result is a slow-cranking engine, which, as you’ve probably guessed, is an engine that isn’t producing a strong spark. Wrong engine oil type, OK, I hear you say, what has that got to do with cold-weather starting? Stay with me here.

Oil, as you know, is graded by its ability to flow (viscosity). Using a heavier grade oil (thicker) won’t cause a problem in the summer when the temps are higher.

Check oil level

But come winter, that heavier oil can resemble a trickle in the engine, causing the engine to turn over more slowly. As we’ve covered earlier, a slow-cranking engine may not create a spark strong enough to start the engine.

To check your oil level and oil type, check out this guide “Can I use 10w30 in my mower.”

Riding Mower Won’t Start When Cold (This is why)