Lawn Mower Won’t Start After Running and Hot (Why + How to Fix)

I’m hoping you are not one of those unlucky people who have to keep their lawn mower running the whole time they’re cutting the lawn. You know, those lawn mowers that if you switch them off when they get hot, they’re not going to start again. If you do have one of these lawn mowers, then don’t worry. My mower has tried this on me, but I quickly took care of it. Here’s what you can do to confidently turn your lawn mower off and back on again when it gets hot.

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Fixing a Lawn Mower That Won’t Start After Running (The Short Explanation)

A lawn mower that won’t start after running usually has an issue with a heat-related problem. Internal and external cooling, altered air-to-fuel mixture, expanded components, and reduced electrical current can all be affected by a lawn mower getting hot. Once a lawn mower cools down, these problems, in many cases, will go away until the engine gets hot and you try to restart.

Why Your Mower Won’t Start After It’s Been Running + Gets Hot (5 Ideas)

A lawn mower engine that is running can mask a bunch of problems. It’s only when you switch off the mower that you discover there is an issue. Usually, the main sign is that your lawn mower won’t start when hot. Here are what I have found to be some of the more common problems making a lawn mower hard to start when hot.

Cooling Fins

On top of your lawn mower, you’ll find the cooling fins hidden beneath a guard. As your lawn mower is running, these cooling fins suck in air through the guard and cool the engine by pulling cool air in and blowing hot air out. Now, this is only possible if the fresh air has a clear passage to pass through.

If your lawn mower has been cutting for several months without this air passage being cleared out, then it’s probably not performing very well. Once you turn your lawn mower off, heat will be trapped in the engine without anything to cool it down. If you take a peek inside the guard or up under the engine cover, you might discover a lot of clippings blocking the airflow passage.

Engine Oil

The oil in your lawn mower’s engine is there to lubricate the internal engine parts. I’m sure you already know that. But once the oil gets low, the internal engine parts begin to rub metal on metal with increased friction. This increased friction will make the temperature shoot up and overheat the lawn mower. Besides extensive damage caused by overheating, your lawn mower will be hard to restart. So, pull out the dipstick and give the engine oil a check. Hopefully, it’s between the lines.

Loose Parts

I’ve been there too wondering, why does my lawn mower not start after running? After doing a thorough inspection, I discovered that loose parts were the cause. My mower had a bit of a vibration in the past that loosened off a few bolts.

Unfortunately, these bolts were around the carburetor. The effect was that the air-to-fuel mixture was off, as the air was getting sucked in. This became a real problem when the mower got hot.

With the heat, the metal expanded and widened the gap between the carb and the mower. So, take a good look at your lawn mower and make sure that everything is nice and tight. Even a slightly loose carburetor can throw your lawn mower off when it’s hot.

Auto Choke

You probably haven’t taken much notice of your auto choke. Being that it’s automatic, why would you? Well, there is a thermostatic arm that moves once a lawn mower warms up and gets hot.

You may have noticed a little arm on the exhaust pipe that moves a few minutes after the engine has been running. This is the thermostatic arm at work. If the choke isn’t working as it should, then it can’t engage or disengage the choke correctly. This basically means that you will be trying to start a hot engine with the choke on. Yep, not easy.

So, observe the position of the arm with a cold engine. Then, start the mower and let it get hot, and check the position of the arm again. You should see it move as the mower gets hotter. If it’s not looking right, then it’s probably not turning the choke off. If you look closely at the thermostatic arm, you might see a scuff or two where it’s been bent out of shape. This is one of a bunch of lawn mower automatic choke problems that I’ve come across over the years.

Ignition Coil

Last on my list is the ignition coil, otherwise known as the flywheel magneto. The coil generates the voltage needed by the spark plug to make a spark as the flywheel turns. Unfortunately, coils become less reliable with age and when they get hot. Resistance builds up, making the spark harder to produce. This is even more of a problem if your engine is overheating. So, if you’re trying to restart your lawn mower when it’s hot, it could be challenging if the coil is on its way out.

To test the coil, you’ll need to disconnect it from your lawn mower and grab a multimeter. Now, coils are all different and designed for specific lawn mowers. So, you’ll need to look up the engine model number and find the ignition coil inspection data that shows the resistance levels of the coil windings.

Example Ignition Coil Inspection Data

Mower – Husqvarna 48″ Tractor Mower.
Engine – Kawasaki FH531V-BS06.
Engine – 18hp V-TWIN XP Series.

-/+	+ A	+ B	+ C
- A	-	2 - 18 K ohms	2 - 18 K ohms
- B	∞	-	∞
- C	2 - 18 K ohms	10 - 30 K ohms	-

Once you have the data information, you’ll need to test the connection between points A, B, and C of the coil. You need to test to ensure you have resistance levels in the range of the data table for your lawn mower coil/s. It sounds harder than it actually is.

You’re basically checking to see that the multimeter reads the same number as the data. If the reading is out of range, then you’ll know your coil needs replacing. The easiest thing to do is get a hold of the inspection data and follow the specific instructions.

How to Fix a Lawn Mower That Won’t Start After Running

Now that you have been through my top causes of why your lawn mower won’t start after running, it’s time to get down to the fixes. So, grab your toolbox, and let’s look at the repairs.

Cleaning Out the Cooling Fins

It looks like someone might have skipped cleaning their lawn mower. It happens to the best of us. Well, the quickest and easiest way to clean out the cooling fin is to use an air compressor or leaf blower if I’m feeling lazy.

A quick blast of air through the top of the cooling fin guard usually does the trick. You’ll want to get out all the grass and debris stuck under the guard and the engine cover. Sometimes poking your fingers up the side of the cover also works.

Just make sure the engine has cooled down first. What I wouldn’t recommend is using a water hose. I’ve seen that done a few times with awful results. Water in the oil and fuel is not a good idea. Thankfully, it wasn’t my lawn mower.

Tools Needed to Clean Out Cooling Fins

Air Compressor / Leaf Blower.

Topping Off Your Lawn Mower Oil

Hopefully, you know what type of mower engine oil your mower takes because you’re going to need some to top off the engine.

First, check the dipstick and see how much oil the lawn mower needs, then grab your oil funnel. Next, place the funnel into the oil filler cap and add a little oil. I find it best just to add an ounce at a time and recheck the dipstick. Because you’re topping off, the mower probably only needs a couple of ounces, and you don’t want to add too much.

Once you think you’re at the right level, turn over the engine and recheck the level. This will push the oil around the mower and fill the gaps in the engine that were missing oil. You’ll probably find the oil level goes down a bit when you do this.

Tools & Parts Needed to Top Up Engine Oil

Mower Oil
Oil Funnel

Tightening Up the Engine

If your mower won’t start when hot because of a loose part, then you’ll need to grab a socket wrench set and a set of screwdrivers. The most likely place that would cause the problem is around the carburetor and the air intake filter. This is a common issue for many push lawn mowers, like Toro and Honda. Luckily it’s a quick fix.

So, remove the cover of the air filter and remove the air filter from the housing. Sometimes these covers are held in place with a screw that you can remove with one of your screwdrivers.

Next, take your socket set and tighten the couple of bolts inside the housing. Usually, there will be two or three bolts inside the air filter holding the housing and the carburetor to the engine. So, tighten them and then test out the mower.

Tools Needed to Tighten Engine Parts

Screwdriver Set
Socket Wrench Set

Fixing the Auto Choke’s Thermostatic Arm

To adjust the auto choke thermostatic arm, you’ll need to start up the engine and get it hot. Depending on the type of lawn mower you have, you might be able to remove the engine cover to make this easier. But if your starter pull cord is attached to the cover, you’ll have to leave it in place.

So, once the engine gets hot, the thermostatic bar should move, then push against the air vane and close the choke on the clutch. This is usually about an inch or so movement of the air vane. If you use a pair of pliers, you should be able to bend the arm enough so that it moves the vane the right amount.

With a bit of play, you should be able to get the arm in the right place to turn the choke off using the thermostatic arm.

Tools Needed to Repair the Thermostatic Bar

Pliers

Replacing the Coil

Replacing the coil isn’t particularly difficult, but it does require accuracy, so you’ll need to have a feeler gauge on hand. If you can get hold of a feeler gauge specifically for setting the gap on a lawn mower’s coil, even better. So let’s go over the steps you’ll need to follow to replace a lawn mower coil.

Steps to Replace a Lawn Mower Coil

Remove the engine cover from the lawn mower.
Rotate the flywheel so that the coil magnets are on the opposite side of the coil.
Remove the stop wire from the coil.
Remove the mounting bolts that hold the coil to the lawn mower.
Disconnect the spark plug cable from the spark plug.
Remove the old coil from the lawn mower.
Position the new coil on the lawn mower.
Reinstall the bolts that hold the coil to the lawn mower and lightly tighten the bolts.
Pull the coil away from the flywheel.
Tighten the bolts so that the coil can’t move.
Insert feeler gauge between coil and flywheel. (feeler gauge gap is engine/coil specific)
Rotate the flywheel so that the magnets meet the coil.
Loosen the bolts holding the coil to the mower so that the coil can move.
Allow coil magnets to attract the coil while keeping the feeler gauge in place.
Tighten the bolts fully.
Remove the feeler gauge from between the coil and the flywheel.
Connect the stop wire.
Refit the housing.
Connect the spark plug.

Tools Needed to Replace the Coil/Magneto

Socket Wrench Set
Feeler Gauge
Replacement Coil

What About a Riding Lawn Mower That Won’t Start After Running – Anything Different?

So, what if a riding lawn mower won’t start after running? Well, other than power and size, there isn’t much difference between a riding mower and a push mower when it comes to the engine. If you follow the same diagnosis process above, you should be able to spot the reason your riding lawn mower won’t start when hot.

Did You Spot What I Missed Out?

V-Twin engines? The Kawasaki engine I mentioned before on the Husqvarna was a V-Twin. This means it has two cylinders, two spark plugs, and, you guessed it, two coils. So, if you have a twin cylinder, you’ll need to test both ignition coils. For you guys with say a single cylinder Briggs and Stratton, you’ll just have the one coil.