What a pain in the jacksie! Halfway through the yard work, she stalls. But don’t panic, you are in the correct place, and very soon, you’ll know what the problem is and how to nail the repair.
The most common cause of a mower that runs for thirty minutes and then dies is a faulty armature. A faulty gas cap is the second most likely cause.
In this post, you’ll learn the most likely cause of a mower engine stall after thirty minutes, and you’ll learn how to test and fix it.
Mower Gas Cap
A faulty gas cap is, as you know, the second most common cause of a mower stalling after thirty minutes. I’m covering it first because the gas cap is really easy and fast to check. All mower gas caps are vented, which basically means the mower gas tank must allow air to enter the tank to replace the fuel that leaves. If the tank is sealed, the fuel becomes air locked, and the mower stalls.
This typically happens after a short period between ten and thirty minutes and also depends on factors like weather and gas tank fuel level.
Need more info on the fuel system, carburetor components, and how they work, you can check them out here.
Commonly gas caps are lost, and the owner may MacGyver up an oil can cap that fits like a glove. While on the face of it, this seems like a coincidence. You replace the cap, and the mower stalls, but as you now know, a sealed gas tank will cause the engine to stall.
Anyway, if you’ve got a makeshift gas cap fitted, it’s likely your problem. But even if you haven’t, gas cap vents do fail, but we’ll eliminate it as a possible with this simple, fast test.
Work the mower in the usual way until it stalls, then go ahead and follow these steps:
- Remove gas cap
- Wait one minute
- Attempt start
MacGyver didn’t get it right every time.
If the mower starts and runs as normal, then your gas cap is faulty, go ahead and replace it. If, on the other hand, the mower still won’t start, you’ll need to check the mower for spark. Checking spark isn’t difficult, and it’s all covered below.
What Is Mower Armature?
A mower armature is a small component fastened to the exterior of the engine; some may refer to it as a coil. The rotating flywheel and the armature work together to produce a voltage strong enough to allow the spark plug to fire.
Armatures commonly fail in two ways, they stop working, and your mower won’t start, or they work when the engine is cold but fail as the engine heat soaks into the windings of the armature. Armatures are solid-state components. They aren’t repaired. They are replaced.
Check out the Amazon link below for common lawnmower armatures.Amazon Lawnmower Armature
Armature – Common cause of hard hot starting and running issues.
Where Is My Mower Armature Located?
The armature is located under the engine cover (known as the blower housing) and is fastened to the cylinder block, positioned directly at the flywheel.
Location – Armature and flywheel
Test A Mower Armature
To test the armature, you’ll need a helper to simulate starting the mower while you check the spark. You’ll also need insulated pliers and a plug wrench. The process is simple, but you’ll need to use caution. Keep hands and feet clear of the mower blade as the engine is cranked over.
As we are chasing an intermittent failure (fails when the mower is hot), checking the spark when the engine is cold won’t be useful (we expect it will work great then). We’ll need to cut some grass until the engine warms up and stalls.
That’s when we’ll check for spark. Now, we’ll need all our tools to hand; allowing the engine cool will, as you know, mean the fault disappears again. Let’s get testing so.
Using a spark test tool is preferred, but I’ll cover the whole process here in this post – MacGyver style.
You may find these videos useful also:
You may find the following post helpful – How ignition systems work?
The spark testing process is as follows:
Remove – Remove the spark plug wire (twist and pull) and also the spark plug.
Fit – Refit plug wire
Ground – Ground the plug wire on any bare metal, but you’ll need to use insulated pliers to hold it in place while the helper pulls on the starter cord.
Attempt Start – Simulate engine start (Helper pulls on the cord) while you view the spark plug.
Repeat – Try another spark plug to eliminate a faulty plug; failing to test with a second plug means you could condemn the armature in error.
No spark means the armature has failed; go ahead and order a replacement. Armatures all look alike, but there are a ton of different types. Remove the old armature, check the part number, and order its replacement by part number, not by mower model.
If you have spark, your armature is good, and it’s likely you have an over-fueling carburetor issue (see below), usually caused by a faulty auto choke system. A replacement carburetor will likely fix the issue but first, check for binding auto choke controls. I’ve covered choke testing and carburetor replacement on this page; it contains several videos – “Choke testing”.
Replacing Mower Armature
To remove the armature, you’ll need a screwdriver, ratchet, and socket set, and you’ll need to remove the following:
- Engine cover (Blower housing)
- Armature fasteners
- Plug wire
- The stop-start armature control wire
Replacing the armature is the reverse. However, when fitting the armature, an air gap between the armature and the flywheel must be maintained in order to generate a sufficient spark. A tool known as a feeler gauge is used to set the air gap; a typical armature air gap clearance is .010 – .014in.
Most won’t have a feeler gauge, and that’s OK; you can use a business card to set the gap. If you need more detailed instructions with pictures, check out this post, “Mower hard to start when hot” it covers the replacement procedure in full with pics.
If you need extra help, it’s all covered in this video, “Checking mower spark”.
Armature air gap
Mower Stalling After Grass Bag Emptying
Some mowers equipped with the auto choke function may have difficulty starting when hot. Typically an operator stops the mower to empty the grass box, and the mower won’t restart. Wait ten minutes or so, and the mower starts.
The root cause is a defective auto choke, causing the engine to flood with too much gas.
Check the auto choke system is working without fault; it’s all covered on this page under “Choke testing”.
Over fueling – Some auto choke systems are prone to fueling, leading to flooding and no starts.
- About the Author
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John Cunningham is an Automotive Technician and writer at Lawnmowerfixed.com.
He’s been a mechanic for over twenty-five years and shares his know-how and hands-on experience in our DIY repair guides.
Johns’s fluff-free How-to guides help homeowners fix lawnmowers, tractor mowers, chainsaws, leaf blowers, power washers, generators, snow blowers, and more.