Lawn Mower Runs For Thirty Minutes Then Dies (This is why)

By: Author John Cunningham. Published: 2020/12/17 at 1:15 pm

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 of between ten and thirty minutes and also depends on factors like weather and gas tank fuel level.

Faulty Cap

Commonly gas caps are lost, and the owner may MacGyver up an oil can cap that fits like a glove. 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

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

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.

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 on hand; allowing the engine cool will, as you know, mean the fault disappears again. Let’s get testing so.

Testing Spark

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:

• Spark testing video
• Plug cleaning video
• Plug gaping video

The spark testing process is as follows:

Result

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

Fitting Armature

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.”

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.”