Lawn Mower Runs For Thirty Minutes Then Dies

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 the 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 on hill

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, 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.

 

Gas cap

Faulty Cap

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. We’ll eliminate gas tank air locks in this easy, simple and fast test.

Gas cap

Vented Gas Cap

Work the mower in the usual way until it stalls, then go ahead and follow these steps:

  1. Remove gas cap
  2. Wait one minute
  3. Attempt start
  • If the mower starts and runs as normal, 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. It’s not 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 fire.

 

Armatures are a solid state component. When they fail they are simply replaced.

Mower Armature

Armature

Common cause of hard hot starting and running issues.

Where Is My Mower Armature Located?

The armature is located under the pull assembly housing, fastened to the cylinder block and positioned directly at the flywheel.

 

Mower flywheel and coil

Located At 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 an 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.

 

Testing mower spark

Testing Spark

The process is as follows:

  1. Remove spark plug wire (twist and pull)
  2. Remove spark plug
  3. Refit plug wire
  4. Ground spark plug (use an insulated pliers)
  5. Simulate engine start (Helper pulls on cord)
  6. Check plug tip for spark

No spark means the armature has failed, go ahead and order a replacement. Armatures all look alike, but there’s a ton of different types.

 

Best advice here, remove the old armature, check part number and order by part number not by mower model.

 

Replacing Mower Armature

To remove the armature, you’ll need a screwdriver, ratchet and socket set and you’ll need to remove the following:

  1. Pull assembly housing)
  2. Armature fasteners
  3. Plug wire
  4. Ground spark plug (use an insulated pliers)
  5. Stop start armature control wire
Fitting mower armature

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 sufficient spark.

 

A tool known a s 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 a more detailed instruction 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 on this step check out the video library where I cover the complete repair.

 

Place the card between the armature and the flywheel, push the armature towards the flywheel and tighten fasteners.

 

Mower auto choke

Auto Choke Fault

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, it’s causing the engine to flood with too much gas.

 

Check the auto choke system is working without fault, it's all covered in the video library.

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Auto Technician and Writer at | Website

John Cunningham is an Automotive Technician and writer on Lawnmowerfixed.com. I've been a mechanic for over twenty years, I use my knowledge and experience to write "How to" articles that help fellow gear-heads with all aspects of mechanical repairs, from lawn mowers to classic cars.