By: Author John Cunningham. Published: 2019/11/05 at 7:30 am
Pulling and pulling and nothing, a mower without a spark is useless. In this post, we’ll cover all the most common ignition system failures.
Mower won’t start any spark? Common reasons a lawnmower has no spark include:
- Plug fault
- Plug wire fault
- Stop/start switch fault
- Coil (Armature) Fault
None of these tests are difficult, and twenty minutes from now, you’ll know why your mower has no spark.
This post will have you covered, but if you need video help diagnosing no spark or help fit a new coil, check out the “Mower won’t start video.”
Checking Lawnmower Spark
Since you’ve checked the spark already, I’m guessing you know the procedure. However, it’s worth pointing out that getting this test wrong can lead to misdiagnosing and replacing the ignition coil or other parts unnecessarily.
Spark testing is, as you know, a simple test. You won’t need any special tools here, but a spark testing tool does make the job easier and totally foolproof.
If you need video help, check out the mower “Mower spark test video,” where I cover the whole process.
For these tests, you’ll need a plug spanner, insulated pliers, screwdrivers, and a spark plug is useful. You’ll also need a helper, as we’re not using a spark testing tool. It can be difficult to crank over the engine and, at the same time, check for spark. With all the tools gathered and a helper on hand, we’ll get right to it.
As we’ll have a helper cranking over the engine, that means the blade will be spinning, and even though the engine’s not running, it can still remove body parts, so you know!
You must use insulated pliers (plastic/rubber-handled pliers) to hold the plug, as the voltages produced are enough to give you a jolt, which isn’t pleasant.
Tools – Plug spanner, insulated pliers, and a spark plug will be needed.
Spark test tools
Step 1 – Remove the spark plug wire by twisting and pulling, then using the plug tool, remove the spark plug.
Step 2 – Reattach the spark plug wire to the plug. Using your insulated pliers, hold the plug threads firmly against the metal of the engine. This is known as grounding. If the plug doesn’t make good contact with the metal of the engine, you won’t get a spark.
Step 3 – While you watch for spark, have the helper hold the bail lever as normal and yank on the pull cord.
If you have no spark, swap out the plug and test again.
If you still have no spark, it is most likely a failed coil, but it is best to check the on/off switch assembly first.
Common Spark Plug Faults
A healthy spark plug is essential for reliability, power, and smooth running. Plugs have a tough job. They carry high voltages and live at the heart of the engine where it’s hottest.
Making matters worse for the plug is its location – right out front of the engine. So, getting shoved into fences and trees is all part of a spark plug’s life, and you thought you had it hard!
These are the most common spark plug faults:
- Wrong plug type
- Dirty plug
- Bad plug gap
- Cracked spark plug insulator
Wrong Plug Type
Plugs areas you know are graded; each engine will have a particular plug code. So even though a plug fits, it doesn’t mean it’s correct. Plugs are graded by heat. The plug should get hot enough to burn off contaminants but not so hot that it pre-ignites. Wrong plug types can cause all types of problems, from hard starting, rough running, hot start failures, etc.
Plug type – Check your plug type with your mower engine maker.
An incorrect plug type will lead to intermittent problems.
Self-explanatory, it’s a plug that’s contaminated by too much gas (flooding), carbon, or oil. All of these will prevent the plug from doing its job. Flooding may be caused for a few reasons – blocked air filter, faulty choke, overuse of choke, tipping mower over on its carburetor side, and carburetor fault. Check out the video “How to fix a flooded engine.”
Carbon build-up in the engine is a normal condition. Fuel type, oil type, maintenance, and plug type all affect how quickly it builds.
Oil on the plug is also common. It’s caused by too much oil, a blocked crankcase breather, a head gasket fault, engine wear, and the wrong plug type. Check out the video “How to clean a plug.”
Bad Plug Gap
A spark plug function is obviously to create a spark, and it can only do this if the electrode gap is correct. The coil has been designed to create a sufficient spark to jump a pre-determined spark plug gap.
- No gap, means no spark
- Gap too small means poor running or no start
- Gap too big means no start and risks damaging the coil
A plug gap tool is used to set the spark plug gap. The electrode is manipulated to the correct size by simply bending it with pliers. Check out the video “How to gap a plug.”
Plug gap – The gap is important. Too small or too big can lead to no starts or poor running.
Cracked Plug Insulator
Self-explanatory too. The insulator is the white ceramic material of the plug’s body, and as said earlier, plugs are at risk of being damaged by bumping into obstacles. If the insulator breaks or cracks, the plug stops working.
Common Spark Plug Wire Faults
A spark plug wire has a few particular problems that affect them, depending on a few variables, like how and where they’re stored.
The common faults I see again and again include:
- Loose terminal connector
- Faulty terminal connector
- Damaged plug wire
Caused by our old friends, the trees, shrubs, and fences. The plug wire terminal that clips to the spark plug becomes loose, and that can cause no starts, poor running, and intermittent starting/running.
The fix here is simple: squeeze the terminal body using pliers to tighten it.
A loose terminal will cause the engine to misfire or not start at all. The quick fix here is to squeeze the terminal until it fits snugly on the plug.
Because this cap was loose, it created arching, which burnt the metal of the terminal cap.
Faulty terminal connector – It’s different but related to a loose connector. A loose connector will often turn into a faulty one as the spark starts to jump inside the terminal, burning it or setting up conditions for corrosion to take hold.
The outcome is the same: no spark or poor running. A replacement terminal can be purchased and fitted to solve this issue.
Damaged Plug Wire
Plug wire rubbing off the engine cover can cause the insulation to wear and the coil to ground. But more often than not, a damaged plug wire means rodents. Mice love wiring insulation, and unfortunately, our furry friends have cost us a coil.
Sure, you can wrap them with insulation tape, but it’s only a quick fix. The long-term repair is to replace.
Damaged wire – Mice love to chew on the wiring insulation.
Common Stop/StartAssembly Faults
Most mower owners are familiar with the bail lever at the handlebars, which must be held to start the mower. Most mowers will use this type of stop/start system; other manufacturers may incorporate the stop/start function with the throttle lever. But apart from this difference, all other components will be very similar.
The main components of the stop/start assembly include:
- Bail/throttle lever
- Flywheel brake assembly
- Stop/start switch
- Coil control wire
Bail / Throttle Lever
Common faults here include disconnected out-of-adjustment or broken levers.
The cables break and stretch, so it’s not uncommon for the bail lever to work, but because the cable has stretched, it doesn’t move the brake assembly to the start position.
Stop / start cable
Flywheel Brake Assembly
Common faults here include cable out of adjustment, meaning the bail lever doesn’t pull the brake to the off position.
This is the on/off switch. It’s fitted at the flywheel brake assembly. When the bail lever pulls the assembly, it pushes on the switch, removing the ground connection to the coil. This allows the mower to start.
On /off switch
Coil control – Here’s a different mower coil control switch. It’s a very simple connection; the contact points must separate before the coil and plug will create a spark.
The Coil (also known as Armature)
The control wire is connected from the stop/start switch on the flywheel brake assembly to the coil, which is fitted to the engine. The coil and plug won’t produce a spark so long as the control wire is connected to the ground (Metal of the engine).
A common fault is the chafing of the control wire on the engine (shorting to the ground); this effect is the same as releasing the bail lever – turns the engine off.
Check coil control wire for chafing, especially anywhere the wiring turns sharply around the engine.
Coil control wire – Coil control is a single wire with a push-on connection. Often, they’ll come loose, and when they do, the mower won’t turn off.
Common Coil Faults
Coils generally work, or they don’t. Occasionally, you’ll get a coil that works when it’s cold and stops when the engine heats up. Coils are solid-state units – they can’t be repaired. Testing a coil and fitting a new one is easy; I wrote a whole post about it right here “Push mower hard to start when hot.”
Check out the video here; it covers spark checking, diagnosing, and replacing the coil. If you need to replace the coil, check out the great deals on the Amazon link below.Amazon Lawnmower Coils
Coils – Lawnmower coils give lots of problems; I replace tons of them.
Can a spark plug have a bad spark? Spark plugs wear out. A spark plug should be changed once every year at the start of the new season. You can check the spark plug for spark by removing it, connecting the plug wire, grounding it off the engine, and turning over the engine.
- 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.