Mower Won't Start No Spark
- Plug fault
- Stop/start switch 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 fitting a new coil, check out “Mower won’t start video”.
It covers diagnosing and repairing all common causes of a no start mower, including no spark.
Checking Lawnmower Spark
If you need video help, check out the mower repair video library where I cover the whole process of checking the ignition system and of course making the relevant repair.
It covers a ton of other common mower repairs you can easily do. It’s packed with 25 years experience, tips, tricks and hacks and the best bit, you’ll save a packet on mower repair bills.
For these tests you’ll need a plug spanner, an insulated pliers, screwdrivers and a spare 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 we get right to it.
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, we'll need to check the on/off switch assembly.
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 plugs 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 electrode
Wrong plug type – Plugs are as you know graded, each engine will have a particular plug code. So even though a plug fits, doesn’t mean it’s correct. Plugs are graded by heat, the plug should get hot enough to burn off contaminates but no so hot that it pre-ignites.
Wrong plug types can cause all types of problems from hard starting, rough running, hot start failures etc.
Check your plug type with your mower engine maker.
An incorrect plug type will lead to intermittent problems.
Dirty plug – 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 can be caused for a few reasons – blocked air filter, faulty choke, over use of choke, tipping mower over on its carburetor side, carburetor fault.
Carbon build up in the engine is a normal condition, fuel type, oil type, maintenance and plug type all effect how quickly it builds.
Oil on the plug is common too, it’s caused by too much oil, blocked crankcase breather, head gasket fault, engine wear, wrong plug type.
Bad plug gap – A spark plugs 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 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 simple plug gap tool is used to set the spark plugs gap. The electrode is manipulated to the correct size by simply bending it with a pliers.
Cracked plug insulator – Self explanatory too. The insulator is the white ceramic material of the plugs 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 effect 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
Loose terminal – Caused by our old friend 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 a pliers to tighten it.
Faulty terminal connector – It’s different but related to a loose connector. A loose connector will often turn into a faulty 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.
Common stop/start assembly faults
Most mowers owners are familiar with the bail lever at the handle bars which must be held in order to start the mower. Most mowers will use this type stop/start system, others 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
flywheel brake assembly
The coil (also known as Armature) control wire is connected from the stop/start switch on the flywheel brake assembly to the coil, which is fitted on the engine. The coil and plug won’t produce a spark so long as the control wire is connected to ground (Metal of engine).
A common fault is chafing of the control wire on the engine (shorting to ground), this effectively is the same as releasing the bail lever – turns engine off.
Check the wire for chafing, especially anywhere the wiring turns sharply around the engine.
coil control wire
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 a solid state unit – 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”
Can a spark plug have bad spark? Yes, spark plugs do 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.