So frustrating, but I’ll bet we’ll get this figured out within minutes; I’m a mechanic, and here’s how the pros do it.
Chainsaw ignition systems commonly don’t spark for two reasons, a spark plug issue or coil failure. Spark plug issues are more likely; cleaning and gapping is the usual fix.
In this post, we’ll cover the top reasons your chainsaw isn’t getting spark; we’ll cover diagnosis and the repair for each.
As a mechanic of over twenty years, I know the obvious is often overlooked, and so I have developed a habit of checking the basics first; it only takes a few moments but often saves a ton of time.
The basics include:
Is the switch on?
I know this sounds too simple, but if, like me, you don’t use your saw too often, you may forget which way the toggle switch goes for ON.
This tends not to be a problem with modern saws because clever designers have combined the choke on/off control.
Worth double-checking those older style toggle switches are in the on position.
Plug wire on?
A loose or disconnected plug wire is common on other small engines such as lawnmowers, but less so on chainsaws because they generally tend to be protected by a plastic shield. But some saws may have a spark plug wire exposed, making debris contact possible and a loose plug wire possible.
Worth checking that the plug wire is fitted securely.
Incorrect spark plug test procedure
I’m guessing you know well how to test the spark plug for spark, but this is for the folks who may be a little foggy on the task.
Most folks check the spark by removing the spark plug and placing it against the saw engine. And that works so long as the spark plug threads are grounded securely against the engine’s bare metal.
If the plug doesn’t ground properly, you won’t get a spark, and this often catches folks out, and they diagnose a failed plug or coil incorrectly.
The best way to test a spark plug and a coil is with an inline ignition system tester. It is designed to stress the system and offers a more thorough test; I’ve covered that test below.
But you can, of course, use the DIY plug on the engine way. Here’s how I do it.
- Remove & check spark plug condition and gap
- Fit plug wire.
- Ignition On.
- Place ground jumper lead on engine & plug threads.
- Crank over the engine and check for spark
If there is no spark, I’ll try a known good spark plug to be sure.
And that’s the low-hanging fruit covered; now we’ll go a layer deeper.
Spark Plug Issue
A spark plug is one tough component; it lives a tough life, what with being electrocuted 200 times per second and close to incinerated inside the combustion chamber.
Check out this post to learn more about how a small engine ignition system works.
No surprise then when you learn they have some weaknesses, and here they are.
Dirty Spark Plug
Because the spark plug center and ground electrode live inside the combustion chamber, they tend to attract more than their fair share of contaminants. As said, they must contend with high voltage, heat, and chemicals.
Engine contaminates can build up around the center electrode, bridging the gap and preventing spark creation.
How to diagnose? Pull the spark plug and check for oily or carbon deposits around the gap.
How to fix it? Clean with a wire brush and some brake cleaner, and use a blow torch to burn off oily deposits if present.
Check out how to clean a spark plug. (It’s a mower, but the process is identical)
Bad Spark Plug Gap
The plug gap is what gives rise to the birth of a spark. Spark won’t jump a gap that’s too big, and it stresses the coil; a gap that’s too small won’t work either, as the spark is weak or not created at all as the voltage goes straight to the ground.
The gap must be within spec for the ignition system to produce a bright blue consistent spark.
How to diagnose? Pull the plug and check the gap using a feeler gauge.
How to fix? Use the gauge screwdriver or pliers to open or close the ground terminal as indicated by the gauge. Typically chainsaw’s spark plug gap is set to .020″ for small saws and .025″ for larger saws, but check your spec in your owner’s manual.
Check out how to gap a spark plug. (It’s a mower, but the process is identical)
Wet Spark Plug
In this case, our plug and ignition system may be working fine, but the plug is becoming saturated with fluid. Typically there are two ways plugs get wet: they get wet from a third party outside the cylinder or in the side of the cylinder.
Both can cause a plug to stop working. A plug that gets wet on the outside will be pretty obvious, rain or spilling a fluid, washing, etc, and not very typical.
The second way is far more typical – a wet plug inside the combustion chamber, and the fluid is a gas/oil mix. It’s known as flooding, and the excess gas extinguishes the spark and sometimes partially fills the cylinder with raw gas.
Flooding may be caused for several reasons, but first, let’s look at how we diagnose a wet plug.
How to diagnose? Pull the plug and look at the condition.
How to fix? The fix is to dry the plug and dry out the cylinder.
The process is as follows.
- Remove the spark plug and dry it with a paper towel.
- Remove air filter.
- Set throttle to full but choke and ignition set to OFF.
- Pull the starter five to six times.
- Allow the saw sit for ten minutes before fitting spark plug.
It’s not uncommon for a saw to flood now and again; simply falling over on its side shouldn’t, but it can cause flooding. But if your saw floods repeatedly, it suggests an underlying issue, likely carburetor metering diaphragm, needle valve or lever issue.
Some common reasons for flooding include:
- Overuse of choke – turn choke off after start
- Blocked air filter – clean air filter
- Stale gas – drain & clean gas tank
- Wrong gas type – drain & clean gas tank
- Too much oil mix – mix gas to correct ratio
- Dirty carburetor – clean carburetor
- Carburetor out of adjustment – adjust
- Carburetor gaskets worn out – replace gaskets
- Metering lever out of adjustment – adjust metering lever
- Faulty carburetor – replace carburetor
- Faulty plug – replace plug
- Bad spark plug gap – re-gap plug
- Faulty coil – replace coil
Wrong Spark Plug
Spark plugs might look alike. Indeed many spark plugs will fit in many machines and appear to start and run just fine – until they don’t. And that’s because spark plugs are graded not just by their physical dimensions but also by heat range.
Plugs are designed to get hot enough to burn off contaminants but not so hot that they pre-ignite.
Go ahead and check that your spark plug is correct; the code is printed on the ceramic insulator; check that code against the recommended code in your owner’s manual.
I don’t assume the plug installed is the correct plug type; I like to check.
The coil is where a small voltage created by the rotating flywheel and magnet is transformed into high voltage and delivered to the spark plug. Because they are under a ton of stress, they fail occasionally.
But as components go, coils can be pretty spendy, so let’s look at some other possibilities before wrapping it up with the ignition test.
Check out this post to learn more about how a coil works.
Coil issues include the following:
Bad spark plug cap or wire
Plug caps are metal wrapped inside a rubber boot. Years of removal and fitting tend to spread the metal tab wings, and they become loose and create resistance to current flow. Chafing of the wire is common, too, usually caused by the wire fastener coming loose and allowing the plug wire to make contact with the spinning flywheel.
How to diagnose? Remove the plug cap and examine it closely, look for chafing, and look inside the cap for signs of corrosion.
How to fix? A damaged spark plug cap may be replaced, but a damaged wire can’t. Finding such an issue means replacing the coil.
Coil Air Gap
A coil is fastened to the engine adjacent to the spinning flywheel. It must be positioned a specified distance from the flywheel; too far, and the magnets can’t excite the coil’s windings, and we have no spark.
How to diagnose? We’ll need to remove the pull assembly and check the air gap. Before checking, we should check our owner’s manual for correct specs, typically between .008″ and .012 “.
The process is as follows:
- Remove pull assembly (4 fasteners usually)
- Use the appropriate feeler gauge (a business card is a good substitute for the feeler gauge in this application)
The fix? Adjust as necessary by loosening the coil fasteners, placing the appropriate feeler gauge or business card between the coil and the flywheel, and pushing and holding the coil against the flywheel whilst tightening the fasteners.
I’ve covered the whole process here – Coil air gap adjusting.
Failed Ignition Coil
You already know these guys give some trouble; it’s not so much that they are troublesome, it’s that they work hard under huge stress; it’s kind of expected as a swa gets older.
How to diagnose? To diagnose, we need to do an ignition system test, preferably using an inline spark plug tester which I cover next.
Ignition System Test
To test the ignition system, we’ll need to use the inline tester, and you’ll find one here on the small tools page.
The test is as follows:
- Remove the plug & inspect.
- Ignition On.
- Fit plug wire to the inline tester.
- Ground the inline tester.
- Pull over the engine and check the window for spark.
A bright blue spark should be evident; anything less than a bright blue spark likely means the coil has failed, and you can see how to replace a chainsaw coil right here.
For a saw that starts and runs intermittently, fit the spark plug and ground the inline tester on the plug; fitting it this way allows us to run the saw and diagnose spark intensity while running.
Here’s a video of the In-line spark ignition system testing (It’s a mower, but the process is identical).
Ignition Switch Fault
Coil voltage always looks for the shortest path to ground.
The ignition switch is set to Kill; it offers the coil a shorter path to the ground and, in doing so, diverts voltage away from the spark plug, and the engine shuts down.
But a faulty ignition switch that fails to disconnect the coil from the ground won’t allow the voltage to reach the spark plug.
While On/Off switches are simple, wiring can come loose, ground the coil, and divert that critical voltage away from the spark plug.
How to diagnose? A visual is usually enough to tell if there’s a problem, but if the switch wire snakes around the machine, removing the ground wire from the coil and spark testing, as previously covered, is the fastest way to eliminate this as a possibility.
If you now have a spark after bypassing the switch system, you know it’s a chafing wire or grounding switch issue.
How to fix? Locate the chafing wire repair and secure it.
Other Possible Causes
There are some other possible causes for no spark, and while they are, they are worth mentioning. Here they are:
Damaged flywheel – a missing flywheel magnet or otherwise damaged flywheel will interfere with the ignition system. So too, will a sheared flywheel key, a spark may still be made, but it’s delivered to the plug at the wrong time.
Dirty spark plugs, incorrect spark plug gaps, wet plugs, and faulty coils are typical causes of no spark. Check the basics before diving into more complex diagnostics.
- 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.