How to Check Motorcycle Spark Plugs

Your bike’s trying to tell you something. Hard starting, a flat spot, a lack of power, or an engine that just doesn’t sound quite right can all point to a spark plug worth having a look at.

Spark plugs are small, simple and easy to overlook, but they’re one of the best little diagnostic tools you’ve got. A quick inspection can tell you a fair bit about how your engine is running — and sometimes point you towards a bigger problem.

What you’ll need to check your spark plugs

• Spark plug wrench

• Small non-metallic brush

• Feeler gauge or spark plug gapping tool

• Your bike’s service manual

Getting the spark plugs out

Start by finding the spark plug caps. They’ll be sitting over the top of the cylinder head, with a rubber boot connecting each plug to the ignition lead.

If you’re working on a multi-cylinder bike, pay attention to which plug came from which cylinder. Keep them in order. That way, if one plug looks different from the others, you’ll know which cylinder might be causing trouble.

Pull the caps straight off the plugs. Don’t yank them around by the ignition lead.

Now grab your spark plug wrench and carefully undo each plug.

A lot of bikes came with a plug wrench in the factory tool kit, usually tucked away under the seat. A flexible or articulated plug wrench can be handy when access is tight, while a deep spark plug socket with a rubber insert makes removing the plug much easier once it’s undone.

Spark plugs come in a range of hex sizes, commonly from around 10 mm through to 21 mm, so make sure you’re using the right one for your bike. If you’re not sure what you need, check the manual or ask one of the volunteers at Machine Alley.

Read what the spark plug is telling you

Once the plugs are out, line them up on the bench and have a good look.

On a multi-cylinder bike, comparing the plugs is particularly useful. If they all look roughly the same, that’s a good sign. If one looks dramatically different from the others, however, you’ve got a clue that something may be going on with that cylinder.

A plug can tell you about fuelling, ignition and even potential engine problems such as oil getting into the combustion chamber.

A healthy plug

A healthy spark plug will generally have a light tan or brown appearance around the electrode and insulator. It should be relatively clean, with no major deposits or damage.

If your plugs look good, the gaps are correct and there’s no obvious damage, there may be nothing more to do. Give them a quick check and put them back in.

A damaged plug

If the electrode is badly worn, rounded off, squashed or broken, or if the ceramic insulator is cracked, don’t muck around with it.

Chuck it and fit a new one.

Spark plugs aren’t expensive enough to justify gambling on a damaged one.

Black and sooty

A plug covered in dry, black carbon deposits can indicate the engine is running rich — in other words, there’s too much fuel in the mixture.

If all the plugs look like this, it could point towards a fuelling issue affecting the whole engine. If only one cylinder’s plug is sooty, you’ve got a more specific clue to investigate.

White and ashy

White or very light, ashy deposits can indicate the engine is running lean, meaning there isn’t enough fuel in the mixture.

Again, look at all the plugs together. One odd-looking plug can tell you considerably more than four identical ones.

Wet with oil

If one plug is noticeably wet or oily while the others look normal, don’t just clean it and forget about it.

Oil where it shouldn’t be can point towards an engine issue that needs a closer look. The plug is doing its job here — it’s giving you a clue.

Cleaning the plugs

If the plug isn’t damaged but has built-up deposits, you can give it a clean.

Use a small, non-metallic brush to gently remove the deposits.

Avoid aggressive wire brushes, grinding wheels or anything else that’s going to attack the electrode. The components at the business end of a spark plug are tougher than they look, but they’re not indestructible.

And remember: cleaning a plug doesn’t fix whatever caused the deposits in the first place. It just gives you a clean starting point.

Check the gap

With the plugs clean, check the electrode gap against the specification in your service manual.

The gap is the distance between the centre electrode and the ground electrode. It needs to be right for the ignition system and engine to work properly.

Don’t guess it.

A feeler gauge will do the job, while a dedicated spark plug gapping tool makes checking and adjusting the gap straightforward.

If the gap needs adjusting, use the proper tool and make small adjustments rather than bending the electrode back and forth like you’re trying to open a tin of paint.

And remember: not all spark plugs are interchangeable. If you’re replacing them, use the plug type and specification recommended for your bike.

Check for spark

Before you screw everything back into the cylinder head, you can do a quick spark test.

Reconnect the plug to its cap and hold the metal body of the plug firmly against a suitable bare metal part of the engine to provide a ground. Hit the starter or kick the engine over.

You should see a clear spark jumping the gap between the centre electrode and the ground electrode.

No spark? That gives you another direction to investigate — ignition wiring, the plug cap, high-voltage lead, coil or other parts of the ignition system could be involved.

Be careful around the high-voltage ignition system while doing this test. Keep your hands clear of the plug and exposed electrical connections.

Back in they go

Once you’re happy with the plugs, refit them in their original cylinder positions.

Start them by hand to avoid cross-threading, then tighten them to the torque specified in your service manual. Refit the plug caps securely and make sure everything is seated properly.

And that’s it.

A few minutes spent looking at your spark plugs can tell you a surprising amount about what’s happening inside your engine.

If the plugs have pointed you towards a bigger problem, that’s where the fun really starts. From here you might move on to checking the air filter, valve clearances, carburettors, ignition system or fuel supply.

Your bike usually gives you clues. You just need to know where to look.