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Tutorial #21: Replace the spark plugs on your motorcycle

A motorcycle engine is like true love: if there isn't that little spark at the start, it will never work... And in petrol engines, no spark without a spark plug! This essential part of the ignition circuit therefore deserves your full attention. Replacing the spark plugs, an operation to be carried out periodically, and for that I explain everything you need to know in this video.

But before getting to the heart of the matter, a little technical point on candles. 2 or 4-stroke engine, same fight: the operation is the same, even if the specificities of the spark plug vary. Depending on the architecture of your engine, there are one or two per cylinder, screwed into the cylinder head.

You know its role: convert the high voltage discharge sent by the alternator into an electric arc to trigger the explosion of the air/fuel mixture. This is the famous spark. This must be sufficiently powerful so that combustion is as complete as possible. With maximum efficiency and minimal pollution.

And for this, your candles must be in good condition. Understand not worn, clean and correctly adjusted. As you can imagine, the only way to get a glimpse of them is to take them apart!

The necessary tools

To replace your spark plugs, you will need the following equipment:

Disassembly: Everything that gets in the way, including the suppressor

The operation is carried out cold, otherwise there is a risk of thread breakage – a matter of the coefficient of expansion of the metals, which is different between the engine cylinder head and the metal base of the spark plug.

Beforehand, you must dismantle everything that hinders access to your candles. I say "your", but if your engine is a single cylinder, it will most likely only have one spark plug. Nothing unusual! Depending on the motorcycle, remove what is blocking access to the suppressor: fairings, various covers, air box, fuel tank, fuel rail… What will take 5 minutes of preparation on a BMW flat will take you several dozen on a Japanese 4-cylinder.

Everything is ready ! All you have to do is remove the anti-parasite. Here too, we come across different configurations: on the most modern machines, it is imposing since it integrates the reel. And if so, you may need a specific accessory to remove it without damage. Don't panic: in general, these extraction pliers are provided by the manufacturer in the tool kit! On older engines, removal is super simple, and done by hand.

That's it, you see the candles! Grab your best blowgun, and remove any debris lodged in the spark plug well. These little tricksters are likely to fall into the cylinder when you remove the spark plug. And a little stone in the engine, it's a joke that doesn't make you laugh for long...

Candle after candle!

That's it, we dismantle the first candle. Proceed candle by candle for the disassembly and inspection steps, this way you will see if it is wrong more on one cylinder than the others without risking an error.

Here again, the simplest thing is to call on the spark plug wrench provided in your motorcycle's repair kit, with the appropriate extension if necessary. This way, you are sure that it is the right impression, with the right depth, and you avoid any risk of breakage!

In short, fit the key on the spark plug, and loosen it without rushing it. Then take out the candle gently. Smell that good smell of mechanics, quite a poem!

Visually check the condition of the spark plug

We can now carry out the visual inspection. The spark plug electrodes, which protrude into the cylinder, must be chocolate color. This is a sign of optimal air/fuel mixture adjustment. On the contrary, if they are whitened, it is because the mixture is too lean and conversely if they are blackened, it is because it is too rich (in essence, eh). In principle, if your motorcycle is equipped with injection, you don't have to worry too much about the issue: the electronics regulate the mixture carefully. It's another thing if your mill is powered by carbs. In this case, take this information into account later, when it is time to adjust your carburetors!

While you are there, also check the condition of the electrodes, i.e. their regularity of appearance: they must have sharp edges, and above all must not be gnawed, sign of advanced wear. If necessary, it is high time to replace the spark plugs, because they burn less well and wear out the ignition circuit!

Also monitor electrode spacing using a set of shims, depending on what the manufacturer of your spark plugs recommends for the model in question. For example, at NGK , the last digit of the reference indicates the spacing in tenths of a millimeter.

If necessary, adjust the gap with a small flathead screwdriver, by prying on the base. Too far apart? Gently tap the ground electrode until the desired result is achieved.

A quick brush with a wire brush to clean it all up, and off we go!

Replacing the spark plugs on your motorcycle: Which reference to choose?

If the spark plug is at the end of its life, it must be replaced. Follow the advice of the motorcycle manufacturer in the mileage maintenance booklet – between 10 and 25,000 km depending on the bike. The same goes for the candle model: we call on the reference suggested by the manufacturer. The candles are designed for precise temperature ranges, and have many specificities: length and diameter of the base, shape and number of electrodes, etc. Of course, the spark plugs must be replaced at the same time, and not individually.

Are the spark plugs new? Make sure that the model is identical to the one in place previously. If necessary, replace the aluminum connection tip on the head of the new spark plugs and the sealing ring on the base.

Come on, let’s put it all back together! Apply a little copper grease on the thread of the base. Engage the spark plugs in the screw threads. They must be very straight. If it engages at an angle and you tighten like hell, you'll screw up the threads. Needless to say, this is not desirable! Tighten to torque, or by hand with the spark plug wrench.

Replace the suppressors, then everything you dismantled beforehand and crack your engine to hear it purr like the premier day. Oh, love…

I hope this little tutorial will help you know how to replace the spark plugs on your motorcycle, or their routine check. Feel free to comment with any questions!

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Editor and tester for Motoblouz , I am an unconditional fan of roads with bends. For me, the motorcycle is a means of escape as well as a means of transport.

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