How to: Check and change your spark plug

How to check and change your spark plug, arguably one of the most important components on your bike

A motorcycle spark plug that needs changing

by Jim Blackstock |

Unless you own a diesel motorcycle, then the spark plug in your petrol engine is probably one of the most key components if you want to, well, use your bike. Of course, there are others – tyres, brakes and so on – but without a spark plug or if yours is failing, you are going nowhere - not even out of the garage.

Unlike the kind of vehicles that benefit from a diesel (self-ignition) engine, such as anything heavy that needs an engine to produce vast amounts of torque (the actual work the engine does), motorcycles use spark-ignition petrol engines. This is because they are generally lighter and don’t need so much torque - they benefit instead from power (a numerical property only, the rate at which work is done).

A spark-ignition engine needs a spark to get it going and this is the job of your spark plug or plugs, depending on the design of your engine. Looking after your spark plug is easy, as is changing it when it eventually wears or deteriorates, though getting to the plugs can be more of a task.

Related: Garage essentials for a basic motorcycle toolkit

If you are lucky, then your plug or plugs are hanging out of the side of the engine – sometimes the case with a single or a transverse twin or a flat twin. However, for V-twins using a traditional north/south orientation or inline motors (twins, triples, four or more) then plugs can be hidden behind radiators or underneath fuel tanks, meaning a degree of disassembly of the bike to access them.

Here, we are looking at checking and changing the plug itself. As there are so many ways to access the plug, you should familiarise yourself with the actual method for your bike in the owner's or workshop manual. Once you have done that and have sight of the plugs, you can join us again.

Here’s how you check and change your spark plug.

Step 1:

Spark plug location
©Photo: Bauer Media

The first thing to do is locate your plug or plugs. On our Royal Enfield Himalayan (a 411cc single-cylinder motor), it is on the right-hand side of the cylinder head. The inlet is on the rear, the exhaust on the front and the plug, on the right.

Step 2:

HT lead removal
©Photo: Bauer Media

Now you need to remove the spark-plug lead. This will generally have a rigid plastic section and probably, like ours here, a rubber seal to prevent water from getting into the electrics. At the risk of teaching you to suck eggs, never go near this with the engine running – if the spark charge gets you, it WILL hurt. With the engine off, try to pull the plug lead off the plug.

Step 3:

Using a screwdriver to remove a stubborn HT lead
©Photo: Bauer Media

The chances are that, depending on access, you may not be able to get the lead cap off by hand. If not, you could try gently levering the bottom of the cap up with a screwdriver, or...

Step 4:

Using pliers to remove a stubborn HT lead
©Photo: Bauer Media

...with a pair of pliers. If you can get access, you could also try gently gripping the plug cap with the pliers and lifting the body off of the plug. Try not to pull on the lead itself, as it may come out of the plug cap.

Step 5:

Exposed spark plug top
©Photo: Bauer Media

Once you have released the plug cap, pull it off the top of the plug and move it out of the way. In our case, we had to remove the rubber seal, then lift the plastic cap off, then re-fit the rubber seal to the plastic cap.

Step 6:

Using a deep socket to remove a spark plug
©Photo: Bauer Media

Now fit your spark-plug tool to the flats of the plug. You may have a tool as part of your bike’s toolkit, but I prefer the solidity of a proper tool for the job. This is from a Gedore socket set, that comes with three spark-plug sockets, depending on their size. Loosen the plug with the socket – it shouldn’t be too tight.

Step 7:

Unscrewing a loosened spark plug by hand
©Photo: Bauer Media

Once you have released the plug, it should turn freely in the thread in the cylinder head. If it doesn’t, then you need to be careful when finally removing it, to make sure that no debris falls into the engine. Might be worth at this stage screwing the plug back in and having a good clean around its seat before you go any further.

Step 8:

Unscrewing the final turns with the socket removed
©Photo: Bauer Media

If the plug is in a tight spot, like it is here, with limited access for chubby fingers, then you should be able to remove the socket and unscrew the plug with your fingers. A healthy thread in the head should allow you to do this. If not, again have a clean around.

TIP: A short length of rubber hose fitting tightly over the porcelain of the spark-plug body can help, by allowing you to turn it to unthread the plug further away from the confines of the tight space.

Step 9:

Removing the spark plug from the engine
©Photo: Bauer Media

Once you have unscrewed the plug completely, remove it. If you are just cleaning and replacing it or replacing it with a new plug, don’t worry unduly about covering the hole in the cylinder head. However, if you are leaving it for a bit, you might want to pop some rolled-up paper loosely into the plug hole to prevent anything from falling into the motor.

Step 10:

A slightly blackened spark plug
©Photo: Bauer Media

Once out, have a look at the plug, as it can tell you quite a bit about your engine. This one has slight blackening but that’s normal – a grey-ish finish is fine. If it’s very black, that can mean the engine is rich. If it looks very white, this could indicate it is lean. If it is oily, that’s generally not good.

Step 11:

Cleaning a spark plug contact with a wire brush
©Photo: Bauer Media

On a plug like this, which is in decent condition, then a quick clean-up with a wire brush is all that’s needed. Just clean around the end, the electrodes (between which the spark is created) and the threads to make sure it’s in tip-top shape. Obviously, if you’re fitting a new plug, then skip this and the preceding step.

Step 12:

Cleaned spark plug
©Photo: Bauer Media

This plug is now clean and ready to have its gap checked before refitting.

Step 13:

Feeler gauges for checking spark plug gap
©Photo: Bauer Media

To check the spark-plug gap (to make sure the spark generated is the right strength), you need a set of feeler gauges. Check the prescribed gap (0.7mm in our case) and find the relevant blade in the set of gauges.

Step 14:

Measuring spark plug gap
©Photo: Bauer Media

Slide the blade in between the two electrodes. If you can’t get it in, the gap is too small and use the blade to open it up slightly - the 90° electrode is soft enough that you can do this safely. If the gap is too large and the blade rattles around, then gently tap the electrode on something solid to close it up slightly. The blade should fit in the gap snugly and not wiggle around. Repeat steps 10-14 for each of the plugs that the engine uses.

Step 15:

Replacing the spark plug
©Photo: Bauer Media

Once you’re happy with the gap, it’s time to replace the clean and gapped plug(s). When re-fitting the plug, always ALWAYS start by hand to make sure you get the thread started properly. If it won’t start or goes tight, stop and start again. A steel plug into an aluminium head is a potential recipe for disaster, cross-threading the head and if you do, you’re in all sorts of trouble. Use the rubber hose to extend so you can thread it in as you did when removing the plug.

Step 16:

Re-tightening spark plug
©Photo: Bauer Media

You should be able to run the plug most of the way back into its threaded hole by hand. When you get to the end, you will need to tighten it. Check your workshop manual for how to do this – it should either give a torque figure or a procedure to get the right tightness.

Step 17:

Replacing HT lead
©Photo: Bauer Media

Then refit the spark-plug lead and cap. You should feel the cap locate on the threaded portion of the top of the spark plug (some engines use a ferrule screwed on to this but most motorcycles use the bare, threaded section) so push it down as far as it will go. That’s it – job done.

Step 18:

Start your engines
©Photo: Bauer Media

Just a quick fire-up of the engine to make sure it runs as it should do.

Step 19:

Check the idle and running of the engine
©Photo: Bauer Media

Make sure the engine idles smoothly and sounds good and that’s it – your plug is good to go until the next prescribed maintenance check.

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