Get your spark plugs firing

Ten steps to checking and changing your bike’s plugs

1 Choose the right plug

Plugs might all look the same, but the devil is very much in the detail. All petrol engines use one or more spark plugs to ignite the fuel/air mix and when you consider the number of times a minute a cylinder fires it’s no surprise they have a hard life. Your manual will let you know how often you should change the plugs and will give you the spec on which type to fit.

Examine the old plug 3

If you’re changing the plug because of poor running or starting issues give it a thorough inspection; a plug that is showing signs of carbon build up and fouling is usually the cause of poor performance. Spark plugs are relatively cheap to replace, so trying to clean up a fouled plug is a waste of time unless you are attempting a roadside repair.

Remove the plugs 2

Make sure you have enough room for a plug socket and ratchet. Depending on your bike, you may have to remove some bodywork and even the tank. Take the plug cap off and move it to one side, then use a dedicated socket to remove the plug. These are extra deep and have a rubber insert which helps extract the plug from the engine when it’s undone.

Mind the gap 4

Before you fit the new plug, the gap between the electrodes should be checked and adjusted. The gap will typically be somewhere between 0.6mm and 0.8mm depending on the model. If you don’t have a specific measuring gauge, use a feeler gauge. Adjustments can be made using a gapping tool, or by carefully using a screwdriver blade.

Not all plug caps are the same 5

Plug cap fitments vary slightly. If you look at the top of the plug, there is a threaded section which may or may not require the fitment of a boss. Check the plug that came out and see whether the boss was retained or discarded. If it is fitted, use a pair of pliers to make sure it is tight.

Can any water get in? 7

There should be two rubber grommets either end of the plug cap to make a watertight seal and these need to be in good condition with no signs of perishing or splits. If they need replacing, it’s usually quickest and easiest to buy a complete cap. When fitting a new one, make sure it is specific to your make and model of bike.

Installing the plug cap 9

Take the grommet off the top of the cap and slide it along the HT lead. Then, firmly grasp the HT lead and screw the cap onto the lead clockwise until it starts to tighten up. When it’s firm, slide the grommet back over the top of the cap, making sure it’s seated properly. Also, ensure that the plug cap is facing the correct direction when it’s fitted to the plug.

Install the new plug 6

Screw in the new plug clockwise by hand until it bottoms out on the compression washer. Then tighten the plug up with a torque wrench to the value specified in your manual, this figure can be anywhere between 12Nm and 20Nm, depending on the type of plug and model of bike.

Need to fit new caps? 8

You can remove the plug cap from the HT lead by unscrewing it anti-clockwise. but quite often, the very end of the HT lead will show signs of degrading. This can be taken care of by cutting back the lead. Using a pair of cutters, trim back around 5mm of HT lead. This will expose a fresh area for the new plug cap to screw into.

Don’t muddle the leads 10

Make sure that the HT leads are being put back on the correct spark plugs and cylinders or the engine won’t run. Quite often, they are marked with a number: one, two and so on. This corresponds to the plug/ cylinder that it fits with and is always done from left to right as you sit on the bike. The cap should go on with a reassuring click.