Spark plugs need replacing at major services. They may also need cleaning and adjustment if you experience any rough running or misfires.
Modern bikes are miracles of packaging, squeezing everything into the most compact, lightweight form possible. Unfortunately, this means getting at things can be a knuckle-grazing nightmare.
Spark plugs are a case in point. On a typical four cylinder machine, the middle two are bound to be tucked well out of view under the tank. So you’ll have to remove the seat, tank and possibly also the fairing to get at them.
Even then, you might find they are buried under the airbox or squeezed tightly between the cam covers.
You’ll need the right sized plug spanner or socket and you may even need a special thin-walled spanner when space is really tight.
When taking out spark plugs, make sure the ignition is off. Take off the plug cap and unscrew the plug.
You can tell a lot about how a bike is running from looking at the spark plugs. On multi-cylinder bikes, they should all look the same. If one stands out, there’s an issue either with that cylinder or all the others.
Plugs should be dry and a nice coffee colour all over. If the plug is wet with oil, there could be a problem with a piston ring or valve seal. If it’s black and sooty, it’s running too rich. If white or blistered, it’s running lean. Anything melted spells pre-ignition and major trouble.
Use a set of feeler gauges to check the gap between the electrodes. The correct gap for your bike will be in the owner’s manual. If you need to adjust it, do not lever against the spark plug tip. You can close the gap by pushing the underslung electrode against a wooden surface but to lever it out you’re best off using a proper gapping tool.
When the correct size feeler gauge slides though the gap with a bit of drag, the gap is set.
Putting the plugs back in is pretty much the reverse of taking them out. The one exception is knowing how much you should tighten them.
There should be a torque setting in your owner’s manual but sometimes the job is so fiddly you struggle to apply a torque wrench. Other times, you might be doing the job at the road side. So...
If you’re fitting a new plug, with an uncompressed sealing washer, screw the plug in by hand until it’s seated against the head. Then, using the plug spanner, give it just over a quarter of a turn to tighten. If you’re refitting an old plug with a compressed washer, make it just over an eighth of a turn.
