Get a bright spark
Bike running badly? Tired plugs could be the problem
1 Old versus new
A spark plug will last for thousands of miles but over that time its performance will decay. This is because, like most bike parts, spark plugs wear out; the gap between the electrodes becomes wider and the angular edges of the electrodes get rounded off. This means the plug needs more voltage to spark, and the overall efficiency of combustion can suffer.
2 How clean are your plugs?
Spark plugs can become fouled (ie their electrode gets excessively oily or sooty) at this time of year when you’re perhaps regularly starting up your bike in lieu of riding it, meaning that the rich air/fuel mixture never gets a chance to burn off. Symptoms are a low rpm misfire or an engine not running on all cylinders. Often this will clear once the bike gets ridden.
3 Check the mileage
Plugs are the only serviceable item on a modern bike’s ignition system, and their lifespan is usually listed in the service schedule. Typical multicylinder bikes need them changed every 7500 miles or two years. Remember, even if your plugs look OK at the service mileage they will be worn past their peak and running them any longer will see performance deteriorate.
4 Get access
The spark plugs are located on top of your engine’s cylinder head and in order for you to gain access to them most bikes will require the removal of the seat, fuel tank and or airbox. You’ll also more than likely need to whip off a few fairing panels too. This Suzuki GSX650F needs to have a coolant filler cap bracket removed to aid access.
5 Free the cables
Shine a torch across the cam cover and see if any of the HT leads or coil connectors are numbered. This is important as they must go back the same way. Four cylinder bikes are numbered 1-4 from left to right, when viewed from the riding position. If they’re not numbered it’s worth making a note of which went where prior to removal.
6 Drive out the dirt
The plug/coil caps on most bikes have a broad rubber flange, this acts as a seal to prevent water and dirt getting down the plug hole. With the plug caps removed make sure that they are in good condition and are not showing signs of decay. Clean with a cloth or brush until all dirt and dust has been removed.
7 Socket to ’em
Always use the correct size and length of spark plug socket. A proper plug socket has a rubber ring inside that grips the top of the plug and pulls it out. If you don’t have a plug socket you can sometimes make do using the correct size 3/8in-drive long-reach socket, and then use the plug cap/coil to remove the loose plug.
8 Mind the gap
Check that the new spark plugs are of the correct variety – reference them against the old ones if necessary. Then, using either a feeler gauge or gap measuring tool, check the gap between the electrodes, for example this Suzuki GSX650F should have a gap that’s between 0.70mm and 0.80mm.
9 If the cap fits…
Fitments vary slightly. Some have a removable terminal fitted to the threaded section, which is necessary on some applications and plug caps, but not others. Reference against your old plugs and consult your workshop manual to check what your bike should have, or else you will find that the plug cap will not fit correctly.
10 Torque isn’t cheap 10
Install the spark plugs and tighten them up evenly by hand at first. Next use a torque wrench set to the correct value as listed in your manual. It’s worth pointing out that spark plugs only need around 11Nm, so they can easily be overtightened if done up without a torque wrench, causing damage to the threads in the cylinder head.