Spark Plug Removal, Inspection and Replacement

Plug replacement is straightforward once access issues are out of the way. With most modern inline engines, you’ll certainly have to remove the tank and probably lift the airbox out of the way. For tools, you’ll need whatever it takes to get the tank (and seat, if necessary) off the bike, plus a specialized plug socket.
Your next task is to remove the spark-plug lead, but first you should clean the area with a towel or compressed air. These days, the spark plug lives in a deep well just asking to collect junk. Carefully twist the lead until it breaks free from the plug, and pull it straight up and out. Take a moment to inspect the plug cap for cracks and abrasions. For bikes with coil-on-plug ignition, carefully remove the electrical connector to the coil and gently lift the coil up as you twist. Some models may have the coil screwed to the cam cover, and if so, remove this fastener first.
Shine a flashlight into the spark-plug cavity to look for debris and the like; if you see any, blow it out now. Slide the socket into the recess and be sure it is securely on the spark-plug hex by gently rotating it. Loosen the spark plug. Note: It should not be terribly easy or hard to remove, and should absolutely not be loose already. Lift the plug out and examine it. Your spark plugs can tell you alot about how your bike is running, if its running too rich but still firing, you will have a black powdery coating.

Too rich

Oil fouled spark plugs indicate a serious engine oil control problem probably not compression rings but oil control rings or valve guides and seals. This coating is conductive and will cause a misfire. This may be observed as fuel and can be dried off if this is the case you have an ignition fault and the plug is not firing.

Oil fouled

Overheating of the spark plug will result in a glazed or glassy finish. Overheating is due to spark plug that is too high in temperature rating for the compression ratio that the motor operates at, Blocked exhaust, muffler or catalytic converter. Also over advanced ignition timing or cooling system too hot.

Overheating

Fouled spark plug indicates a low cylinder compression this leaves unburnt residues on the plug due to a lack of cylinder efficiency such as oil and lead where used. This may lead to running on as the plug heats up and is capable of combustion alone with no spark.

Low compression

A plug in good condition at correct mixtures and timing will show a good efficient burn at the correct temperature and little wear.

Good

Installation is straightforward, but take your time. Check the electrode gap (Look in owners handbook for correct gap,or a manual if you have one). Slip the plug back down into the well and begin threading only by hand. Aluminum heads have fragile threads–and the spark plug is steel– so what’s going to give is the expensive part if you cross-thread it. Cinch it down by hand and torque to spec. Even if you use your torque wrench rarely, this is the time. Follow the service manual recommendations, but in its absence, torque spark plugs with 8mm threads to 6-7 foot-pounds; 10mm threads to 7-9 foot-pounds; 12mm threads to 11-14 footpounds.
Take your time putting it all back together and be certain that you have the plug wires going to the appropriate plugs and that they seat fully. Listen for the click, click, click as the clip locks to the threaded tip. For individual-coil models, it doesn’t matter which coils go where–they’re identical–and, usually, the wiring harness is built so that it’ll only fit one way.