1. First, gently remove the spark plug wires from the stays.
2. Gently pull upwards on the protective rubber boot - NOT on the plug wires to remove the boot and hoods from the plugs.
3. Pull the boot away from the plugs and push it off to the side.
4. I like to use a blast of compressed air to clean out the spark plug wells. This prevents any dirt or debris that may have collected in the wells from falling into the engine when the plug is removed. It is VERY important that nothing fall into the engine - severe damage can occur to the pistons and cylinders if unwanted debris falls into it! It's also a good idea to clean out the well drains - you can see the small holes just below each plug. This is to allow rainwater to drain away out of the well. If the wells are clogged, rainwater can collect in the wells and short out the plugs, causing the engine to miss or stall.
5. Using an appropriately-sized spark plug socket and a ratchet, or the spark plug tool in the motorcycle's tool kit, loosen the plug from the engine, turning counter-clockwise.
6. Once the plug has loosened, remove it the rest of the way by hand. The threads in the soft aluminum engine head are EXTREMELY soft and easily damaged, so I like to do most of the work with the plugs by hand to give a better feel.
7. Once the plugs are removed from the engine, set them on a clean surface so that they can be analyzed.
8. These plugs, from my engine, are in reasonably good shape after 3,500 miles. The side electrode (curved part) and center electrode (metal part on top of the white insulator) should be a light brown or grayish color as shown here. Quite a bit can be diagnosed about the health and condition of the engine from the appearance of the spark plugs - see the end of the article for details on "reading" spark plugs.
9. The new spark plug comes with a the electrodes gapped to a specific size. This may or may not be the correct size for your bike, so you need to "gap" the plugs before installing them.
10. Using a gapping tool, push the appropriate size wire between the electrodes. You should not have to push the tool hard to get it through - it should slide between the electrodes easily, but there should be no space for the gap tool to move when it is between the electrodes.
GL1000 (all years): 0.6 to 0.7 mm (0.024 to 0.028 inches)
GL1100 1980-1981: 0.6 to 0.7 mm (0.024 to 0.028 inches)
GL1100 1982-1983: 0.8 to 0.9 mm (0.031 to 0.035 inches)
GL1200 (all years): 0.8 to 0.9 mm (0.031 to 0.035 inches)
GL1500 (all years): 0.8 to 0.9 mm (0.031 to 0.035 inches)
GL1800 (all years): 1.0 - 1.1 mm (0.039 - 0.043 inches)
11. Use the gapping tool to gently bend the side electrode closer or farther away from the center electrode until the gap is perfect.
12. Most spark plug sockets have a rubber interior that will grip the plug tightly so that it won't fall out. Spark plugs have porcelain insulators, and if dropped on a hard surface, this insulator will crack, causing the plug to fail. Any spark plug that has been dropped must be scrapped! Be careful not to drop your plugs!
13. Place the plug securely into the plug socket.
14. Spark plugs are subjected to tremendous heat, and changes in temperature, and they will have a tendency to seize in the head of the engine. This is bad, as the soft aluminum threads can be damaged trying to extract the plug. For this reason, it's important to use high-temperature anti-seize compound on the plug threads before inserting it into the engine.
15. Apply the anti-seize compound to all of the threads of the spark plug.
16. Gently insert the new plug into the engine, being careful not to bump the side electrode. If you do bump it, you should re-gap the plug and try again.
17. Once the plug is in the well, I like to start threading it in by hand, to be absolutely sure that it is not going in cross-threaded. Because of the soft aluminum head, it is very easy to cross-thread a spark plug. If you feel any resistance at all, back the plug out and try again. There should be NO resistance when threading the plug in correctly.
18. Once you have threaded the plug in by hand several turns, put the spark plug tool on and continue to screw the plug gently into the head until it stops. This means the crush washer has contacted the engine. Once you get to this point, turn the spark plug tool exactly 1/2 turn, and remove the tool - the plug is now fully tightened. A plug that is tightened too tight can overheat and fail, or destroy the threads. Because the crush washer only crushes once, I do not re-use spark plugs in my bike. Honda does not publish a correct torque value for torquing plugs, so the only way to know that they are torqued correctly is to use new plugs, and to use the 1/2 turn method of tightening.
19. Once the new plugs are installed, replace the boots on top of them, and push into place to make sure they make contact correctly, and that the wells are sealed.
20. Gently reinsert the plug wires into the stays, one at a time. You're done!
How to read spark plugs
Normal
Brown to grayish-tan color and slight electrode wear. Correct heat range for engine and operating conditions.
Worn
Symptoms: Rounded electrodes with small amount of deposits on the firing end. Normal color. Causes hard starting in damp or cold weather and poor fuel economy. Plugs have been left in the engine too long. Replace with new plugs of the same heat range.
Carbon Deposits
Symptoms: Dry, sooty deposits indicating a rich mixture or weak ignition. Causes misfiring, hard starting and hesitation. Make sure the plug has the correct heat range. Check for a clogged air filter or problem in the fuel system. Also check for ignition system problems.
Ash Deposits
Symptoms: Light brown deposits encrusted on the side or center electrodes or both. Derived from oil and/or fuel additives. Excessive amounts may mask the spark, causing misfiring and hesitation during acceleration. If excessive deposits accumulate over a short time or low mileage, install new valve guide seals to prevent seepage of oil into the combustion chambers. Also try changing gasoline brands.
Oil Deposits
Symptoms: Oily coating caused by poor oil control. Oil is leaking past worn valve guides or piston rings into the combustion chamber. Causes hard starting, misfiring and hesitation. Blue smoke will be seen coming from exhaust. Correct the mechanical problem causing the condition and install new plugs.
Gap Bridging
Symptoms: Combustion deposits lodge between the electrodes. Heavy deposits accumulate and bridge the electrode gap. The plug ceases to fire, resulting in a dead cylinder. Remove the deposits from between the electrodes.
Too Hot
Symptoms: Blistered, white insulator, eroded electrode and absence of deposits. Results in shortened plug life. Check for the correct plug heat range, over-advanced ignition timing, lean fuel mixture, intake manifold vacuum leaks, sticking valves and insufficient engine cooling.
Preignition
Symptoms: Melted electrodes. Insulators are white, but may be dirty due to misfiring or flying debris in the combustion chamber. Can lead to engine damage. Check for the correct plug heat range, over-advanced ignition timing, lean fuel mixture, insufficient engine cooling and lack of lubrication.
High Speed Glazing
Symptoms: Insulator has yellowish, glazed appearance. Indicates that combustion chamber temperatures have risen suddenly during hard acceleration. Normal deposits melt to form a conductive coating. Causes misfiring at high speeds. Install new plugs. Consider using a colder plug if driving habits warrant.
Detonation
Symptoms: Insulators may be cracked or chipped. Improper gap setting techniques can also result in a fractured insulator tip. Can lead to piston damage. Make sure the fuel octane is high enough. Use care when setting the gaps on new plugs. Avoid lugging the engine.
Mechanical Damage
Symptoms: May be caused by a foreign object in the combustion chamber or the piston striking an plug that is too long. Causes a dead cylinder and could result in piston damage. Repair the damage and correct the cause.
Coolant Leak/Head Gasket
Symptoms: Coolant is entering the cylinder, either through a leaking/failed head gasket, or a cracked block. The coolant "steam cleans" the plug, so the plug appears clean and shiny.
You need an 18mm deep well socket.