1. Look for black specks on the porcelain insulator under the center electrode. This would be evidence of pinging or detonation.
2. Look at the arm of the ground electrode and note where the color change on it occurs. The further out toward the end of the electrode, the hotter the engine is running (indicating lean). If the arm is discolored all the way to the end, that cylinder may be running extremely hot.
3. Inspect the insulator for any signs of blistering. Blistering would indicate a dangerously lean mixture. A normal tan/yellow coloration should be observed on at least one point on the insulator where it faces the intake valve. This would indicate a correct mixture and burn rate.
4. Inspect carbon build up at the end of the plug threads where it is exposed to the cylinder. Some carbon should be found there, but it should not be caked up or wet.
The fuel injection system of the Wing does a very good job of keeping the mixture correct, so the plugs in this bike normally look good when pulled, so most of the time you don't have to worry about it. As long as you don't have a mouse nest in the airbox, or a bad sensor you should be ok. This bike does exhibit some pinging in lower gears under load, so you may see some black specks on the insulator, but unless you find it really heavily speckled, don't worry about it. If you're worried about pinging, just go up one octane grade in gas.
I've pulled plugs from literally hundreds of GL1800's and I've not seen any that were a cause for alarm. But I do recommend regularly replacing them because the electrodes will round off and they will go out of gap. Worn plugs will make the secondary electrical system work harder which stresses the coils and plug wires, and it will make the bike harder to start which puts added strain on the battery and starter, and it can also cause a slight drop in gas mileage. As cheap as plugs are, if you have removed them, you should just go ahead and replace them.
