The Yamaha Royal Star is outfitted with Yamaha's 1294cc V4 engine. On the V4, there are two spark plugs per cylinder, for a total of eight plugs, with four being accessible behind and under the fuel tank, and the other four being mounted behind the fairing, accessible with a socket extension on your ratchet. During routine maintenance, all spark plugs should be replaced all at once. If a single spark plug has been damaged, however, just the one spark plug can be replaced without having to remove the rest of them.
Remove the spark plug boot from the spark plug you wish to remove. The spark plug boot is black rubber on the Yamaha Royal Star, located at the top of each cylinder. It is removed by simply pulling the boot straight off of the spark plug. You will hear an audible "pop" when the spark plug boot disengages from the spark plug head.
Insert the spark plug socket onto the spark plug, then the eight inch socket extension, then the socket wrench. Loosen the spark plug by turning the wrench in a counterclockwise motion, being careful not to hit the motorcycle fuel tank with the handle of the socket wrench. The spark plug will stick a little at first, but then will come out easily. Avoid allowing anything to drop into the spark plug hole while the plug is removed.
Insert the head of the new spark plug into the rubber o-ring inside the spark plug socket, then apply a small amount of dielectric grease to the threads of the spark plug to help prevent cross-threading. Gently thread the new spark plug into the spark plug hole until it is finger tight. Cross threading will be indicated by the spark plug's resistance to being threaded after just one or two turns of the spark plug. If this happens, remove the plug and adjust the angle of the plug when inserted into the hole.
Thread the spark plug into place, then tighten it down gently, not applying too much pressure. Reinstall the spark plug boot by simply pushing it down over the spark plug head until it snaps into place.
Change all the spark plugs at once when performing maintenance service to the motorcycle.
Do not attempt to force a cross-threaded spark plug into the cylinder. With the angle off, misfires of the cylinder and loss of compression are practically guaranteed.