Spark plugs are a regular maintenance item that have a definite effect on the performance and response of your engine. The Honda maintenance manual calls for the replacement of spark plugs at 8,000 mile intervals - I make a habit of changing mine as part of my winter maintenance routine. Fortunately, GL1500 spark plugs are cheap: Cyclemax sells a set of the proper 6 NGK plugs for $15. Unfortunately, unlike the four-cylinder Wings, there is a bit of disassembly involved to get to the spark plugs.
1. The first step is to take off the lower cowls. If you have any accessories that need to be removed before you can remove the lower cowls, do so now. In my case, I have a set of wind wings that I need to remove.
2. I also have a set of air horns that need to be disconnected, loosened, and pivoted out of the way.
3. To remove the fairing front cover, press inward at the center of the top of the cover. This will expose the posts seated in the grommets at the top corners.
4. Gently pull the top corners away, one at a time, to unseat the posts from their grommets. Note the black tab that is holding the front of the lower cowl in place - it's important that the lower cowl is fitted back behind these tabs when it is replaced.
5. Once both posts are unseated, remove the front cover.
6. Remove the screws on either side of the under cover.
7. While holding the under cover in place, remove the center screw.
8. Pull the under cover away.
9. Gently pull the inner edge of the lower cowl screw cover away from the screw head.
10. Be very careful removing these covers - pulling them back too far will snap the tab off of the outer edge. See how the tab hooks into the cowl. Gently rotate the inner edge back, then pull it straight out.
11. Remove the lower cowl screw.
12. Gently remove the lower cowl deflector. It pulls out approximately one inch at the bottom, then is pulled straight down to disengage the top tab, then is pulled straight back to disengage the front tab.
13. This is the top tab that needs to be disengaged.
14. And this is the front tab.
15. The side marker light trim needs to be removed next. OEM Honda trim is black plastic and looks different than my aftermarket chrome trim. My trim has screw caps that must be removed to access the screw heads.
16. Remove the three screws holding the trim and the marker light in place.
17. Once the screws and marker light are free, the lower cowl will easily pull away. Disconnect the cornering lamp wire and remove the lower cowl. Repeat steps 9 through 17 for the other side of the motorcycle.
Replacing The Plugs
18. It is good practice to replace each plug one at a time, instead of pulling the wires from all of the plugs and doing them at once. This keeps you from accidentally plugging the wrong wire onto the wrong plug, which at best will make your engine run terribly, and at worst can actually damage it. Gently pull the spark plug boot (NOT the wire) until it comes free of the spark plug.
19. It's essential that no dirt or foreign debris fall into the cylinder when the spark plug is removed. If you have an air compressor available, use it to blow the debris out of the spark plug well before removing the plug. You can also use a shop towel to wipe away grime or debris.
20. Using a spark plug socket (there should be one in your bike's tool kit), gently loosen the spark plug, then remove it from the engine. You might want to check out the "how to read spark plugs" section of How to remove, analyze, gap and replace your spark plugs to check on the health of your engine.
21. New spark plugs should be gapped to 0.031 to 0.035 inches - although it has been a long time since I saw new plugs from the manufacturer that weren't already gapped correctly.
22. Use a gapping tool to adjust the gap if necessary.
23. Always apply anti-seize to the threads of the plug before installing it. This will keep the steel plug from seizing in the aluminum head of the engine.
24. Rub the anti-seize around the circumference of the threads.
25. Insert the spark plug into the well and start threading it in by hand until you are sure it is not cross-threaded. Cross-threading a plug in an aluminum head has dire consequences!
26. Place the spark plug socket over the plug and continue to thread it in by hand until it is finger-tight.
27. Using a socket wrench, tighten the plug exactly 1/2 turn. This will compress the crush washer and correctly torque the plug. Note: This is for NEW plugs only - if you are replacing old plugs, the crush washer will have already been crushed, and tightening a full half turn past finger tight is a good way to break a plug off in the head. In this case, use a torque wrench and torque the plug to 12 ft-lb. However - being that a set of new plugs are quite cheap, I never bother to reuse them - once they come out, I always put new ones in.
28. Replace the spark plug boot over the plug and push it into place. Again make sure you put the correct boot over the correct plug!
29. Place the lower cowl back into position. Ensure the innermost tab (with the hole in it) fits behind the black tab protruding down from the radiator grill mentioned in step 4. This holds the front of the lower cowl in place.
30. Replace your side marker light and trim, and screw it into place.
31. Replace your screw caps (if you have them).
32. Replace the lower cowl deflector. Insert the front tab first, then the top tab, then slide the deflector to the left to line up the screw holes.
33. Tighten the lower cowl screw.
34. Replace the lower cowl screw cover - insert the tab first, then press fit it over the screw head.
35. At this point you would repeat the procedure to install the lower cowl on the other side of the bike. Once that is complete, lift the under cover into place.
36. Insert the center screw to hold the under cover in place.
37. Ensure the posts at the front, on the bottom of the lower cowls fit into the holes on the top of the under cover (visible at the front edge where the lower cowl and under cover meet) - on both sides! Once the posts are in place, replace and tighten the screws on either side.
38. Make sure the tabs at the bottom of the front cover fit behind the ridge of the under cover, and that the tab on the bottom center of the front cover fits into the slot in the under cover. Align the front cover, then press the posts on either side into their respective grommets.