When I got my '85 Shadow 1100 in 1995, one of the things I noticed about it was the distinctive look of the two black and chrome horns under the large round headlight. What I didn't yet know was that only one of the horns (the high note) on my bike worked. This was annoying, but no big deal compared to some of the other used-bike problems I had to manage in the first months of ownership.
As I got more familiar with life in the Honda streetbike zone, I became aware that there were quite a few mid-80s Hondas that had this "face" in one variation or another, always with the same round black horns. I also became aware that Honda motorcycle parts from your friendly Honda dealer are among the most overpriced objects on earth, and got turned on to used parts suppliers and the aftermarket.
Then one day the other horn quit. I checked that power was getting to the horn terminals. I cleaned the terminals. I played with the adjusting screw (see below). No beep. I stopped in at my friendly local independent moto shop/junkyard (751 Motorsports in central North Carolina) to buy some stuff, and asked for used replacements. After about 20 minutes of rooting around in boxes, we found no less than ten dead black/chrome Honda horns of both pitches. Not one of the horns from donor bikes worked. Honda wanted something like $35 each for new ones, and they probably wouldn't last any longer than the originals did. After considering the price and looks of aftermarket horns, I decided to attempt a repair of the stock ones.
So, one chilly November day, I carried both horns inside and examined them to decide what to try. I found that the horns didn't draw any current, which indicated an internal open circuit. It was time to attempt surgery. A word of warning: Don't loosen that big nut on the back of the horn that holds it to the mounting bracket-- just leave the bracket on the horn. The stud that the nut is threaded onto is an adjustment and will turn. It's best not to mess with this. The only adjustment you want to play with is the little screw with a locknut off to the side of the case.
This normally isn't a big deal. Many horns (especially older cars horns) are held together with screws and come apart easily. They're simple devices, not much can go wrong with them, and when something does it's pretty easy to fix. But the horns Honda used on my bike (made by Mitsuba), while still being simple, are held together with a big crimp seal around the outside of the horn--the old "sealed for life" problem. So the challenge is unsealing the thing.
First, remove the chromed plastic trim ring around the outside of the horn. It just snaps off toward the front (diaphragm side) revealing the crimp ring that holds everything together. Now for the fun part: You have to somehow remove the crimp ring without destroying it or the horn. I took a small triangle file to mine and filed a radial groove through the crimp ring, carefully stopping when I hit another piece of metal. A Dremel tool would be perfect for this job. The only tricky part of the job is filing the front face of the crimp without notching or scratching the diaphragm. Don't be paranoid about getting a little more than just the crimp, but try to minimize any unnecessary metal removal.
At this point get out a big flat-blade screwdriver and pry between the crimp and resonator (that disk on the front with the silver button in the middle). If you successfully filed all the way through the crimp, the crimp will be pried off the edge of the joint. Continue around the resonator until the crimp is enough larger in diameter than the rest of the horn and can be slipped and wiggled off of the horn (note that the crimp is much easier to remove toward the front).
Once the crimp is removed, all that holds the horn together is a paper gasket between the diaphragm assembly and the case. Gently pry it apart at this joint, being careful not to put a permanent bend in the diaphragm, which is quite thin. The paper will tear. Don't worry about this--just make sure that when you reassemble everything it ends being the same thickness as before.
Having split the diaphragm from the case, you can now lift the diaphragm assembly from the case. You'll see that the diaphragm has a metal slug in the middle (which is riveted to the silver button on the front) with a piece of brown plastic around it. Looking in the case, you'll see a flat thing on one side by where the terminals come out of the back, and a coil down in the middle. You'll also probably see some rust and corrosion. This is why the horn quit working.
Theory time: In a working horn, when you push the horn button, you apply power to one of the terminals on the back of the horn. The other terminal is grounded. The current flows through a set of points (mounted on the flat thing in the case) and through the coil to ground. The current flowing through the coil generates a magnetic field which pulls the metal slug on the diaphragm toward the coil. When the slug (and the diaphragm) moves toward the coil, the plastic ring on the outside of the slug presses against a tang of metal which has one of the points on it. This pushes the one point away from its mate, and breaks the electrical connection. The magnetic field disappears, and the diaphragm springs back out, taking the slug with it. When the slug goes out far enough, the points close and the slug and diaphragm are again pulled toward the coil. This cycle repeats as long as you hold the horn button down. It all happens fast enough that it makes the "beeee..." noise we hear.
Now it's time to check and repair. All the stuff to check is in the case. If you've got one, you can use an ohm-meter to check resistance across the terminals, across the coil (check at the solder pad on the points and at the direct-connected terminal), and across the points. You want to see that it has a total resistance of no more than about 5 ohms between the terminals on the case. Both of mine came in at about 1.5 ohms in working order. If the coil is bad, you'll have to replace the horn unless you want to get into coil rewinding. What you'll probably find (or have to assume if you can't test) is that the points aren't conducting anymore because they've got corrosion on their contact faces. Unfortunately, the points are riveted in place and a points file won't fit in there. The cure is to take a small strip of fine-grit sandpaper (I used 600 grit) to the points, pulling it back and forth across them. Be sure to flip the sandpaper over so that the abrasive side gets to work on both points. You'll notice that the sandpaper turns a nasty brownish color with the stuff that's being rubbed off the points. When this stops getting worse and you've done both points, you're done. Recheck with the ohmmeter (if available). You should see that the points now conduct fine, and open circuit when you push on the metal tang that the moving point is mounted to.
You can clean the inside out a little if you want to. Compressed air is probably the best thing--I don't know what solvents the plastic inside can cope with, and water is a bad idea. The real cure is to keep the thing from getting damp inside again. We'll get to that in the reassembly process:
Put the diaphragm assembly back into the case, aligning the tears on the paper to where they came from so that the thickness of the paper doesn't change from before you opened it up. Your filing marks on diaphragm and case should be helpful here. Now put the crimp back over the joint. Rapping the whole horn against the countertop helps to get the crimp pressed over the joint. You'll find that the crimp is a little springy, and doesn't want to close up the gap where you pried it apart. Not a problem. Just get it as tight around the horn as you can--the gap shouldn't be more than about 1/8".
Now get out your silicone rubber gasket sealer, bathroom caulk, or whatever. Smear some on your finger and work it into all the gaps where two pieces of metal come together. This means all the way around front and back of the crimp ring where it meets other surfaces, and the mount and nut assembly on the case. Don't mess with the little adjustment screw yet, you have to complete assembly and test before that can be sealed.
To finish squeezing the crimp ring onto the horn, just snap the plastic trim ring on. Presto! you're done. Now to test.
Go out to the bike and hook up the connectors to one horn (I'm assuming you cleaned them before you found out the horns were bad). Turn ignition on and beep the horn. If you're lucky, it'll make a nice loud beep. Whether it did or didn't loosen the locknut on the small screw about two turns, then turn the screw out until it turns freely. Beep the horn. You should get a "blert" sound like the horn is trying to beep but can't manage it. Screw the screw in just past where it touches a little and it'll beep. Adjust until the beep is loudest (if you go too far in it'll just stop beeping), then hold the screw in place and tighten the locknut. Beep the horn a couple more times to make sure it's reliable.
If you have to do the other horn, disconnect this one so you can hear the other one better. If both work, hook everything up, beep the horn once and enjoy that two-note chord--a real rarity on motorcycles. Also observe that it sounds just like a Honda car... Smear some sealer on the adjustment screw and you're done. Now you can ride with a little less fear of the blind cage drivers, at least. You're on your own where the deaf ones are concerned.
Disclaimer: There is no guarantee or warranty to this procedure. You're on your own in trying it. Your horn may not work any better after this procedure than before. This procedure is not approved by anyone I know of, especially including Honda. Horns are important for safety. If you're not sure that your horn will work when you need it, then get one that will. This procedure may void manufacturers' warranties. My employer has nothing to do with this in any way.