Project Vespa Smallframe - AC Horn

The sound of the original DC horn operating on the clipped AC through a rectifier and capacitor filter, just wasn't satisfying me.  It doesn't sound the same as operated from a battery.  There had to be a way to make the horn louder.  After all it is a safety issue.  If you blow your horn, you want to be heard.

I had purchased a 12VAC horn from India via eBay.  It is a nice build quality, but of course it was made for a Primavera, and the hole pattern was too small to fit the unique 100 Sport.  I had pondered this for months, and really didn't give it any action until this rainy weekend.

Since the reproduction 12VAC horn hole pattern was smaller, I could potentially graft it to the original 100 Sport horn cover.  This would require removing the rivets, and make a paper pattern of the adapter.  I could then cut out the adapter ring from whatever metal I happened to have laying around the shop.  I could use the same soft-aluminum wire to make replacement rivets to hold the whole assembly together.  I had nothing to lose, the DC horn could still be put back together if I wasn't satisfied... I had already taken it apart once before.

Things came apart easily.  No photos taken during the Dremel tool cutoff disc work.

I got to work drafting the paper model of the adapter ring.

Here are all the parts that will be used.

After crafting each aluminum rivet one-by-one, we have a completed AC horn mechanism grafted to the original horn cover.  It sounds nice and loud.

Project Vespa Smallframe - Horn Repair

Rehabilitating the horn is actually a pretty easy task. The rivets can be ground off of the back side so that they press out with a flat punch, allowing the unit to be disassembled. I did this with a Dremel cutoff wheel. The inside was really corroded, which prevented any good contact from being made. The two terminal screws can be removed and the mechanism cleaned. The adjustment screw is also removed from the rear side. I suggest running the screw the whole way in first counting the number of turns so that the adjustment can be returned nominally to the original place.

Current flows through contact directly connected to the coil on one side. The other terminal is on the opposite side of a set of contacts. The contacts are normally closed completing the circuit. When the current flows through the coils they act as electromagnets which open the contacts stopping the current flow through. The contacts close again completing the circuit. This cycle repeats as long as voltage is present at the terminals. The adjustment screw adjusts the tone of the horn.

I used a wire wheel to clean the components. The final step was to use some contact cleaner such as CRC 2-26 which is safe for the insulator components. Assembly is the reverse order as disassembly.  

The rivets were made of soft aluminum. I just happened to have a soft aluminum wire (typically used on a residential satellite television dish). This is very malleable and makes good material for rivets. The wire is gripped in a set of pliers and a ball-peen hammer can be used to flatten the end to form the head. The wire is nipped off with a set of cutters to an appropriate length. This makeshift rivet is then inserted into the original hole and pounded flat to secure the horn.

Beep Beep

Work begins

The S7 Deluxe does run as received.  Not well, and not with much oil pressure, but it does run.



I have no KEY!!! There was a temporary toggle installed by the previous owner, so that had to go.  I want a key and I want it to work.  So, I took the keyswitch apart.  All you do is unsolder the nut on the center of the switch assembly, then unscrew it.  Then poke each of the tumblers from front to back and it pops out.  I found a key that fit and then ground down the spots that didn't quite make the tumblers line up.  (It happened to be a key to the back door of my grandmothers house which we changed over 30 years ago, and I was given the key to play with.)  I still had it tucked away in a box... never throw anything away!  Only one spot on the key had to be built up.  I've got a key that works ... Fantastic!

I've finally had a few evenings to devote to the S7.  There are so many things to do its hard to know where to start.  The carb has been exhibiting lean tendancies with the needle in the full rich position, so I tore that apart first.  The bottom of the carb was improperly assembled with the gasket material bunched up and not allowing the piece to fully seat.  This caused an air gap along the inner assembly of the carb.  Ah Ha... one problem solved!  It was a chore to take apart.  I had to mount the carb in the BIG bench vise, heat the bottom with a torch and gently apply pressure to unscrew it.  The bowl needed cleaned too, it was full of junk.  The main jet was clean, and the mixture screw and jet got blown out as well.

I then started wiring the back of the bike.  The original wires were shot.  I got the taillight re-soldered, but had to stop since I didn't have a replacement grommet for the fender.  I will probably remove the fender to change the tire anyway... and to clean up 50 years of dirt from the fender.  A set of barrel connectors between the fender and right-side electrical box are probably a good idea to allow easy removal of the fender without disconnecting all the wires at the terminals.  One frapping screw on the brake switch is stuck.  I don't want to force it and am reluctant to use the scrap of wire that's attached to it.  It may have to stay since it only needs to reach the horn.

Ahhh the horn.  I didn't want to pay for an original, so a VW horn is being used.  It looks silly because its all bright shiny and new.  Oh well.

I used an ez-out to removed a sheared screw from the dipper switch.  The insulation of the wires was bad, but not completely gone, so some liquid electrical tape did the job temporarily reassembling the whole thing.  The horn button was re-attached as well.  Not sure its getting a good earth, so that may have to be re-visited.