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Anatomy of a 400/4 Horn (also CB250/360 G5)

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Promised that I will look at fixing a 400/4 style horn to go with a bike I am selling.

The pictured one was a dead as a dodo .. I totally sealed it up in 1986 and then nickel plated it hoping the electrolyte didn't get inside. Fast forward many years it doesn't work any more  so, fearing  the worst, I therefore  decided to open up after bead blasting and take a peek inside with a view to fixing it.. No real corrosion but contacts oxidised so had to move springy contact bar over to gain proper access to clean contacts. You have to de-crimp the outer (mild steel ) ring to gain access. i didn't make a particularly neat job but it will tidy up OK. The coils seem to be mega reliable so don't burn out in my experience

Busy at 'mo but I will do a write up when I have time on how to totally disassemble for replating in zinc.

Would be a nice sideline for Kent400 !!!!

Meanwhile a couple on annotated pics:-

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Excellent timing Ash !

I keep looking at my project 400's horn and wondering if it's salvageable, so your 'how to dismantle' write up would be very useful - I've not even managed to remove the plastic cover yet as it seems very brittle and likely to crack as soon as I start to lever it off ! A guide and any tips would be most welcome !


Well ...if you look here these are interesting. The Honda Shadow horn internals are basically identical to the 400/4 horn.

In the disassembly video clip, he mentions that it's almost impossible to get at the points to clean them but I found a way around that. If you look in my photo's I moved the main contact bar away from the other 'springy'  contact bar by lifting it slightly and rotating so that the contact is visible through the large threaded hole in the centre. You now have access to clean both contacts properly and then carefully reposition the contact bar so that  both contacts are aligned.

Detains on 'tuning' it will follow. Re-crimping is harder to do than on Honda gauges (speeedo/tacho) but it is possible to do a neat job with a bit or care and patience. Another method of contact access (and also if you wish to re-plate the horn) is to drill out the copper rivet that retains the contact bar and either find a replacement copper rivet or use a brass or copper   nut and bolt,   if you are not too fussed about originality.

One of the two YouTube video clips shows drilling of a hole to inject contact cleaner at the correct spot inside the horn. Mine had pretty tough to remove oxidation on the points but the method may work, as once you get current to flow and the horn oscillates, the highly inductive load's back EMF arcs across the contacts and the contact erosion effectively self cleans them (just like ignition contact breaker points). my main reason for learning to disassemble these horns was to enable me to fully zinc plate the  steel parts for my 400/4.

Perhaps we should ask Ozzy Osborne how to adjust the horn after re-assembly as that was his job at Lucas in Birmingham  :) . It's basically  a case of adjusting the large finely threaded pole piece / nut by screwing in and out  to give maximum armature force and correct movement and then adjusting the M4 screw/locking nut to achieve oscillation by making and breaking of the contacts.

The black plastic cover does get brittle. Best bet it to get it as hot as possible, without melting it  and insert a thick knife blade and gradually prise it off.

I brought a totally original but dead 400F horn to life today for my own 400 using  a Honda-4  ignition coil. I was originally going to de-crimp it to clean up the points inside but with this method it's non-invasive.

Trick is to connect both HT cables of the ignition coil to the the horn terminals. Connect the black/white wire of coil to a 12v battery and clamp the tang of a coarse file onto the negative terminal of the battery. Connect a piece of wire to the other terminal of the horn  (blue or yellow wire depending on which side coil) and strip the other end back about 10mm and twist the copper strands together. Then repeatedly brush the wire against the file teeth so that you create lots of makes and breaks of the current through the primary of the coil and correspondingly  lots of HT pulses are generated on the HT leads, which discharge through the oxide layer inside the points of the horn. This repeated action soon breaks though the layer of insulating oxide on the horn points, so that conduction is soon restored. You then disconnect the HT leads and test for current flowing through the horn again. If not reconnect the HT setup and keep trying. Ideally you would also connect a points condenser between the battery negative and the coil wire to give a better build up of HT but in my case it wasn't required.

McCabe-Thiele (Ted):
Interesting process Ash especially the coil connecting- nothing like as complicated but I have revived old car horns for a short period by connecting one side to the negative side of a fully charged 12 v battery then rapidly connecting & disconnecting the live terminal so it progresses from a dull click to eventually making the horn sound. Only seemed to work for a short period on a seldom used car horn. I thought it was due to the back EMF somehow reviving the contacts.


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