REVIVING THE MORTAL COIL

I take this title from an article in the Classic Motorcycle July 1990 called “Rejuvenating the Immortal Coil” by Roy Greene, which set me onto the experiment described. I give thanks to Mr Greene and have followed closely his guidance and have changed the title only because my coils are anything but immortal and I have stuck closer to Hamlet.

All this waffle derives from some efforts to give my James ‘Comet’ reliable and easy starting. On Club events it’s OK, once I get it going it runs well. But several times at home, if I have popped out on some errand, it has caused embarrassment and I have had to use the run and push start - not very becoming in the village High Street.

I put this behaviour down to an ailing HT ignition coil, so I looked around for easy options. There is no battery on my James, so coil ignition was out of the question. The article describes in detail the process (well known to many) of rewinding the primary and feeding its output to an external HT coil and I thought that I would give this a go. Rather than destroy the working (but fickle) coil in the engine I did my experiment on an old coil that was rattling around in my box of 1F bits.

The first part is easy. If you are working on the coil in the engine you have to remove the flywheel and take off the armature plate with all the coils and contact breaker. Remove the screws coming through the back of the armature plate which secure the two cheek pieces holding the coil and unsolder the connection to the contact breaker.

With the coil free from the armature plate, unsolder the little HT cap and pull off the paxolin sleeve that covers the windings. Under this I found a treacly mess that was the old shellac and tape all gunged together and cut this off. The outer HT winding is thousands of turns of very fine wire which can be cut off in chunks using a craft knife. You will eventually come to the primary winding which on my coil was 145 turns of cotton covered wire which as best as I could measure (because of corrosion) was 22 swg. Strip all this away and clean up the armature and side cheeks.

To start rewinding the primary you obviously need insulated wire and you will have to search around your locality for a suitable supplier. In Guildford I went to a shop called Basic Electronics, who sold me enough to wind the 145 turns that were needed. I was told by someone that you could recover wire from a disused electric motor but this seems a bit dodgy to me. Roy Greene interleaved the windings with cotton tape coated with shellac to impregnate it. I found the tape in my wife’s haberdashery basket and used button polish for the source of shellac.

The process starts by soldering the wire to the iron core. Scrape the insulation off the wire, form a small loop and solder it on. I used Baker’s Fluid to help tin the core and a small blow lamp flame to warm up the core because my 65Watt soldering iron was not man enough to do the job on its own. Take care to ensure a good electrical connection at this point because it will now be covered by the rest of the windings and you will never see it again. To lay the windings on the core Roy Greene used a lathe - but I haven’t got a lathe. I used an electric drill lightly clamped in the vice as a rotating holder for the coil core. then by rotating the chuck with the left hand and laying on the wire with the right hand a neat result was achieved. At the end of each layer I put a wrapping of tape around the windings and coated it generously with shellac before winding on the next layer of windings in the other direction. I’m not sure, with modern coated wires, that this business with the shellac is necessary but I played safe and took the writer’s advice.

I found that I was laying about 44 turns per layer and I put on four layers pluss a few turns to get back to the exit point. This is a brass ferrule in one of the side pieces and I managed to take the scraped end of my windings through this ferrule and solder it in place. This provides a solid point of connection for the lead to the contact breaker. Once I was satisfied that the windings were all secure I laid on a generous coating of fast setting Araldite and occasionally rotated the coil by hand to ensure that it did not all droop down to one side. When this was set I Araldited the outer sleeve into place. Now comes the stage of reconnecting the coil (See Fig 4)

When the ‘new’ coil is all set it has to be bolted back into position on the armature plate between the two cheek pieces. The connection can now be made between the new primary and the contact breaker, preferably with a new wire. Be careful that the little spring clip that keeps the rocker arm on its post is securely fixed as this is part of the earth path. Another wire from the same point has to be taken out through the back of the armature plate and up the rear frame tube to the CB terminal of the externally mounted HT coil. Use a modern 6v coil for this, mine had been recovered from a Japanese bike at some time. I mounted it out of sight on the frame of the saddle, but any convenient site would do provided that it is properly earthed. I left the old HT lead in, for appearances sake, and just taped the open end to the new HT lead.

Does it work ? Yes, on the limited use that I have put it to so far it seems to be a success. If you do not want to go to these lengths but like the general principle you can get the kit for doing the same thing from Simon Bateman at Nametab Engineering (01527 870834). He will supply a rewound coil, the new HT coil and all the connections for £70. Either way let’s hope that you get easy starting with that fat blue spark.

John Hawthorn

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