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The majority of cyclemotors and autocycles of the 1950s relied on a flywheel magneto to provide the ignition (Vincent and Lohmann being two of the exceptions). Wico-Pacy, Miller and others made units suitable for our machines and both made versions with and without lighting coils. The construction of all of these was very similar but here I will cover one of the most common types, the Wipac "Bantamag".
This is perhaps one of the simplest magnetos ever made. It consisted of a back-plate supporting a double wound HT/LT ignition coil, contact points and a brass cased cylindrical condenser; a shaft-mounted cam and a Mazak rotor containing cast-in magnets. There are no lighting coils and the Bantamag is very reliable. However, its spark is not too strong so a close eye must be kept on its adjustment.
It is possible to adjust the contact breaker through the holes in the rotor. Turn the rotor in the direction of your engine rotation until either the hole marked "CCW rotation" or "CW rotation" is in front of the contact set, as appropriate for the engine. There is no adjusting screw so loosen the contact set fixing screws slightly and open or close the points by moving the contact set mounting plate with a screwdriver. They should be set to 0.018" (0.45mm). When they are set correctly, tighten the screws and re-check the gap. Whilst you are checking the gap it is a good time to re-lubricate the felt cam pad (when did you last look at yours?) Either use grease (if the magneto rotor is off) or ordinary engine oil will do. Place the nozzle of an oil can hard up against the pad and gently pump in oil until the pad is just saturated.
The timing of the Bantamag is achieved by slackening the two screws holding the back-plate onto the engine casing and rotating the back-plate. Turn it in the direction of engine rotation to retard the ignition (hotter running, better slow speed performance, better starting) or opposite to the direction of engine rotation to advance the ignition (cooler running, better high(?) speed performance). When the desired timing is achieved tighten the two screws hard. If they come loose they can wreck a magneto in seconds (I know, I've done it!)
All of these adjustments can be carried out with the rotor on or off, but the operations are easier with the rotor removed. To remove the rotor, undo the central fixing nut and remove the washers. Then carefully screw on the extraction tool so that its fixing screws project just below the surface of the rotor, Finally tighten the extractor and, when there is some load on the screw, give it a smart tap with a hammer. When refitting ensure that the Woodruffe key is properly in place and has not fallen out. If the rotor continually breaks keys when in use, gently lap the taper of the rotor onto the shaft with fine valve grinding paste, clean all the parts, and sprinkle some French chalk onto the taper before refitting it.
There are no major problems with the Bantamag. As with most magnetos, it is very gap-sensitive on its contact breaker and it is worth the trouble to ensure that the contacts are clean and the gap correct.
As all Bantamags are now very old a condenser fault is not unheard of. Replacements are not now available but a modern electronic capacitor can easily be fitted into the old brass casing and will give a good performance. Use a capacitor of 0.1 to 0.2 microfarad, of a non-electrolytic type.
Coil failure is very rare without mechanical damage. The coil can be checked by measuring its resistance. The high tension (HT) coil should measure between 3,000 and 4,000 ohms from the plug lead to the magneto back-plate. The low tension (LT) coil should only measure about 3 to 5 ohms, so a continuity check should suffice. This is measured across the contact breaker when the points are open.
If the rotor is kept away from the magneto, or is subjected to severe shock or heat, it can become demagnetised. At present I know of no cure for this with this type of rotor, and another one should be found.
The use of a modern capacitor in the condenser can greatly increase the strength of the spark available from one of these units and has no known detrimental effects. The only other improvement easily achieved is to increase the available timing adjustment range; this is very small on the Bantamag, and often a better engine performance can be achieved with a more advanced timing than this unit can give. Carefully file out the mounting screw slots to provide the extra movement. Usually only about 1/16" or 1/8" is required. The very adventurous can also advance the ignition by opening the contact gap to around 0.025" (0.64mm) but this is not recommended for the uninitiated, as it can produce complications.
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