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SOME THOUGHTS ON THE AUSTIN 7 MAGNETO In most cases the Manufacturer sets the contact opening at the optimum in full advance and keys the circuit breaker to the armature. All that is necessary is to locate the key in the circuit breaker to the slot in the armature and tighten the centre screw that holds the breaker to the armature, which connects the contacts to the coil and the condensor in rotating coil magnetos. In the case of the BLIC magneto, which is a rotating magnet type, the cam is from memory mounted on a parallel shaft with a key that holds the timing gear, so no adjustment is possible. If I'm wrong, I will apologise. The ML magneto also has a rotating cam and magnet. It's cam is mounted on taper on the end of which is a mark across the end of the shaft. This Mark is offset and will only line up in one position on rotating armature magnetos. If the key is missing or flattened - it is only a piece of the breaker pushed out to make a key to engage in the slot in the armature shaft. The procedure to set the optimum position is as follows: first set the contact gap to 012' then, with the cam ring in the full advance position turn the shaft in the direction of rotation until the trailing edge of the coil lamination or magnet leaves the pole shoe. Then turn the shaft back until there is a gap of a maximum of 1/8". It is at this point that the contacts should be just opening. In retard there will be a much bigger gap between the pole piece.
If the pole pieces cannot be seen - as is often the case - proceed as follows: in full advance turn the shaft th the direction of rotation, as the armature is in the magnetic resistance can be felt, on further rotation you will feel the release of the magnetic field as it changes polarity through the coil as the armature comes under the influence of the magnetic field. A few trial runs through this operation will give you a more exact feel of the reversal effect. It is at this point that the magnetic field collapses through the coil to generate a current in the primary winding of the coil, which is then transformed into a high voltage in the secondary winding. Similar to the operation of a transformer used to reduce or raise the voltage in an alternating current system. ML and BLIC magnetos do not have slip rings, only rotating coil magnetos are blessed with them. The bearings in all revolving coil magnetos are insulated from earth. If the earth brushes stick at the HT, current returns to the coil via the bearings with resultant pitting of the bearing surfaces. With regard to extende pole pieces, they were not in use when A7s were fitted with magnetos. They did give a better spark in retard than the ones used in those days. Allan Hale Austin 7 Club, Victoria. Inc 3 Feb-06 |
