The 'B' and 'C' machines have magneto ignition and Miller lighting, while the 'D' models have coil ignition with all-Lucas equipment. As the location and method of mounting of the distributor/contact-breaker assembly or magneto is the same on all Vincents, the equipment is interchangeable. With the aid of factory-supplied parts, it is possible to fit the 60-watt output Lucas generator in place of the 50-watt Miller instrument. The dynamo on the 'Comet' is driven from the timing chest but the addition of a second camshaft in the twin-cylinder design prevents this and the generator is driven by a sprocket meshing with the middle row of the triplex primary chain.
If the coil ignition equipment only is fitted, the Miller dynamo is capable of dealing with the extra 2 amp demand. The Miller regulator is adjustable in emergency. Sidecar owners can usefully wire another battery into the circuit in parallel. The standard battery is a 13-amp. hr. item.
Servicing of the magneto is best left to a specialist but simple workshop tasks, like renewal of brushes, are well within the scope of the amateur. At the drive-end of the Lucas magneto is an earthing brush which must be kept clean and the ring free from grease. Failure to do this may result in premature failure of the drive-end bearing.
Platinum contact-breaker points are used on the vee-twin magneto to boost the less intense spark of this type of instrument. Tungsten points are fitted to the single-cylinder magneto.
The armature firing interval is 155°/205°. A one-degree error at the armature equals a two-degree error at the crankshaft. A cure can be effected by care-fully stoning the cam-ring. Ignition timing should, therefore, be checked at both cylinders.
The automatic advance-and-retard mechanism should be examined for signs of rusting every 5,000 miles. If the machine seems to be pinking too much at low speeds-when everything else is in order-the control springs should be renewed. It is possible to make a manual conversion but parts for this are not supplied from Stevenage. A rev. counter can only be driven from a manual magneto. Attaching a driving dog to the armature bolt may result in the bob-weights of an automatic magneto being unable to throw the armature fully advanced.
Standard contact-breaker gaps should be maintained. The magneto and the distributor are both mounted on a triangular flange on the back of the timing-side crankcase wall; one paper gasket, without compound, is used.
To free the timing unit, break the taper by means of the self-extracting nut on the armature. To remove the magneto, undo two plain nuts and one sleeve nut, and slide the magneto off towards the left.
Coil-ignition machines can be equipped with a sports-type Lucas coil and the sparking plugs resct to a gap of 0.032-in. The result is slightly better acceleration and certainly better starting. A check on H.T.-lead insulation can usefully be made in the dark when small blue sparks jumping to metal parts denote breakdowns.
Many riders tend to judge the efficiency of an engine by how much ignition advance it uses. In reality, the criterion is how little! More than the minimum advance that is necessary only produces premature bearing failure of the bottom-half. E7/6 pistons are used with 40°, E7/7 with 38° to 39°, and E7/8 with 37° to 38°. Insufficient advance results in heavy fuel consumption but not much loss of performance until the ignition lag reaches a high value. A retarded engine runs hotter than a correctly timed one.
The cause of sparking difficulty with the magneto-equipped twins is the 50° angle between the cylinders used on the post-war twins pre-war 'A' models were 47°) The 180°/180° nature of a magnetic field cannot work well with a 155°/205° firing interval and spark production suffers accordingly. It is weakest on the No.2 cylinder, that is the front one. Fouling of the earthing brush at the drive end of the Lucas magneto is a particular handicap in this context and it is important to keep this clean and, indeed, the entire instrurnent in better condition than that of a single or parallel-twin.
The Miller dynamo is an exceptionally robust component that seems to work happily even when soused in oil. The owner can strip the generator easily to clean the cut-out points or to fit new brushes but for anything more involved than this he is advised to send the component for specialist attention. Similar advice applied to the Lucas generator. Here the cut-out is in the separate c.v.c. unit.
Both types of control units can be reset to cope with changed output or load. The Miller set is improved if the regulator is mounted on a pad of sponge rubber. Miller cut-out points may stick open, so breaking the generator supply; a persistent offender can be temporarily dealt with by boring a small hole in the cover and manually operating the points with a matchstick.
In an emergency the two leads to the Miller unit can be broken and the ends leading from the dynamo joined together. The output, nonmally limited to some 8½ amp., is then lifted to 14 amp. or more. Prolonged use in this state causes the generator to overheat and throw the solder off the commutator. A parched 'dry' battery of the Varley type can produce symptoms similar to those given by a recalcitrant regulator.
The 3-in. dia. Lucas dynamo fits into the 3-in. Miller cradle but, of course, not into the 3½-in. Miller assembly. The conical light-alloy adaptor supplied as a distance piece controls the lateral disposition of the Lucas dynamo and therefore, the correct meshing of the sprocket in the middle run of the primary chain. It may be necessary to machine the adaptor to allow the sprocket to move over to the left in order not to foul the right-centre row of links. No paper washers or felt seals are used in this conversion but the oil-thrower is retained.
Conversion parts are also available for fitting a Lucas F700 block-lens light-unit into a Miller shell and the later type of Miller unit can also be substituted for the old.
Now available with internal parking light, the Lucas P700 unit gives an exceptionally useful beam for fast riding. It can be fitted to a Miller shell by bending extra flanges on the shell rim with pliers and using a Luras SLR7OOS rim, lined with rubber, to join unit to shell. Naturally, a P700 goes straight into a Lucas shell. Marchal units can also be used with adaptors and these arc available for both makes of shell.
It is possible to modify the battery carrier so that the battery is moved over close to the left side of the machine, so permitting the rear float chamber to be mounted on the left. This is advantageous as, being on the same side now as the front one, it is possible to lean the machine on the left prop-stand without the carburetters flooding. Alternatively, the modified carrier can be used to mount a larger battery. The band of the composite strap is in stainless steel.
'Monobloc' carburetters fitted to 'C' models call for the battery to be moved in any case.
An earthing cap to a magneto can be easily fitted; standard Lucas parts are available for this as they are for a manual advance-and-retard conversion to an "automatic" magneto. But, unfortunately, owing to the disposition of the control cable and the direction of rotation of the magneto, only a tight-wire advance can be achieved. Failure of the spark to cut when the 'kill' button is pressed can nearly always be traced to a fouled earthing brush.