Brightspark Magnetos

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Which is better, automatic or manual?

The advantages and disadvantages of the various sorts of ignition control can be summarised as follows:

Fixed timing

Fixed timing ensures that the magneto always delivers its sparks at the optimal position of the armature to the magnet in the instrument's housing - the point of maximum flux change. This ensures maximum spark intensity at all times. It's a big 'Plus'.

An ATD in combination with a fixed-timing magneto therefore has the advantage over a manually-controlled cam that retarding the timing is done otherwise than by altering the internal timing of the magneto.

However, fixed timing with an ATD has the disadvantage that no allowance can be made for load, as the only factor governing the amount of advance applied is engine speed. A manual lever can always be retarded as necessary, even if the sparks weaken slightly in the process.

Special considerations apply in the case of V engines. Although a magneto can perform satisfactorily in a somewhat retarded mode internally (vis à vis the optimal fully advanced setting), in V engines one spark will inescapably be 'retarded' in relation to the other because magnetos are not designed to work assymetrically. Therefore, ATDs were adopted early by V engine manufacturers as they avoided the need to weaken additionally the spark supplied to the 'Number Two' cylinder. In a 60° V twin with a half-engine speed magneto, one spark is inevitably supplied 15° 'late' at the magneto - adding to it by using a manual cam can push the magneto to its limit of operation or beyond.

Manual Systems

It follows then that manual advance and retard systems suffer from the disadvantage that they demand interference with the optimal internal timing of the magneto to obtain 'retard' for starting or other purposes (such as hill-climbing under heavy throttle at modest rpm), while they have the advantage that the operator can at least make small timing adjustments to cater for road conditions, throttle position and load.

Many 'sporting' versions of British motorcycles retained manual operation long after their more mildly-tuned siblings went to ATDs. Probably to give the rider some control as described, possibly also because the sporting rider liked to have an array of knobs and levers to play with, rather as car owners favoured additional gauges and gizmos so they could have more to worry about while out and about.

Further issues relate to wear and sloppiness in the cam mechanisms of manually-controlled magnetos. With fixed timing, the cam can be positively located in position by the maker and will stay there. A manual magneto requires contra-rotation of a steel cam in (usually) an alloy housing. Despite efforts to lubricate with oiled felt, wear inevitably occurs over time, and some degree of chatter is induced. This chatter will not always be evident when conducting static timing tests, but dynamic tests reveal a sometimes considerable amount of variation in the firing intervals on multi-cylinder engines. Slop in the fit of housing to cam is to be avoided if at all possible, and the locating peg which limits movement of the cam needs to be set correctly to ensure the internal timing is preserved.

We devote a good proportion of our energy to trying to eliminate such variations on multis, by careful measurement of firing angles, contact breaker performance and the condition of bearings and housings. It is not rare to find as much as 10° of error between cylinders - at the crankshaft - on twin cylinder magnetos, which is a recipe for rough running at best, and mechanical woe at worst on a high compression engine used hard. It is, however, in the nature of decades-old machinery for there to be some small degree of error - the trick is to reduce it to the bare minimum.

General Pros and Cons

As far as we are aware, no magneto was ever fitted with a secondary system of advance and retard using inlet manifold depression to override or offset the advance offered by an ATD (or indeed to vary a manual setting). The lack of such a parallel means of control may have extended the life of manual control on motorcycles way beyond its prevalence in the automobile industry. Assuming variations in pressure in a typical inlet manifold are up to the task, it would theoretically (at the price of interfering with the internal timing on an otherwise 'fixed' magneto) be possible to do so, using a manual cam on a fixed-timing magneto, or a dual means of control on a manual magneto. We'd be fascinated to hear from anyone who has managed to do this - and what effect it had on the road manners of the machine in question.

ATDs generally advance the engine to its maximum by 2000-2500 rpm; most operators of manual systems run their engines in the fully advanced position most of the time, except for starting and tick-over or occasionally when loads are heavy. This contrasts with the more sensitive advance curves to be found with many modern electronic ignition systems, and has to be included as a disadvantage.

However, these and any other negatives are outweighed, we believe, by the beauty of having an independent ignition generator which is not dependent on the sorts of charging systems to be found on the majority of elderly vehicles which have little power to spare for coil ignition on top of lighting requirements and battery needs.

On balance, an ATD in good shape is probably the easiest and most foolproof way of managing the timing, but it's horses for courses, and personal preference and originality will usually be the deciding factors in the decision whether to go 'auto' or 'manual'. That and the question of space under the timing cover in many motorcycle applications.


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