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The difference between automatic, manual and fixed timing
lies in how the ignition timing changes - or doesn't - according to engine
speed.
A magneto with fixed timing and a fixed drive train provides
neither the operator nor the engine with any means of varying the instant that
the spark(s) occurs at the sparking plug(s) relative to the position of the
crankshaft & piston(s). Therefore, at all speeds the engine will run with the
same ignition setting. Such set-ups are typical on, e.g., stationary engines and
agricultural machinery designed to run at reasonably constant speed whilst in
service. They are not ideal on machines designed for road use, where a degree of
speed-related control is desirable, and a substantial amount of retard may be
required for starting. The amount of ignition advance required for running at
high speed is always considerably greater than that required for starting.
A magneto with manual advance and retard has a cable or rod
which connects to the magneto's contact-breaker cam and which can be controlled
by the operator, for example by a lever on the handlebars of a motorcycle or on
the steering column of a motor car. By adjusting the lever, the cam is
moved in relation the contact breaker, thus delaying or advancing the moment at
which the contact breaker points are opened.
A magneto with fixed timing may, however, be equipped with an
Automatic Timing Device (ATD), which varies the moment the spark is generated
according to engine speed. Typically, an ATD offers between 11° and 18° of
ignition advance at the magneto as speed rises (which equates to double that at
the crankshaft in any engine where the magneto runs at half engine speed). It is
to be noted that an ATD operates solely according to speed. Most types of ATD
(e.g. Lucas and BTH) are provided in the drive train between the engine and the
magneto, typically mounted on the input shaft of the magneto. However, it is
known for example in some models of SEV magneto for the ATD to be an integral
part of the magneto and to adjust the position of the cam relative to the
contact breaker in dependence upon the magneto speed.
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Brightspark Magnetos