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Brightspark Magnetos

NEW ... Take a look at some of the equipment we use in our workshop for magneto servicing and overhauls.

 

 

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FAQs

What's the difference between automatic, manual and fixed timing?

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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