![]() ![]() I will use the BSA M20 as the example here but the basics apply to all This section details the procedures to follow and includes a few hints and tips. the system is adequate, if not perfect.Īs part of a restoration or after carrying outwork on the engine or magneto it will periodically be necessary to However, in the case of the older engines, with their lowĬompressions, mild cam timing etc. Modern electronic ignition systems progressively and infinitely alter the spark timing throughout the rev range to suit piston speed Speeds, to aid starting and whilst making slow running adjustments.īy modern standards this entire system is regarded a too This is the reason why the ‘RETARD’ facility is only ever used at low engine Using the lever the spark can be ‘RETARDED’ ,or made to occur later, allowing the piston the time to arrive at the optimum The manual advance and retard lever provides the solution to this ‘slow speed’ However, at slow engine revs (under about 1000rpm) the piston is moving far more slowly than it would be at say 5000rpm.Ĭonsequently although the explosion time is constant, the time taken for the slow moving piston to reach the ‘optimum’ position is longer and it effectively arrives at the correct position after the explosion has reached its full When the spark ignites the mixture the time taken for the explosion to reach its full potential is basically constant at all engine There is also another factor to take into account and that relates to the manually operated advance and retard facility So, if the spark is incorrectly set in relation to the piston position and is occurring too early it is too ‘ADVANCED’ and if it is occurring too late it is too (before the top of the compression stroke) produces the desired result…with the explosion reaching its maximum potential as the piston arrives in the optimum ![]() Retard, and in the case of the BSA M20 a figure of 7/16” before top dead centre Speeds, this time delay is treated as constant for machines fitted with a manually operated advance and At engine speeds over about 1000 rpm, despite the variation in piston Due to this slight delay in the explosion reaching its full potential the spark is delivered slightlyīefore, or in ‘ADVANCE’ of the piston reaching the optimum position for power Small, but measurable amount of time to reach its full potential after the spark It is to achieve this state that the spark is ‘timed’ so that the maximum force is exerted when the piston is in the correct ![]() (and thus correctly positioned to be driven down the cylinder). To produce the maximum effect the piston must be just past the top of the compression stroke as the explosion reaches its full When the spark occurs and the fuel air mixture is ignited the resulting ‘explosion’ drives the piston down the cylinder and the force of the explosion is transmitted to the crank to produce rotary Some confusion can arise from the terminology used relating to ignition timing so the following is a brief guide to theīasics. A firm grasp of the principles involved and the reasons for the various settings can be helpful in diagnosing problems and carrying out Correct setting of the ignition timing is one of the key factors in achieving reliable starting and efficient running on any
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