Impulse magneto
 

On a hand swing engine fitted with an impulse magneto (No 1 mag),does the impuse carry on working up to a certain lowish rpm, or does the impulse mechanism stop immediately the engine fires?
Number 2 mag is normal operation and is switched on after engine is running on 1.
Lister:)

Lister,

The impulse coupling is a spring loaded mechanical coupling which delays rotation of the magneto during an engine start. After a short period of rotation the device allows the magneto to catch up...in effect spinning the mag shaft faster than the engine is turning it. The faster it turns, the hotter the spark and the bigger the voltage impulse...making for easier starting. After the engine is running rotational forces move the spring loaded arms, which make the impulse coupling work at low speeds, out of the way. The impulse coupling only works at very low engine speeds.

When you (carefully!) pull the propeller through in the normal direction of rotation, by hand, the mechanical click you hear is the impulse coupling winding up and then releasing.

Thank you SNS,I should have been more explicit.
I do understand how the impulse works, but wondered at what engine speed the impulse mechanism "retracts"
Would it continue to "trigger" at low speed tickover,say 300-500rpm or would it finish as soon as engine fired up?
I assume the strength of the springs and weights via centrifugal forces play a part, much as on an old type distributor for advance and retard.
Lister:)

Lister,

The actual speed at which the spring loaded arms of the impulse coupling retract and lock up the mag vary with the spring strengh, but generally the impulse coupling should stay engaged at least to 75-125 rpm, and should be fully disengaged by 450 RPM.

Thank you SNS,so I need not worry that the impuse would still be working at low rpm tick over.
Lister:)