If a car magneto has an advance mechanism it's usually very obvious
since it generally rotates the entire housing. You can't just move
the point base (or at least you can't if you want a good spark). The
point opening has to be timed to the position of the rotor relative
to the magnet, otherwise it opens when too little current is flowing
through the coil and the voltage produced is lower.
You can get away with moving the cam around a little on a motorcycle
magneto because it only has one or two sparks per rotation and
therefore the magnets can be wide. The cam on a Lucas mag is a hollow
cylinder that is eccentrically ground on the inside--the points run
against the inside, and the spark retard mechanism manually rotates
the cylinder in the housing. But even then the mechanism is just for
starting and does result in a weaker spark when it's used.
All of this is a sore point for me--the magneto for a Vincent is
always problematic because it's a 50 degree V twin. The spark is
compromised no matter what you do and gets much weaker in at least
one cylinder when you use the retarding mechanism--a big problem when
you're trying to kick start a high compression 1000cc V twin (1200
cc's in the case of my hotrodded Vincent) with a very lumpy cam and a
very primitive quadrant style kick mechanism. I'm giving up on period-
correct fixes and replacing the guts of my magneto with an electronic
ignition system. I've had enough embarrassing moments starting this
beast. Oh for an electric starter like the RTV clone motor had.
Bill Babcock
billb@bnj.com
bnj Website for Babcock & Jenkins, a direct and interactive agency
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On May 3, 2007, at 8:16 AM, John Herrera wrote:
> Amici,
>>
>> The shop I work at has an XK-120 with a
>> Vertex Scintilla magneto ignition in for repair.
>> The drill for starting it is: Pull fast idle cable,
>> Pull starting carb solenoid switch on, Turn on
>> key switch, Push starter button, Then turn on
>> magneto toggle. I'm not familiar with this magneto,
>> is there any advance built into it? Just wondering.
>>
>> Bill
>> Triumph Rescue
>
> I don't know if this will help, but:
>
> If it's like an aircraft magneto, there is no advance in it. You
> static-time
> an aircraft magneto to a given degree before TDC and it fires there
> every
> time. The spark event doesn't change.
>
> There is a spring-mechanical coupling device (called an impulse
> coupling)
> between the mag and the drive gear that retards the spark until TDC
> during
> starting. It disengages once the engine starts and the mag fires at
> the
> setting BTDC. Don't know if car mags have impulse couplings.
>
> Aircraft engines run in a very small RPM band, so car mags might be
> different.
>
> John
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