Exactly right. On every car I've ever owned (including TR3s), the stock
setup is for the vacuum advance to be connected to 'ported' vacuum. The
port in the carburetor (or throttle body) is positioned such that when the
throttle is closed for idle, the port is in front of the plate, and sees
little or no vacuum. As soon as the throttle plate moves, it crosses over
the port, which now sees manifold vacuum. So curb idle give no vacuum
advance, but somewhere around very fast idle the vacuum advance goes
(almost) to full advance.
To answer Jack's question : more vacuum gives more advance, but it's ported
vacuum, not manifold vacuum.
Unless of course you have a vacuum retard unit, which is another can of
worms, found only on cars imported to the US in the early to mid 70s.
Randall
59 TR3A daily driver
> -----Original Message-----
> From: Jim Altman
>
> Perhaps the question really relates to where you are getting
> your vacuum.
> In my car the vacuum advance is pulled off the carb inside
> the butterfly so
> that vacuum and advances increases with throttle. The high
> manifold vacuum
> is blocked at idle by the butterfly. At idle advance is zero
> and goes up
> from there.
>
> -----Original Message-----
> From: Jack Brooks
> Sent: Friday, October 15, 1999 8:05 AM
> Subject: RE: Vacuum advance - Clarification
>
>
>
> Randall,
>
> Listers, can anyone shed some light on this issue?
>
> Thanks,
>
> Jack Brooks
> 1960 TR3A
> 1974 Norton 850 Commando
> Hillsdale, NJ
>
>
> > Jack :
> >
> > I'm not sure if I said one of those things that "just
> didn't sound right"
> > <g>, but I think Barry glossed over the point that most if
> not all auto
> > vacuum advance systems do NOT advance the spark at idle
> (unless the engine
> > is overheated). Efficiency at idle is not a concern (it's 0 by
> > definition),
> > and a retarded spark makes for a smoother, more stable idle
> speed.
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