Jack :
Maybe a little ASCII art will help (if you view it in a fixed pitch font
<g>) :
Throttle closed
---------------
|
|
intake X manifold
|
|
-------^-------
port
Throttle open
---------------
/
/
intake X manifold
/
/
-------^-------
port
As you (maybe) can see, when the throttle plate is closed for idle, the port
is on the outside, where there is little or no vacuum. When the throttle
opens, it moves across the port, putting the port on the inside, where it
'sees' the intake manifold vacuum.
Randall
> -----Original Message-----
> From: owner-triumphs@autox.team.net
> [mailto:owner-triumphs@autox.team.net]On Behalf Of Jack Brooks
> Sent: Friday, October 15, 1999 3:15 PM
> To: Jim Altman
> Cc: Triumph List
> Subject: RE: Vacuum advance - Clarification
>
>
>
> Jim,
>
> Now you've got me confused. If your car has the vacuum
> pulled off the carb,
> inside the butterfly (between the engine and the butterfly?)
> the vacuum will
> approach zero as you open the butterflies and will build a
> negative pressure
> as the butterflies close. The butterflies, restrict the flow
> of air into
> the manifold, causing the vacuum to become greater. This
> would mean that
> the vacuum would decrease with throttle, not increase, right?
>
> Later,
>
> Jack Brooks
> 1960 TR3A
> 1974 Norton 850 Commando
> Hillsdale, NJ
>
>
> > 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.
> >
> >
> >
> > Jim Altman jaltman@altlaw.com Illigitimi non Carborundum
> > http://www.altlaw.com/metro/jaltman.html
> 69-TR6#CC28754L(O) W4UCK
> >
> >
> >
> >
> > -----Original Message-----
> > From: owner-triumphs@autox.team.net
> > [mailto:owner-triumphs@autox.team.net]On Behalf Of Jack Brooks
> > Sent: Friday, October 15, 1999 8:05 AM
> > To: Randall Young
> > Cc: Triumphs (E-mail)
> > Subject: RE: Vacuum advance - Clarification
> >
> >
> >
> > Randall,
> >
> > The bottom line is that at idle, the vacuum advance is pulled in
> > by the high
> > vacuum in the inlet manifold changing the timing. When you open the
> > throttle, you let in more air and bring the vacuum in the
> > manifold closer to
> > zero, allowing the vacuum advance to move back to it's static
> > position. So,
> > the question is whether the engine is advanced or retarded
> as the vacuum
> > decreases or increase.
> >
> > 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. Also,
> > > for most engines, the centrifugal advance is no longer working
> > at idle, so
> > > the idle timing needs to be retarded for the slowest
> possible speed, not
> > > just the nominal idle speed.
> > >
> > > Randall
> > > 59 TR3A daily driver
> > >
> > > > -----Original Message-----
> > > > From: owner-triumphs@autox.team.net
> > > > [mailto:owner-triumphs@autox.team.net]On Behalf Of Jack Brooks
> > > > Sent: Thursday, October 14, 1999 2:23 PM
> > > > To: Triumph List
> > > > Subject: Vacuum advance - Clarification
> > > >
> > > >
> > > >
> > > > A few days ago I was following a few posts about the way a
> > > > vacuum advance
> > > > works. Some of what was said, just didn't sound right.
> > >
> >
>
>
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