On my '79B the vacuum advance was on the manifold, not the ZS carb. I
put a Weber down on it, and the manifold had no port other than for the
brake booster. For a while, I ran no vacuum advance, just the
mechanical. If set for 15 deg advance at idle, I would get pinging
under acceleration at higher rpms. The Weber has two vacuum
connections, but measuring with a meter showed that both were ported
vacuum, not manifold vacuum. I drilled and tapped a fitting into the
manifold near the carb, and connected that to the distributor. Better
response at low rpms when the throttle is open (low vacuum), no pinging
under acceleration (again, low vacuum when the throttle plates are
open).
As far as the "perverse vacuum retard" on emission MGs you mentioned,
there is a vacuum valve/solenoid assembly mounted on the pedal box. It
is activated by a switch on the transmission engaged in fourth gear.
This switch allowed the vacuum valve/solenoid device to apply vacuum to
the dizzy only in fourth gear. I guess (but not sure) that it released
the vacuum (allowed air in) when not in fourth. The vacuum line is not
connected, but the valve is mounted just to have somewhere to put the
connector from the wiring harness.
--
Keep on keepin' on!
Wayne Kube
1979 MGB
Plano, TX
Texas MG Register http://www.tmgr.org
NAMGBR UK Site http://www.mgcars.org.uk/namgbr
Robert Allen wrote:
>
> Todd Mullins wrote:
> >
> > Robert Allen writes:
> >
> > > Vacuum advance increases fuel economy and provides for smoother running
> > > when cruising. It does nothing (for or against) performance. When using
> > > Vacuum Advance, it must be connected to *ported vacuum* which is a port
> > > just aft of the throttle plates on the carb -- not manifold vaccum as it
> > > used by the power brakes. You want vaccum advance to go away as soon as
> > > the throttle position increases.
> >
> > For reference, the box-stock Lucas dizzy on my box-stock '74 uses
> > manifold vacuum, taken from one end of the balance pipe. It's
> > definitely closer to the block than to the carb butterfly.
>
> By golly, you're sure right. MGs use manifold vacuum for their
> distributor and the carbs are CD -- constant depression (or continuosly
> drips) so there will always be vacuum on each side of the throttle
> plate. Curious. That would imply that vacuum advance contributes to
> total advance even under periods of moederate acceleration. From a
> traditonal carb perspective, that is pretty strange.
>
> D.D.B. said he has a downdraft Weber which is a traditional venutri carb
> where ported vacuum might be available. Mechanical advance is set
> (easily with a Mallory) to maximize performance and, then, vacuum
> advance, while cruising at a steady part throttle, would increase timing
> under light load conditions which, as in my original post, is supposed
> to provide increased economy and smoother engine operation. Transition
> to full throttle would negate vaccuum and drop total advance to the
> mechanical advance setting -- thus preventing engine knock.
>
> Now I'm wondering how much vaccuum is present in the log manifold under
> acceleration. Maybe I'll go lift the hood of the Triumph with the two
> Strombergs and see if I can figure it out.
>
> Also, there is the perverse "vacuum retard" on emission MGs (that I have
> zero experience with) so I'm curious where that vacuum source is. I
> understood that the "retard" feature was supposed to be active only at
> or near idle so I wonder where the vacuum source sees idle. Or I could
> be wrong again.
>
> Bob Allen, Kansas City, '69CGT w/DCOES, '75TR6 w/Strombergs
|