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Re: Vacuum Advance

To: "Barney Gaylord" <>,
Subject: Re: Vacuum Advance
From: "Ptegler" <>
Date: Thu, 1 Feb 2001 15:55:40 -0500
...have to add a bit of 'technology flip-flop' info that
has taken place over the years.
As Barney described...manifold vac goes with retard
ported goes to adv.

But there were years/models that did use a vac adv.
pot on the dist. while hooked to the manifold rather than
ported to the carb throat.
The idea 'at that time' was with max vacuum at idle...
you'd get max advance. No engine load at idle with max advance
meant a cleaner idle burn. As you opened the carb...
(accel) the vac dropped off a bit, so it wouldn't ping.
Then up at cruise... the high manifold vac would
then again adv. for max burn time.

The reason this format is no longer used is one of component
'slop' and timing of cam/dist/spark etc.
If just a wee bit off...the systems start fighting each other 
very inefficiently.

If anyone has been following my  stumble/surge problem....
this very subject was the root of the problem.

The aging systems were the 'stumble' I had.
I've gone to an older model dist, (different vac adv. curve)
and now use ported vac, and the problem is totally gone 
now. A side reward was also better power at the lower 
rpm ranges.   

So ...just because you  rebuilt everything to spec for your model year
doesn't mean it's the best setup for your aging toy.

The cat's out of the bag Robert L. ...   :-)

...web pages to come.

Paul Tegler

----- Original Message ----- 
From: "Barney Gaylord" <>
To: "R. O. Lindsay" <>
Cc: <>
Sent: Thursday, February 01, 2001 2:39 PM
Subject: Re: Vacuum Advance

At 10:46 AM 2/1/2001 -0600, Rick Lindsay wrote:
>   The original designs for vacuum advance used a port on the carburetor
that was connected to a small opening in the throat of the carburetor. ....
As the throttle is opened, the vacuum rises (depresses?) and the vacuum
advance module goes to work. ....
>   .... Newer designs have a vacuum port on the inlet manifold rather than
the carb.  That port shows pretty strong vacuum even at idle.  My car has
BOTH ports. Which method of applying advance is BETTER for my engine and my
application? ....

The method ported from the manifold uses a different distributor, which can
be thought of as "vacuum retard" distributor.  The vacuum from the manifold
is high at idle and holds the distributor in a retarded position.  When you
open the throttle the manifold vacuum drops, allowing the timing to
advance.  You need to use whichever port is appropriate for your
distributor.  Or conversely, for those who do not have both ports, you need
to use whichever distrubutor is appropriate for your existing port.

Barney Gaylord
1958 MGA with an attitude (and no vac Mallory dizzy)

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