I have an 80 with the DGV conversion. You wan to get your vacuum from the
manifold not a port on the carb. As for using the current setup, I would
suggest tapping another hole on the other side of the manifold from the
brake servo close to the carb end of the manifold. My servo tap has a
special valve on it which closes when the vacuum drops and this might
prevent proper function of the advance unit. Get a vacuum tester and check
the vacuum. You should have high vacuum at idle and low at open trottle. If
the needle in the guage bounces, try to tap further back towards the carb.
The valves produce a presure wave when they close which is strongest on the
head side of the manifold. You want to try to avoid that. In my manifold
there are two tap areas on either side of the manifold pick the one closer
to the carb and you should be fine.
Chris Reichle
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From: mgs-owner
To: mgs
Subject: Vacuum advance questions
Date: Thursday,September 19,1996 5:50AM
Dear SOLers -
Need some advice from the Wisdom of the Web. When I rebuilt my '79B
engine, I changed from the ZS carb to a Weber DGV (thanks, Bob!!). But,
I don't know how the vacuum advance was originally connected (hoses
missing) or how it should be connected now. The large vacuum line from
the manifold to the brake servo is connected to a port on the intake
manifold - that works properly. The DPO or someone had put a tee
fitting in this line - there was a piece of hose attached to the free
arm of this tee. That's where I have connected the vacuum advance
unit. The car runs well - make that great! - but I don't know if the
advance is really right. There are also two vacuum fittings on the carb
- both capped right now. The distributor is the original '79 Lucas with
the amplifier (breakerless electronic). The amplifier is inop (of
course) and has been replaced with an external Crane electronic
ignition. I have tested the advance by putting a vacuum pump on - it
does work freely.
I have put a timing light on and checked at various RPMs - but with
the
mechanical advance built in the dizzy, I can't really tell what is going
on with the vacuum advance. I seem to remember some discussion here
about a year ago - (Denise - was that you?) but didn't save it.
Here are the questions:
1. Does the distributor vacuum advance require manifold vacuum - i.e.
vacuum greatest when the throttle opening is closed. This would give
the least vacuum at high RPM when the throttle is widest.
2. Should the advance be greatest at high RPM/wide throttle position?
This would seem to indicate ported (throttle body??) vacuum that would
increase as the airflow speed increases through the venturi.
3. So, what is the correct hookup for my carburetor/distributor
combination?
Thanks for your advice-
Wayne Kube
'79B
Plano, TX
BTW - there is a picture of my car at
http://www.mgcars.org.uk/MGB/mgbpics.html
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