[Top] [All Lists]

Re: Vacuum advance, or lack thereof...

To: John Steczkowski <>
Subject: Re: Vacuum advance, or lack thereof...
From: Graham McCann <>
Date: Thu, 24 Jul 1997 09:56:02 +1000
At 07:47 pm 22/7/97 -0700, John Steczkowski wrote:
>'75 MGB
>Of of the PO or PM (previous mechanic) removed the vacuum line for the
>vaccuum advance, at least they put caps on the distributor and the
>intake manifold so I didn't have a vaccuum leak.  The question is, what
>are some reasons for doing this, that is, what are some of the things I
>should expect to see when I hook it back up?
>I guess, bare minimum, I'll get some vaccuum line, hook it up, get a new
>timing light (the one I have is broke) and set the timing properly.
>Ever since I got the car (a couple of weeks and maybe 200 miles), I have
>noticed that it is pointless to rev very high, because I'm not getting
>any more power. Could that be a result of not getting any advance at
>high revs? Or is it a result of no power anywhere in the rev range :-)
The vacuum actually retards the ignition under load, not advances it.
Power comes from retarding the ignition.  In many of the rubbernose series
engines owners found that the engine would idle much smoother if the vacuum
was disconnected.  The problem is that the vacuum line comes from the inlet
manifold, not the carby throat and thus is a much greater vacuum,
especially at idle.  I think this was done to ensure that the engine would
clear unburnt fuel better and so pollute less.  The SU carby was a very
'dirty' idler, hence the change to other carbies on the US specification
cars.  The vacuum unit spring has a much higher rating to cater for the
greater vacuum.

If you refit the hose and note the change in timing setting as you refit
the hose you will find that it will wind on about 10 degrees of advance.
This retards to the desired setting when you accelerate because the vacuum
falls away under engine load but can mean that the car idles very roughly.

My experience is with the SU carby spec, because that is all we got here,
thank goodness, but I suspect  that the vacuum pickup was from the manifold
for your cars.

Hope this helps.


97 MGF

Graham McCann  Rivett, ACT. Australia
Phone/Fax: +61 6 2889055

<Prev in Thread] Current Thread [Next in Thread>