You can't take it off the carb port and expect to get the 'usual' readings,
it has to come direct off the inlet manifold like the later vacuum advance.
Use a blanking plug near the middle. You should get in the high teens at
idle, progressively less as you open the throttle, and low to mid 20s on the
overrun. I used to set both mixture and timing using a vacuum gauge on a
single SU carb A-series engine and got better results than the 'standard'
figures. Tried using it on my twin SU B with no benefit, so don't bother
anymore. Timing is set with the damper opened for greater sensitivity,
advance until you can see intermittent 'tics' on the needle, retard until
they cease, then retard a further 3/4" Hg. Mixture was set to the highest
reading, but you can judge that with a rev counter if not the Mk.I ear.
----- Original Message -----
From: "Dereck C" <derex39@hotmail.com>
To: <mgs@autox.team.net>
Sent: Wednesday, March 20, 2002 4:15 PM
Subject: 70 B vacuum reading
> Where is the best place to get a vacuum reading? I tried the vacuum port
> (dist advance) on the rear SU (HS4). I get around 5lbs at idle and up to
> about 15 when rev'ed. What numbers should I expect.
>
> I want to try a reading at the manifold. Does it matter which port I
attach
> to? There are about four of them.
>
> I vaguely remember someone talking about how they tune their engine based
on
> the timing that gives the most vacuum. Is that a good approach to take?
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