The vacuum advance only brings in advance when there is vacuum ie when you
are at part throttle or idling. If you want to hook it up you should tap a
hole in the weber manifold closer to the carb if you have the option
otherwise shock waves from the valves closing will influence it. The ported
vacuum on the DGV is above the Butterfly plate and will appear to work fine
except at idle where it will look to the distributor like you are at full
throttle with no vacuum. Tap the hole and insert a fitting. That will save
you some gas and allow you to lower your idle speed adjustment which may
help any runon problem you have. At idle, the fuel air mixture is burning so
slow that 15 to 20 degrees advance may be added by the vacuum advance unit.
Adding 15 to 20 deg advance with the vacuum advance unit will raise your rpm
a bit and you will have to turn down the idle adjustment a bit. The net
result is that you are using less gas at idle with the vacuum advance unit
active and the likelyhood of runon after ignition shutoff under these
conditions is less likely.
The reason your not jumping off the line against those minivans (aside from
the obvious...) is that you probably do not have enough advance at lower
rpm. To test this, set your distributor a few degrees more advanced. If you
feel a little more power off the line you needed more advance at lower rpm.
The only problem is that now you probably have a ping up around 3000 or so
because your mechanical advance is still adding advance. Lighter springs
would bring the advance in quicker and allow you to maintain your current
timing at 2500 but increase the timing advance in the lower rpms. I
originally was going to write a detailed explanation of how and why to do
this but as it amounted to over a page and a half of dry reading I decided
to give you the option of taking my word for it or replying me directly for
more info as to not bore the list( like I haven't done that already... ). In
between my responsibilities at work it was also taking a few days to finish.
Chris Reichle
creichle@nsc.msmail.miami.edu
-----
From: mgs-owner
To: mgs
Subject: Re: Cams and Distributors
Date: Tuesday,February 11,1997 1:25PM
Chris, friends, etc:
I've got a very similar set-up on Rags: 79 engine, header, free-flow
exhaust. However, I've got a Lucas 25D dizzy on the engine ... one from
an older (pre-75) MGB. I've got a Weber DGV 32/36 carb on the car.
Right now, I've got the vaccuum advance on the distributor closed off,
idle advance set at 15deg BTDC @ 1500 rpm. I'm not interested in
replacing the cam. Two questions:
1) Has anyone any experience with changing the idle advance, and what
are some good settings? Rags is pretty happy with 15 deg, but I lack
power on takeoff (witness being smoked by a minivan last week).
2) If I were to attach the vaccuum advance to something, does anyone
know where on the DGV it should go? And what might be the practical
advantages of this? Rags has pretty good power from 2500 rpm to around
4800 or so, when it starts to level off. I usually shift around 3800
or so in normal traffic.
Thanks!
Corey
75 MGB 'Rags'
|