elementary manifold vacuum question

[am018]

Twin carbs ? this is a well known problem;  try drilling both manifold carb
flanges  join them togther using a tee fitting and small bore pipe then add a
bit more restriction  you might also need to add a small volume can  of the type
fitted to the vac adavance pipes on some cars. The idle speed should be about
900 to 950 rpm and mixture should be slightly rich.
Also note twin carb  A and B series engines  tend to idle on  2 cylinders
because of the port design  which plays havoc with the mixture distribution --
can't remember if it is the inner or outer pair  --- If you   do a plug lead
pull off test at idle youll see what  I mean.
After all that if you still get trouble  look for a  tight/bent/burnt valve. For
some strange reason B are also very prone to sticky mechanical advance
mecahanisms -- check the bob weights are free.

Andy Macfadyen

Stowe, Ted-SEA wrote:

> I have a question for the collective. I think I have forgotten vacuum 101. I
> am trying to make sure everything is hunky dory before I strap my gm
> throttle body onto my car.I have a 77 mgb 1800 cc.
>
> this has siamesed intake and exhaust ports unfortunately.
>
> when it is cold and I hook up a vacuum gage to the intake manifold, at about
> 600 rpm the needle fluctuates between 15-19 inches. meaning that the needle
> itself is nearly invisible I can see the trend. if I pinch the hose to the
> gauge, I can dampen down the 'pulses' and get it to stay steady. at higher
> rpms this effect goes away completely.
>
> as it warms up ,at 1000 rpm, it will fluctuate between 16-18 and eventually
> just settle down to a buzz at about 17+.
>
> the engine runs fine and idles fine by all appearances, no shaking or
> anything.
>
> I have no idea if the cam is stock or not.
>
> I remember that one is supposed to actually restrict the vacuum line to
> resolve this issue, I am just wondering if this sounds like word valve
> guides or seals.
>
> I don't know if the gm 1800 cc I have this from had this type of issue and I
> am wondering how this would affect the sensors on the throttle body/ECU. I
> don't seem to remember this as an issue on other engines.
>
> thanks for all the great support btw.
>
> Ted Stowe