elementary manifold vacuum question

[Stowe, Ted-SEA]

Tom thanks for the info.
yes this is a 5 port head, I have a pierce manifold and a downdraft Weber on
it, the car came that way. I don't know if I have the original head or cam.
due to the fairly awkward manual choke , when it starts up cold it runs at
about 600+ rpm, a few minutes later it's doing it's normally 1000.
I did a compression test last night and it came to be
130 130 125 135
1   2   3   4

I am trying to track down an id on the head via casting numbers.


this manifold just has 2 large 'pipes' which go to each intake port, these
come from a pretty large base which meets up with the downdraft carb. so
there is no balance tube as such. I am measuring this from a port on the
manifold, but come to think of it, this is about 1 inch out of the front
head intake port, so the other port isn't really a major part of this
reading.

interesting problem.

thanks, Ted.
-----Original Message-----
From: Tom Parker [mailto:tparker at nznet.gen.nz]
Sent: Monday, November 09, 1998 1:32 AM
To: Stowe, Ted-SEA
Subject: Re: elementary manifold vacuum question


Stowe, Ted-SEA <StowT at PerkinsCoie.com> wrote:

>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.

Sorry, started this post thinking you had an A series engine, read 1000cc
not
1800cc, but maybe it's relevant.

This is with a  5 port head, right? Are you running a twin carburettor or
twin
barral carburettor? If so, does your inlet manifold have a balance pipe
running between the two inlet ports? I was told that if you hook up a brake
booster to one of these engines, you must use both inlets, otherwise you get
too much pulsing in the brakes.

Whether the normal balance pipe between the two branches of a twin SU inlet
mainfold is enough to balance the vacuume, I don't know.

>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.

600rpm sounds a bit low for warm up and any type of vacume measurement. You
might be seeing the individual sucks, at 600 per second if the gauge only
sees two cyclinders.

--
Tom Parker - tparker at nznet.gen.nz
           - http://www.geocities.com/MotorCity/Track/8381/