Hi fellow SOLers, I'm new to the list and find it very enjoyable and the
signal-to-noise ratio very respectable.
I thought I would ad my $.02 to the PCV thread. First a little background,
I have a 69 MGCGT with 2 SU HS6s but the concepts will apply to any LBC
with multiple carbs.
Scott's Theory of Operation is right with one addition. The PCV valve also
acts to meter the amount of air (or blowby) allowed to be sucked into the
carbs. The PCV valve will only allow a small amount of air flow at high
vacuum (idle) and relativly more flow at low vacuum (WOT). The PCV valve
that you select should DEFINATLY be for an engine with the same number of
cylinders and preferrably for one of the same displacement.
I dare say that if you connected the breather hose to the intake man-
ifold (without a PCV valve) that the engine wouldn't run, there would be
a large vacuum leak. In the stock arrangement the small brass tubes on an
SU carb are "ported" vacuum, ie: they are closed or covered by the
throttleplate at idle and get uncovered as the throttle is opened. This is a
relativly crude way to meter the flow and can contribute to other problems
with the carbs. If you connect a PCV valve to the brass tubes I'm not sure
that you will get the full effect of the PCV valve, although it probably
won't hurt anything.
Another problem with the stock configuration are the different lenghts of
hoses from the breather (or PCV valve) to each carb. Air being like water
(ie: takes the path of least resistance) the carb that has the shortest
path will get the most air (assuming that demand exceeds supply) and will
tend to run relativly lean. I experianced this on my MGC, the symptom was
that when I used a Colortune the rear 3 cyls were always leaner at part
throttle then the front 3. On the "C" the PCV hoses run from a breather on
the rear of the engine over to the rear carb and branches off to the front
carb. No amount of fiddling with the carbs would fix it until I removed
the PCV hoses and blocked off the brass tubes on the carbs, then everything
was fine. I assume that the same condition could exist on other cars of
the same vintage.
I "fixed" the problem much the same way that Scott recommended
with one exception. Instead of splitting the hoses and connecting them
to the tubes on the side of the carbs I connected the hose to the intake
manifold (the conection were the gulp valve used to be), and blocked off
the brass tubes on the SUs with plastic caps. This eliminates the problem
of unbalancing the carbs mentioned above. I used a "modern" PCV valve from
a GM 6cyl (for my 6cyl MGC) and I noticed less smoking and a much more
consistant idle right away.
If you want to make this modification but would like to keep your LBC as
original looking as possible you could use one of the SMITHS PC valves
from an early 60s LBC. These use a spring and rubber diaphragm to control
the flow. They probably work just as well as a modern PCV valve if you
keep them maintained. I am planning on replacing mine with a SMITHs but
I havn't found one for a 6 cyl yet.
I guess I rattled on long enough here.
PS: Any fellow MGC owners out there?
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Llyn Allen | Lifetime member SA
Support Engineer | (Ship-fitters Anonymous)
Cray Research Inc. |
Herndon, VA | 1969 MGC-GT steel wheels/4speed O/D
email:llyna@oregon.cray.com | 1969 MGC-GT wire wheels/auto trans
(703)713-1488 | AMGCR #267
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