Here's part one of a general response about SU carbs and ZS carbs. I'm not
the author, but I've posted this here a few times and it seems to be
accurate. It helped me immensely. I'll post part two shortly.
-----Original Message-----
From: DANMAS@aol.com [mailto:DANMAS@aol.com]
Sent: Tuesday, April 20, 1999 10:34 AM
To: jak0pab@jak10.med.navy.mil; Spitfires@autox.team.net;
triumphs@autox.team.net
Subject: Re: ZS carb question! - long-winded response, part 1
In a message dated 4/19/99 6:16:48 PM Eastern Daylight Time,
jak0pab@jak10.med.navy.mil writes:
> Next question, could someone explain to me what the
> difference is between a normal carb and a constant depression carb. And
> why do we put oil into it?
Patrick,
I'm an electrical rather than a mechanical, but I'd like to take a shot at
your question if I may. Keep in mind, I don't understand things very well,
so
I have to break them down into bite sized pieces if I want to make any sense
of it. That means this will be a long response. I'll describe the operation
of the carburetor in layman's terms, because that's the only way I
understand
it, so my description may not be 100% technically accurate.
First of all, take a look into the throat of your carburetor, and look at
the
needle/jet assembly. You'll notice the needle moves in and out of the jet as
the piston moves up and down. You'll also notice that the needle is tapered.
As a result, when the piston is down, as in idle, the larger diameter of the
needle almost completely fills the jet, leaving very little room for fuel
flow. On the other hand, when the piston is up, as in high RPM operation,
the
needle is pulled out to where only the thin tip remains in the jet, allowing
for a large opening in the jet for fuel flow. Basically, you have an
infinitely variable jet, between the limits of the needle movement.
Next, look through the bore of the carburetor, from the inlet to the outlet.
You'll notice that the inlet and outlet bores are about the same, but the
piston creates a restriction to air flow through the carburetor. The
restriction of the piston forms what is known as a venturi. A venturi has an
interesting property, in that air flow through it creates a reduced pressure
in the throat of the venturi. In America, we refer to a reduced pressure
such
as this as a vacuum, while our British cousins use the term "Depression."
The
magnitude of the depression depends on two things: the rate of air flow and
the size of the restriction. For a given air flow, a smaller restriction
gives a larger depression. For a given restriction, a larger air flow gives
a
larger depression.
As the needle/jet assembly is located in the throat of the venturi, the
depression created by the flow of air "sucks" fuel out of the jet (I put the
term "sucks" in parentheses because it is not technically correct, but for a
layman's explanation, it'll do). For a given depression, the amount of fuel
drawn out of the jet will depend on the position of the needle, and for a
given needle position, the amount of fuel will depend on the depression. The
rate of fuel flow will increase as the needle is withdrawn or as the
depression increases.
Now, on to the meaning of the term "constant depression," or CD, as in
"Zenith-Stromberg CD carburetors." Within the body of the carburetor are
several internal passages, connecting various parts of the carburetor to
various ports in the air flow path. Without going into a great deal of
detail, the passages and ports are arranged to adjust the position of the
piston to engine and air flow conditions such that the depression in the
throat of the carburetor, and thus seen by the needle/jet assembly, remains
constant under all operating conditions. At high engine speeds, and the
corresponding high air flow, the piston is up in the carburetor, creating
less of a restriction. At low engine speed, and the corresponding low air
flow, the piston is lower, creating a larger restriction. The design of the
carburetors is such that the depression produced at the low air flow/large
restriction creates exactly the same depression as the higher air flow with
less of a restriction.
As a result of the constant depression operation, the only factor
determining
the fuel flow is the position of the needle. The "suction" is the same at
all
engine speeds, but when the needle is out, the larger opening in the jet
allows more fuel to be drawn.
This is about as far as I can get with one post, and have it get by Mark's
server, so I'll talk about the dashpots in part two.
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://members.aol.com/danmas/
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74
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