Indeed, in fact just about every Japanese motorcycle today with Mikuni
carbs work on
exactly the same principle. Had to work on my suzuki 750 carbs the other
day, and I
felt right at home :)
steve
Tony Rhodes wrote:
>
snip
> I just do not know why EVERYBODY didn't use the CD principle for carbs!
> I heard that George Skinner who designed the SU carb (Skinner
> Union) was a Scotsman. Makes sense. Waste-not, want-not.
>
> -Tony
>
> Message text written by INTERNET:triumphs-owner@autox.team.net
> >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.
> <
--
Steve Chandler - Chandler O'Bagy - steve@obagy.com
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