Here is a good read.
From: pharvey@mipos3.intel.com (Paul Harvey)
Newsgroups: rec.autos.tech
Subject: SU/Hitachi variable-venturi carbs theory of operation
Date: 12 Feb 92 19:28:41 GMT
"These carbs are very simple and very reliable relative to multiple-
jet/air-bleed type fixed venturi carbs. They have only one jet!
So how do they work?
Through a sliding piston arrangement, they maintain the same air velocity
through the venturi, independent of air flow, by varying the size of the
venturi. So as the air flow increases, the piston moves up and the size
of the venturi increases and the air velocity remains the same!
Now, if the velocity through the venturi is constant, that means that the
vacume on the fuel bowl is constant, (Bornoulli's law), and that means
you can exactly meter the correct amount of fuel into the air. A sliding
needle is attached to the piston and it is machined with a slight taper
so that as the piston moves up, more fuel is metered out of of the jet
port so that the air-fuel ratio is kept constant, independent of air flow.
The port is threaded, and a dial turns it so it can be moved up or down
relative to the needle so the idle mixture can be adjusted. Note this
also affects the mixture at all speeds.
An engine needs fuel enrichment during acceleration, so this is done
by a one-way oil damper on the piston. The oil damper delays the piston
on its up-stroke, so in this case the velocity through the venturi is
higher than normal and more fuel is sucked out of the jet port than normal,
which gives you enrichment! The thickness of the oil changes the duration
of the enrichment period as well as the ratio of enrighment.
That's it!
Compare this to a modern Weber-type carb sometime. I'll let someone else do that!
Sure, there are a few negatives!
This type of carb is not easy to modify relative to a Weber-type which
can me modified in ten-thousand different ways. The only things that can be
varied are the jet and needle valve, the idle mixture, and the damper oil!
Of course, this means if you modify your engine, you probably will not
need to modify your carb to compensate.
The machining of the piston needs to be precise, so some British manufactured
carbs are a problem. Of course, Weber-type carbs need more precise machining!
You do not get the instant throttle response of a good Weber setup, but
acceleration is limited by the speed at which the engine can push the car, but
most people like fast throttle response anyway!
Air leaks around the throttle bushings cause them to perform even worse than
Weber-types with the same problem.
And this carb never really made it through the emissions era, there
are some emissions versions, (73-74 Datsun Z), that mostly are junk and
cannot be maintained.
But there is nothing like the woosh sound of these carbs, and just imagine
how much fun you can have asking people what type of oil to put in them!"
http://yarchive.net/car/carb_oil.html
http://yarchive.net/car/index.html