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Operation of a simple Venturi
The Mikuni slide type carburetors described in this manual are also known
as “variable venturi” type carburetors. A venturi is a restriction within the
carburetor designed to speed up the air lowing through the carb. It is at
the point of the smallest cross section that the incoming air low will have
the greatest velocity. As air low speeds up to pass through the venturi,
it loses pressure. It is at the point that the pressure within the carb throat
will be at its lowest; this is called “depression.”
At this point of maximum depression, the fuel will be introduced to the air
stream. In order to transfer fuel from the fuel chamber into the carburetor
venturi, a small tube is placed into the venturi which connects to the fuel
load in the chamber to the venturi. The fuel chamber is open to the atmospheric
pressure (15psi approx.) and the pick-up tube is exposed to the depres-
sion within the venturi. The difference in pressure will attempt to equal-
ize through the pick up tube, pushing fuel from the fuel chamber into the
venturi. The fuel is mixed with the incoming air and delivered to the engine
down stream. This is the basic principle by which all carburetors operate.
It is important to remember that is the pressure difference between the
fuel chamber and the venturi which pushes the fuel into the intake stream.
The presence of engine vacuum alone is not enough to draw fuel into the
intake port. Without the atmospheric pressure in the fuel chamber, the fuel
could not be delivered into the intake port.
The above model will serve a single speed engine very well with the
venturi becoming the throttle. The engine will run at one speed only, de-
pending upon the size of the venturi. because motorcycles are operated
at various engine speeds, there needs to be a way to control the speed of
the engine.
In order to throttle down this wide open carburetor design, Mikuni has
inserted a slide which crosses the carburetor throat to control air low to
the engine, limiting engine speed according to the slide lift. This slide lift
varies the cross section of the carburetor bore at the venturi point. This is
how slide carburetors become known as “variable venturi.”
These variable venturi type carburetors are very good at
maintaining high air speeds with in the carburetor throat and
generally offer better overall performance other carburetor
designs.
From this simple fuel delivery circuit, carburetor designers
will add and divide this circuitry in order to tailor the mixture
available to the engine, enabling it to run accurately at a
wide variety of speeds and loads.
Simple Venturi Design
Venturti Operation
Variable Venturi Design
Throttle
Throttle Valve (Slide)
Engine
Vacuum
(Intake Port)
Mixture
Pick up Tube
(Needle Jet)
Main Jet
Air at
Atmospheric
Pressure
Atmospheric
Pressure (15psi)
Maximum Depression Zone
Fuel Chamber
Mixture
Pick up tube
(Needle Jet)
Atmospheric
Pressure (15psi)
Air at
Atmospheric
Pressure
Engine
Vacuum
(Very Low pressure)
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