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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


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 Valve (Slide)



(Intake Port)


Pick up Tube

(Needle Jet)

Main Jet

Air at




Pressure (15psi)

Maximum Depression Zone

Fuel Chamber


Pick up tube

(Needle Jet)


Pressure (15psi)

Air at





(Very Low pressure)