Carburador

Carburetor

TOYOTA 4A-F ENGINE

REPAIR GUIDE

See Figures 1 and 2
The carburetor is the most complex part of the fuel system. Carburetors vary greatly in
construction, but they all operate basically the same way; their job is to supply the correct
mixture of fuel and air to the engine in response to varying conditions.
Despite their complexity in operation, carburetors function becauseof a simple physical
principle; the venturi principle. Air is drawn into the engine by the pumping action of the
pistons. As the air enters the top of the carburetor, it passes through a venturi, which is
nothing more than a restriction in the throttle bore. The air speeds up as it passes through the
venturi, causing a slight drop in pressure. This pressure drop pulls fuel from the float bowlthrough a nozzle into the throttle bore, where it mixes with the air and forms a fine mist,
which is distributed to the cylinders through the intake manifold.

Fig. 1: Exploded view of the 4A-F carburetor air horn assembly

Fig. 2: Exploded view of the 4A-F carburetor float bowl and throttle body assembly

There are multiple systems (air/fuel circuits) in a carburetor that make it work: Float
 Main Metering
 Idle
 Low-Speed
 Accelerator Pump
 Power
 Choke system

The way these systems are arranged in the carburetor determines the carburetor's size and
shape.
Carburetors all function in the same fashion; larger engines have larger carburetors to move
more air and fuel, but the principle is still the same. Older units don't have as many external
linkages andcontrols to manage emissions and driveability, but the principle is still the same.
It's important to remember that carburetors seldom give trouble during normal operation.
Other than changing the fuel and air filters and making sure the idle speed is OK at every
tune-up, there's not much maintenance you can perform on the average carburetor. Quality of
fuel and presence of water in thesystem will affect the carburetor; dirt particles in the fuel can
clog the jets and water causes rust and corrosion. If t he vehicle is to be parked or stored for a
long period of time, drain the carburetor to prevent the evaporating fuel from gumming up the
system.
The carburetors used on 4A-F engines are conventional 2 bbl, downdraft types similar to
domestic carburetors. The main circuits are:primary, for normal operational requirements;
secondary, to supply high speed/high load fuel needs; float, to supply fuel to the primary and
secondary circuits; accelerator, to supply fuel for quick and safe acceleration; choke, for
reliable starting in cold weather; power valve, for fuel economy.
It is important to know that carburetors seldom give trouble during normal operation. Other
thanchanging the fuel and air filters and making sure the idle speeds are correct at every tune up, there's not much maintenance you can perform on the average carburetor.

PRELIMINARY ADJUSTMENTS
The following adjustments are to be made with the carburetor removed from the engine.
Adjustments on carburetors interrelate; if you change one setting you may affect other
adjustments. Therefore,these procedures must be performed in the order presented. Read
these procedures thoroughly before continuing.
Float
See Figures 3, 4, 5, 6, 7 and 8
It will be necessary to remove the air horn from the carburetor to gain access to the float.
Follow the procedures outlined in the carburetor overhaul.
1. Remove the float, needle valve, spring and plunger.

2. Remove the pin clip from the needlevalve.

3. Install the needle valve, spring, and plunger onto the seat.

Fig. 3: Installing the float and pivot pin to the side of the carburetor

4. Install the float and pivot pin.
5. Allow the float to hang down on its own weight. Check the clearance between the
float tip and the air horn without the gasket on the air horn. Float level should be 0.283
inch (7.2mm).

Fig. 4: With...