The carburetor supplies the correct mixture of fuel and air to the engine under varying conditions.
Despite their complexity in design, carburetors function because of 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. The pressure drop pulls fuel from the float bowl through 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.
All carburetors except the C–W are 2–barrel, down-draft carburetors which supplies the best air/fuel mixture under any operating condition. The C-W is a 1–barrel carburetor.
The carburetor uses a progressive linkage between the primary and secondary circuit. For optimum performance plus fuel economy, the secondary circuit of the carburetor is used only at high engine rpm. Normal low speed operation is handled by the primary circuit.
On later models, the carburetor is provided with a coasting by-pass system which helps control exhaust emissions during deceleration.
A hand control (manual) choke was used on Subaru models until 1973. From 1974 until present an automatic control choke is used. The automatic choke and a throttle chamber heated by engine coolant, to prevent throttle bore icing, help the Subaru start and run well in the coldest conditions.
The basic systems of the carburetor are:
On the following pages illustrations will help you better understand the systems and what functions they perform.
If you are planning to clean, rebuild or replace your carburetor be sure you understand what is necessary. Read all instructions, have all parts/tools on hand and keep everything as clean as possible. Remember the gasoline mileage and the performance of your Subaru depend on how well you do the job.