Post by DOCTOR FIELDS on Sept 13, 2010 22:50:34 GMT -5
Now that you know how a carburetor works, we will go into detail on the main circuits of a carburetor and how they work.
Carburetors have five (5) main circuits. Idle Circuit, Primary and Secondary Circuits, Fuel Enrichment (Power Valve) and Accelerator Pump Circuit.
The idle circuit simply provides fuel at idle. The primary circuit delivers fuel in proportion to the throttle angle of the primary butterfly(ies). Once the secondary butterfly(ies) are kicked in, the secondary circuit initiates fuel flow. The power valve (fuel enrichment circuit) adds fuel only at Wide Open Throttle (WOT) and is typically found on the primary side. The power valve operates off manifold vacuum. At idle and partial throttle, vacuum keeps the valve shut. When the vacuum drops, the power valve opens, adding the equivlent of 7 to 10 jet sizes of fuel. This allows the use of smaller jets for driveability and throttle response, adding extra fuel only when needed. Popular 2 barrel and 4 barrel carbs. have a variety of aftermarket power valves that are designed to open at different vacuum ratings. This allows the engine builder to fine tune the fuel curve.
The accelerator pump circuit is the only circuit not affected by airflow. It is designed to help speed up fuel flow when the intake charge "stalls", by squirting a small amount of fuel. A "bog" or "deadspot" is caused when the volume of air entering the carb. is so great that the carb. signal stalls and no fuel can be delivered. If you go back to Part 1, where we discussed how the fuel is drawn out of the bowl by a pressure difference, you can picture how this stall occurs. A good carb. signal relies on the venturi effect - high velocity / low pressure. When the butterflies are suddenly opened, a large amount of air begins to flow through the venturis. The initial velocity (speed) of this airflow is slow and the pressure is close to or at atmospheric pressure (14.7 psi). The accelerator pump squirts a small amount of fuel to feed the engine until the velocity increases and the pressure drops enough to start pushing fuel out of the bowl.
In Carb. Tech Part 3, we'll get into carburetor sizes and how to select the right one.
Carburetors have five (5) main circuits. Idle Circuit, Primary and Secondary Circuits, Fuel Enrichment (Power Valve) and Accelerator Pump Circuit.
The idle circuit simply provides fuel at idle. The primary circuit delivers fuel in proportion to the throttle angle of the primary butterfly(ies). Once the secondary butterfly(ies) are kicked in, the secondary circuit initiates fuel flow. The power valve (fuel enrichment circuit) adds fuel only at Wide Open Throttle (WOT) and is typically found on the primary side. The power valve operates off manifold vacuum. At idle and partial throttle, vacuum keeps the valve shut. When the vacuum drops, the power valve opens, adding the equivlent of 7 to 10 jet sizes of fuel. This allows the use of smaller jets for driveability and throttle response, adding extra fuel only when needed. Popular 2 barrel and 4 barrel carbs. have a variety of aftermarket power valves that are designed to open at different vacuum ratings. This allows the engine builder to fine tune the fuel curve.
The accelerator pump circuit is the only circuit not affected by airflow. It is designed to help speed up fuel flow when the intake charge "stalls", by squirting a small amount of fuel. A "bog" or "deadspot" is caused when the volume of air entering the carb. is so great that the carb. signal stalls and no fuel can be delivered. If you go back to Part 1, where we discussed how the fuel is drawn out of the bowl by a pressure difference, you can picture how this stall occurs. A good carb. signal relies on the venturi effect - high velocity / low pressure. When the butterflies are suddenly opened, a large amount of air begins to flow through the venturis. The initial velocity (speed) of this airflow is slow and the pressure is close to or at atmospheric pressure (14.7 psi). The accelerator pump squirts a small amount of fuel to feed the engine until the velocity increases and the pressure drops enough to start pushing fuel out of the bowl.
In Carb. Tech Part 3, we'll get into carburetor sizes and how to select the right one.