Carburetor Tuning Basics - Night School

Primary side main fuel metering begins off-idle and goes through wide-open throttle. The stronger manifold vacuum holds the main metering rods down in their jets. A spring counterbalances the vacuum pull, and when vacuum decreases, the spring pressure moves the piston upward. Tapered metering rods, attached to the piston, move upward and allow more fuel to pass through the jets. This is a smooth balance between a calibrated spring, sized rods, and jets, and the amount of manifold vacuum present. When power is needed, the fuel mixture enriches. At cruise and light loads with higher manifold vacuum, the rods remain lower and provide less fuel flow.

Secondary power on four-barrel carburetors like the Quadrajet rely on vacuum signals, secondary metering rods, and an air valve. The throttle valves may be wide open, but full fuel flow will not occur until the low manifold vacuum signals the secondary air valve to open. The air valve opens and simultaneously lifts the secondary metering rods. This increases fuel flow and richens the fuel mixture.

Other carburetor designs accomplish the same results as the Quadrajet with different means. The Holley carburetors, for example, use their familiar power valve. Fuel flow increases when manifold vacuum drops to the power valve's setting. Regardless of design, the goals are the same: 1) provide richer fuel mixtures when load increases, and 2) allow for more fuel flow under wider throttle openings.

Common to nearly all carburetor designs is an accelerator pump. This mechanism is part of the accelerating system. Some accelerator pumps have cup seals and press fuel out of a round well. Other carburetors, like the Holley or Motorcraft square-flange units, use a diaphragm. The aim with the accelerator pump is to provide a smooth, powerful surge of power when the throttle opens quickly. The added "shot" of fuel is necessary because fast throttle openings mean more airflow with a lag in fuel flow. (Air is lighter than fuel, and fuel flows slower when the throttle opens rapidly.) Without the accelerator pump, there would be a momentary lean condition under hard acceleration.

Surely important is the cold start circuit. This is necessary because the air/fuel ratio must be richer for a cold engine. Combustion is poor, and a 14.7:1 stoichiometric air/fuel ratio would cause stumbling, stalling, and poor idling in a cold engine. To compensate, the carburetor has a choke valve above the primary throttle bores.

Chokes can open manually, electrically, or by engine heat. The earliest choke valves were hand-controlled by either a cable or rods. Heated chokes with a bimetallic coil spring and engine heat source became popular in the '30s. Electric chokes, essentially a variation of the bimetallic coil choke, came about in the '60s with the emergence of emission controls. Each design serves the same purpose to reduce airflow while maintaining fuel flow. An enriched air/fuel ratio is necessary for starting and operating a cold engine.

Troubleshooting And Rebuilding
Understanding the functions of a carburetor increases one's troubleshooting skills. By knowing what each circuit of a carburetor does, a tuner or troubleshooter can focus on the area of the carburetor that causes a problem. While a carburetor overhaul will generally provide wide enough coverage to eliminate most circuit quirks, the skilled rebuilder looks for specific components that create trouble symptoms.

An important aspect of carburetor rebuilding is linkage adjustment. Air bleed vents, secondary release linkages, choke rod clearances, and accelerator pump settings are crucial to carburetor performance. It is not unusual for a factory-built carburetor to have incorrectly adjusted linkages.

This month's photo illustrations will increase your knowledge of carburetor circuits. There is more to restoring a carburetor than cleaning the parts. Join me at my workbench, and we'll walk through the functions and needs of a carburetor.