The main metering system is designed to supply the leanest fuel mixture for cruising in the 35 MPH and over range. Operation is simple. Fuel from the main metering jet enters the main well and is mixed with air from the high speed air bleed. Engine vacuum pulls this air/fuel mixture and discharges it through the booster venturi and into the manifold through the throttle bores.

On a street vehicle optimum jetting can be determined by driving at various steady speeds and taking vacuum and spark plug readings. Manifold vacuum will increase the closer you get to ideal jetting; it will fall off once you get past this point. The ideal color for the spark plug porcelain is light brown or tan. A color lighter than this indicates that the carburetor is jetted lean; a darker color indicates that the carburetor is jetted rich.

Holley jets are number-stamped on their side for identification purposes. A higher relative number indicates a larger jet size. Changing to a larger or smaller jet will either richen or make leaner the carburetor's fuel curve from part throttle to full throttle, respectively.

NOTE: Jump two sizes when changing the carburetor jetting. There is approximately a 4-1/2% flow difference from one jet size to the next and one size won't make that much of a difference. If you must go up or down 8-10 jet sizes then you have a problem either with the fuel delivery system or the carburetor is wrong for the application.

Main Metering System

Side View Main Metering System.
Showing Idle Tube in Main Well.