WHAT IT IS

The main metering circuit in a carburetor is responsible for delivering the primary fuel supply to the engine under most operating conditions (beyond idle and high-speed/load scenarios). Here's a basic breakdown of how it works:

  1. Venturi Effect: When the throttle is opened beyond the idle point, air flows faster through the carburetor's main venturi (the narrowest part of the carburetor's throat). This increased air speed creates a drop in pressure.
  2. Jet: The main metering jet, which is a precisely-sized hole, allows fuel to be drawn out from the float bowl due to the low pressure in the venturi.
  3. Air Bleeds: These introduce air into the fuel before it's drawn into the venturi, ensuring that the fuel is properly atomized. This is essential for efficient combustion.
  4. Booster: Located above the venturi, the booster further atomizes and mixes the fuel with air. It uses the Bernoulli's principle: as air speed increases in the venturi, pressure decreases, drawing fuel from the main metering circuit through the jet and into the booster where it's mixed with air.
  5. Transition: As the throttle is opened and engine speed increases, there's a transition phase between the idle circuit and the main metering circuit. Ports near the throttle plate allow additional fuel to be drawn in to ensure there isn't a lean condition during this transition.

The main metering circuit is carefully calibrated to deliver the right amount of fuel for a given amount of air, ensuring the air-fuel mixture is optimal for combustion in the engine. This calibration can be adjusted for different conditions or performance goals by changing the size of the main jet.

Over time, it's not uncommon for parts of the main metering circuit, especially the jet and other small passages, to become clogged with dirt or varnish, especially if a vehicle has been sitting for a long time. Regular maintenance, cleaning, and proper tuning are essential to keep the carburetor functioning efficiently.

HOW IT WORKS

When air flow is high enough to create enough pressure differential in the boost venturi for fuel flow to start from the main nozzle, the transfer point has been reached and the carburetor starts metering from the main metering system.

The fuel passes through the main metering jet into the main well where it rises in the main well passage and idle pickup tube. The fuel continues up the main well tube to the horizontal crossbar in the air horn and to the main discharge nozzle. At this point, air is bled into the fuel by the two air bleeds in the top of the crossbar channel. The mixture is then discharged through the main discharge nozzle into the boost venturi. Here, the air/fuel mixture mixes with additional air and moves to the main venturi of the carburetor and into the intake manifold.

The calibration of the main metering jet and air bleeds throughout the main metering range. Therefore, no adjustments are necessary in the main metering system.

PARTS

Single metering jets

Custom jet assortment (sold in pairs, ideal for multi-carb setups):

Main well tube

REBUILDING & TECHNICAL

Replacing the main well tube:

https://youtu.be/nFmDtYQR5dU?si=gKzadDOpcPFrelNn