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Purpose and applicability of article

This article is intended to explain the function (as opposed to the often erroneously perceived function) of the idle mixture screw(s) used on most post 1935 down-draft carburetors. It does not cover carburetors with inverse idle circuits where the idle mixture screw meters air only. The inverse idle circuit may readily be identified by the reaction of the engine to a change in the adjustment of the idle mixture screw. Turning the screw IN will cause the engine to run richer with the inverse idle circuit, as the screw is reducing the amount of idle air.

What the idle mixture screw does NOT do

Adjusting the idle mixture screw does NOT change the mixture of the carburetor! Please read on.

The typical idle circuit

The typical idle circuit on many modern downdraft carburetors will consist of: (A) an idle jet, (B) the idle bypass, (C) the idle restrictor, (D) the idle air-bleed, (E) the idle passage/port, and (F) the idle mixture screw. Additionally, SOME carburetors may have an addition idle air bypass built into the throttle body (often a measured hole in the throttle plate).

The idle circuit is generally activated by vacuum under the throttle plate, which causes a low-pressure area in the idle passage/port thus causing fuel to flow through the idle jet.

The idle jet is set at some constant value by the carburetor manufacturer, thus flowing a specified amount of fuel. The fuel enters a passage that has the idle bypass (generally at right angles to the passage). The idle bypass is also a constant, and adds a predetermined amount of air to the fuel. To mix the fuel and air, the passage narrows (called the idle restrictor) which accelerates the fuel and air beginning the mixing process. The passage again widens (generally to the original diameter), and the idle air-bleed now adds additional air in the mixture. The resultant mixture passes through the idle passage to the idle port, and subsequently into the throttle area of the carburetor. The AMOUNT of idle mixture metered through the idle port is controlled by the idle mixture screw.

So, the idle mixture (percentages of air and fuel) are controlled by FIXED idle jet(s), idle bypass(es), idle restrictor(s), and the idle air-bleed(s). The amount of mixture is controlled by the idle mixture screw.


Compare the idle circuit to a shower with three controls: (A) hot water, (B) cold water, and (C) volume (some would call this one pressure).

The analogy would be hot water with fuel, cold water with air, and volume with the idle mixture screw.

Why is this important???

The fixed values for the idle jet, bypass, restrictor, and air-bleed were engineered for a set of engine conditions which probably no longer exist anywhere. The composition of fuel has changed (even if ethanol is not present, other compounds are present which change the atomization characteristics of the fuel), carburetors are migrated from one application to another, enthusiasts modify engines (changing the camshaft duration has one of the more dramatic effects on the idle circuit). These changes can cause either an erratic idle, or a dead spot upon acceleration (often misdiagnosed as a defective accelerator pump). Often, the enthusiast tries to adjust the mixture by adjusting the idle mixture screw for more volume (which does give more fuel, but also more air). Generally, the idle circuit will need to be modified (at least on an original equipment carburetor, maybe not so on an aftermarket unit) for best performance.

Modification of the idle circuit

The most common method of increasing the fuel in the idle circuit is to replace the idle jet with a larger one if the jet is replaceable; or drilling the idle jet slightly oversize if the jet is not replaceable. CAUTION! Once a fixed jet has been drilled TOO large, it cannot be “undrilled”!

One other method of modifying an idle circuit is to add an additional idle air bypass in the throttle plate. This is done to smooth the transition from the idle circuit to the off-idle or idle transition circuit. This modification is often required to provide additional air at idle for an engine with a camshaft of large duration. This modification should be done as a last resort.

When modification doesn’t help

No amount of idle circuit modification is going to make a smooth idle and transition to the main circuit if the carburetor(s) is way too large for the engine. Before making any changes to the idle circuit, where the carburetor is not original to the engine, a little homework as to the applicability of the carburetor might save a lot of time/trouble/expense.


This article, as with all other articles found on this website, have been written to place in the public domain and copied if desired. We only ask that you credit The Carburetor Shop LLC when you copy the article.