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Carburetor Tune Up Guide

previous page: Float Circuit


The low speed circuit controls all the flow of fuel going into the engine during idle and light load speeds. It partially controls the flow for light load speeds, between twenty and thirty miles per hour. Above thirty miles per hour, it stops operating completely.

Low speed systems in general, consist of the low speed jet, economizer, air bleed, idle passage, upper idle port and the lower idle port, which contains idle adjusting screw or needle.

During idle and light load speeds, gasoline flows from the float bowl into the low speed jet opening, which is drilled to a definite size to meter the amount of gasoline for this range of engine operation. The fuel travels through the jet into the idle passage. Here it is mixed with air from the air bleed and vaporized.

The mixture then flows down the idle passage, through the lower idle port and into the throat of the carburetor under the throttle valve.

The idle needle or adjusting screw is used in the low speed circuits for the purpose of adjusting the mixture. This helps to compensate for changes in the engine operation due to wear.

When fuel of different grades is used, the idle mixture must be corrected. The idle adjusting screw does this. Since the low speed system also contains an upper idle port, its function is explained in the following: The mixture emerging from the lower idle port is not sufficient to operate the engine at speeds above idle, that is, engine speeds up to twenty miles per hour. Because we cannot change the mixture ratio for idle, we must use a larger quantity of this mixture. To secure this increase, as the throttle valve opens, it uncovers a portion of the upper idle port and more of the mixture enters the throat of the carburetor and flows into the engine. As we continue to open the throttle, more mixture continues to flow until the upper port is entirely uncovered, supplying sufficient fuel to operate the engine, up to the point where the high speed circuits come into operation. Since the low speed circuit plays so great a part in proper engine operation the passages and air bleeds must be kept clean and the dimension must not be tampered with or changed. Worn or damaged idle jets must be replaced. Loose jets or wrong sized jets make trouble shooting a dif­ficult problem. When in doubt use new parts and be certain of a proper repair job.

We must always bear in mind at idle speed, only the idle system should operate. This is not true in cases of badly worn carburetors or carburetors that have been badly mishandled by incompetent, inexperienced mechanics.

On the early WA-1 Carter Carburetors the by-pass and air bleed holes were drilled vertically in the casting. This caused them to accumulate dirt quickly and upset the idle system to such an extent, that the engine could not be made to idle regardless of any amount of adjusting on the idle screw. This condition, however, was eliminated in the later production carburetors, by drilling the top openings horizontally in the casting.

A good rule to go by in setting a carburetor idle is the following: Turn the idle screw in until the engine runs smoothly. If the engine responds to this adjusting, you can be sure the carburetor is operating as it should. Also, it has been found in many cases, an idle needle turned into the casting so tight that it has ridges cut into it. This is absolutely unnecessary. If, when the idle needle is fully closed, the engine still continues to run, thoroughly check for other trouble.

Carbon accumulation around the upper idle port will also cause trouble when it is changed from idle to off-idle speeds up to twenty miles per hour.

To check for any leaking or bleeding in the carburetor during idle, remove the air filter and look down inside the carburetor throat. It should be perfectly dry if the carburetor is working properly with the engine at idle speed.

CAUTION! Do not accelerate the engine while making this last test, as a pop-back can cause serious personal injury.

Shrunken gaskets can also cause a poor idle. The Carburetors on late model cars, that have two screws holding the body to the throttle flange, have trouble with the screws working loose, allowing air to bleed into the carburetor throat and lean the mixture out. This will definitely affect the performance of the engine.

next: High Speed and Power System


Table of Contents

The Carburetor and Its Purpose

Tune-Up of the Gasoline Engine