carburetor Tune Up Guide

This is the text of a booklet published in the late 1940's titled "Hygrade Motor Tune-Up and Carburetor Manual". It covers the operation and service of one and two barrel carburetors, as well as some outstanding real-world information on how to make your car run well using simple tools. Especially interesting is the section on setting up Buick Compound Carburetion.


A carburetor is a mechanical device on an internal combustion engine, for the purpose of mixing air and gasoline into a combustibly fine vapor, in automatically changing proportions, depending on the operating conditions of the engine.

As an example, an engine that runs continually at one speed, day in and day out has need only for a carburetor of the simplest construction. One that has only to mix air and gasoline in one fixed ratio. However, when the demands of the engine are changed and it is desirable to run it at variable speeds, the carburetor must mix air and gasoline in different proportions and therefore, its construction must be more complex.

With this in mind, the throttle valve was added to gain better control of the flow of vapor into the intake manifold. Likewise, the mixture ratio for fast engine operation was not suitable for idling speed and an idle circuit was added.

To compensate for the lag that developed, when the engine speed was suddenly changed from idle, to a faster degree of operation, an accelerating pump circuit was incorporated in the carburetor construction.

To aid in cold engine starting, the choke circuit was another addition. All of this was the basic development of the present day carburetor.

As years passed refinements were added to meet the increasing demands for more power and greater engine performance. Fuel economy was also a large governing factor in carburetor design.

To better understand how a carburetor automatically changes its mixture ratio to meet the operating demands of the engine, the theory of carburetion is explained as follows: A carburetor, to state it simply, operates on a difference in pressures; atmospheric and pressure below atmospheric.

Atmospheric pressure, the scientists have proven, is 14.7 pounds per square inch at sea level. This pressure completely surrounds us and because the pressure is the same externally as it is internally, we cannot feel it with our bodies. However, if for some reason the pressure inside our body were reduced and the pressure outside remained at 14.7 pounds per square inch, our body would be steadily pushed together as the pressure inside was decreased. This same reasoning can be applied to the carburetor on an automobile engine. The gasoline is Pushed through the carburetor.

The engine is a sealed unit, so far as the combustion chamber and intake manifold are concerned. The only way air can enter the engine at atmospheric pressure, is through the carburetor throat or air horn. When this air reaches the throttle valve or plate, its flow is reduced to practically zero if the throttle valve is closed.

Assuming the valve is in a closed position and we turn the engine over with the starting motor, the piston in the cylinder moves downward. In doing this, it creates a space in the cylinder to be filled by air under atmospheric pressure. The air in the intake manifold moves into the cylinder. However, since the throttle valve is closed, air cannot flow into the intake manifold fast enough, or in a large enough volume, to bring the pressure up to 14.7 pounds per square inch. Therefore, we have a low pressure in the manifold.

This low pressure is commonly called vacuum. As we continue to crank the engine, this vacuum increases or the pressure in the manifold continues to de­crease. Since the pressure on the top side of the throttle valve remains the same (i4.7 lbs.), it is also exerted against the fuel in the float bowl of the carburetor. This forces the gasoline through the idle circuit of the carburetor, and because the discharge end of the idle circuit is under the throttle valve in the low pressure area (intake manifold side of the throttle valve), the mixture flows into the manifold and combustion chamber. If the ignition would be turned on at this point, the engine would operate under its own power. Since the pistons would travel up and down in the cylinder, at a faster rate of speed than with the starter turning the engine over, the vacuum in the intake manifold steadily increases, until it reaches between 18 and 20 inches, as measured with a vacuum gauge on an average six cylinder engine at idle.

In as much as the engine has such high manifold vacuum, we would be inclined to wonder why the pressure (atmospheric), being exerted on the gasoline in the float bowl, would not cause the gasoline to flow through the carburetor and into the engine like water flowing through a faucet. However, this cannot happen, due to the many ways the gasoline is directed or "steered" through the carburetor by the metering openings in the jets and passages in the unit itself.

Since it is now understood what causes the gasoline to flow through the carburetor, let us proceed to discuss the construction, circuits, and controll­ing factors, that are the reasons the carburetor can automatically change its mixture ratio for the different demands of the engine.

next: Carburetor Construction and Circuits


The Carburetor Doctor