The model G two bore carburetor has an idle system to supply the correct air/fuel mixture ratios to the engine during idle and low speed operation. The idle system is necessary during this period because air flow through the carburetor venturi is not great enough to cause fuel to flow from the main discharge nozzles. Each bore of the carburetor has a separate idle system. They consist of idle tubes, idle passages, idle air bleeds, and the idle mixture adjustment needles or screws, and discharge holes.
In the conventional idle system, at idle speeds the throttle valve is cracked slightly open, allowing a small amount of air to pass between the throttle valve and bore of the carburetor. Since the engine requires very little air for idle and low speeds, fuel is added to the air by the application of vacuum (low pressure) from the intake manifold, directly through the idle system to the fuel in the carburetor floa tbowl. With the idle mixture needle holes located in a high vacuum (low pressure) area below the throttle valves and the fuel in the float bowl vented to atmosphere, the idle system oeprates in the following manner:
Fuel from the float bowl flows through the main metering jets into the main fuel well. It is then picked up and metered by the calibrated orifice at the tip of the idle tubes. It then passes up the idle tubes and is mixed with air from air bleeds located at the top of the idle tubes and in the idle cross channels in the venturi cluster casting. The mixture then passes downward in the idle channels through a calibrated restriction to the off idle discharge holes located just above the throttle valves. Here the fuel mixture is again bled with air and then moved to the idle needle holes where it is discharged and blends with the air passing the slightly open throttle valves and enteres the engine manifold as a combustible mixture. The idle mixture needle controls the amount of fuel mixture which enters each carburetor bore. Turning the mixture screw clockwise (inward) decreases the fuel discharge (gives a leaner mixture) and turning the screw counter-clockwise (outward) increases
(enrichens) the fuel mixture. The number, size and location of air bleeds in the idle passages in the cluster castings, are determined by the engine idle requirements. These will vary between carburetor models.
On some applications, lower idle air bleeds are used in the idle system to supplement fuel flow after off-idle operation and during the main metering system operation. During idle they act as air bleeds. Operation of the lower idle air bleeds is explained under the main metering system.
As the throttle valves are opened and more air is entering the engine to increase engine speed, additional fuel is needed to combine with the extra air. This is accomplished by the off-idle discharge holes. As the throttle valves and the extra fuel needed is supplied by these holes.
The idle and off-idle holes supply sufficient fuel from engine requirements until air velocity is high enough in the venturi area to obtain fuel flow from the main metering system.
NOTE: Some carburetors are slotted off-idle discharge ports in place of the conventional off-idle discharge holes. Either method gives the correct air/fuel mixture ratios, the type upsed in usually determined by engine requirements.
The idle air by-pass system is used on some two bore carburetors to allow the throttle valves to be completely closed during curb idle operation. This prevents gum and carbon formation, which may form around the valves, from disrupting engine idle.
The fuel flow in the system is basically the same as described in the conventional idle system. However, the idle air which normally by-passes the slightly open throttle valve is passed around the closed throttle valves through an idle air by-pass channel.
In this system, idle air is taken from the carburetor bore above the thottle valves, by-passes around the closed throttle vales, through a separate air channel and enters the carburetor bore just below the throttle valves. The amount of air which is supplied to the engine is regulated by an idle air adjustment screw located in the idle air by-pass channel. The idle air adjustment screw is mounted in the float bowl casting at the rear of the carburetor unit. Turning the screw inward (clockwise) decreases the engine idle speed and turning it outward (counterclockwise) increases the engine speed.
In order to obtain sufficient idle air for stable idle speed adjustment, a fixed idle air bleed is necessary. The fixed idle air bleed in the 2-bore carburetor is accomplished by a drilled hole through each throttle valve. The fixed idle air bleeds maintain a constant idle air flow for part of the idle air requirements, while the idle air adjustment screw regulates the remainder of the idle air. Thus, the engine speed can be adjusted by the idle air adjusting screw.
The idle air compensator is used on some carburetor models to offset enrichening effects caused by excessive fuel vapors from fuel percolation, during extreme hot engine operation.
The compensator consists of the thermostatically controlled valve usually mounted in the area above the main venturi or at the rear of the float bowl. The valve closes off an air channel which leads from above the carburetor venturi to a point below the throttle valves.
The compensator valve is operated by a bi-metal strip which senses temperature. During extreme hot engine operation, excessive fuel vapors entering the engine manifold cause richer than normally required mixtures, resulting in rough engine idle and stalling. At a certain pre-determined temperature, when extra air is needed to off-set the enrichening effects of fuel vapors, the bi-metal strip bends and unseats a valve which uncovers the air channel leading from the carburetor venturi to below the throttle valves. At this time, just enough air is added to the engine to offset the richer mixtures and maintain a smooth engine idle. When the engine cools and the extra air is not needed, the bi-metal strip closes the valve and operation returns to normal mixtures.
In order to insure proper idle adjustment, the valve should always be closed when setting engine idle speed and mixtures.
An important design feature used in the Rochester 2 jet carburetors is throttle body venting. Its purpose is to give quicker hot engine starting after the engine has been shut down for a short period.
During extreme hot engine operation the fuel in the carburetor tends to boil and vaporize due to engine heat. Some of the fuel vapor tends to reach the carburetor bores and condense on the throttle valves and seep into the engine manifold; by venting the area just above the throttle valves, hot engine starting time can be reduced to a minimum, on applications where the carburetor is exposed to extreme engine heat.
There are two methods used in venting the throttle bore area:
The located of the vent holes are such that they will not disrupt engine idle or off-idle operation. They are located above the throttle valves on the side opposite the mixture screws, in an area where the transfer from idle to main metering will not be affected.