It seems reasonable that if we are able to supply enough fuel for idle and introduce a little more for slightly higher speeds, this isn’t going to be enough for cruising speeds. Here we are talking about Classic Cars Carburetor Main Metering System.
So, we need an additional fuel passage. Besides, we haven’t even made use of the venture effect which we spent so much time discussing.
The main metering system controls the economy range of the carburetor.
It consists of a main jet and a main nozzle with air bleeds in it. The main jet is a very accurately machined opening, which controls the fuel flow through the main well in which the main nozzle is located.
An air bleed in the main well and the air bleeds in the main nozzle keep the mixture constant through out the operating range of the main metering system for maximum economy.
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The main metering system operates as follows:
As the throttle valve is opened, air velocity through the venturi increases, which in turn decreases the pressure in the venturi at the outlets where the main fuel nozzle is located.
This will cause the fuel to be pushed by external pressure through the main metering jet and up the main nozzle. As the solid fuel enters the nozzle, it is mixed with air through the calibrated (air bleed) holes in the nozzle.
This air aids in atomizing the air/fuel mixture for improved distribution. The mixture continues through the passage and enters the air stream at the venturi.
At this point, it is mixed with the incoming air and is carried past the throttle valve and into the manifold for distribution to the engine cylinders. The venturi principle is used over the widest range of engine operation.
Main metering provides a proper and economical fuel/air ratio for cruising and normal load conditions, approximately 14-17 lbs. air to 1.1b fuel.
But if load on the engine increases and maximum engine power is required, all air available for combustion must be used.
By simply adding more fuel to the air, the fuel/air mixture can be increased to approximately 12-14 lbs. of air to 1 lb. of fuel.
This produces the most power from the burning mixture in the cylinder, which in turn produces the maximum engine power.
Article Source : This article courtesy should goes to Auto Mechanics Autodology – Technical instruction manual by System Operation Support.