Engine Maintenance and Modification Bradley Artigue

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FIAT 124 Spider Engine Maintenance + Modification • A Venturi (E). The purpose of the venturi is to increase the depression acting on jet (D) to favor the vaporization of the gasoline sprayed from the jet during engine operation. • A spray tube or nozzle (C) through which fuel flows from float chamber to calibrated jet (D). • A throttle (F) that regulates the amount of fuel/air mixture drawn in by the engine. Updraft carburetors were understandably impractical. Carburetors quickly progressed to down draft types, with an important improvement known as air bleed correction. 4.2 Air Bleed Correction Emulsion tubes mixed air with the fuel prior before entering the carburetor. This air bleed correction (figure 3) made operation more efficient. Figure 3: Air Bleed Correction. A Float Chamber Vent , B Fuel Bowl or Chamber, with Float , C Spray Tube, D Fuel Jet , E Venturi , F Throttle , G Air bleed jet, H Well for emulsion tube, I Emulsion Tube A perforated emulsion tube (I) enables air and fuel to be mixed prior to being sprayed into the carburetor barrel. This results in fuel “misting” into the barrel. The arrangement allows for larger jets and supply tubes and a wider range of tuning. Air enters the emulsion tube through an air bleed correction jet. The jet is removable, allowing for precise adjustment of how much air is allowed into the emulsion tube well. There is one well, emulsion tube, and jet per carburetor barrel. 12
FIAT 124 Spider Engine Maintenance + Modification 4.3 Idle Speed Control An additional improvement is an idle speed device (figure 4). This device allows your engine to maintain the lowest rpm rate at which it will keep running. Da T Db D 4 2 1 3 Figure 4: Idle Speed Circuit. D main fuel jet, Da idle speed air jet, Db idle speed fuel jet, 1 idle speed mixture orifice, 2 transition device, 3 idle speed mixture adjusting screw, 4 throttle setting or idle speed adjusting screw The throttle is nearly closed and fuel is drawn from below the throttle plate, rather than from the main jetting. The idle speed device is an independent circuit on the carburetor, sharing the fuel bowl with the main circuit but having its own mixture setting, jetting, and an idle speed screw to regulate throttle position and engine speed. 4.4 Transition We’ve reviewed the main and idle stages of a carburetor, so let’s take a look at how your carburetor transitions smoothly from the idle stage to the main stage. In the side of your carburetor barrel are holes, known as progression holes or transition orifices. Movement of the throttle past these holes draws fuel into the engine (specifically, a drop in pressure draws fuel through the holes as the throttle passes), creating additional speed while building up to the main stage. 13
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