Controlling Fuel Pressure Settings
Fuel pressure should be set between 6 and 8 psi for a gasoline carburetor. An alcohol carburetor is a different animal with very different requirements. The Alky carburetor will require 4 to 5 psi at idle and 9 to 12 psi at wide open throttle. Remember, fuel pressure is not a substitute for volume! If the fuel bowls are not full, the pressure is meaningless. In fact, fuel pressure is simply an indication of the amount of restriction in the fuel system.
Regulators and Bypasses
Most electrical fuel pump systems require the use of a fuel pressure regulator. One regulator is sufficient in many applications. The use of two regulators is recommended in high horsepower engines to avoid excessive fuel restriction and provide adequate volume. With mechanical fuel pumps, and some electrical pumps, a bypass is preferred rather than a regulator. A diaphragm bypass without an idle bleed is recommended when constant fuel pressure is needed from an electrical or mechanical pump. A belt driven fuel pump, using gasoline or alcohol, requires a diaphragm bypass with an idle bleed. Higher pressure mechanical fuel pumps delivering alcohol, such as the 15-psi BG Six-valve, require a throttle bypass to supply the variable fuel pressure required by the carburetor.
Setting the Idle Mixture & Idle Speed
The engine should be at optimum operating temperatures. Lightly seat the idle mixture screws, then back them out 1-1/2 turns to establish a starting point. With the engine running, slowly turn the mixture screws in or out as needed to establish the best idle quality. Do this twice. The first time is a coarse adjustment; the second one is a final fine adjustment.
If idle quality can’t be adjusted properly, or if the idle mixture screws can’t be backed out far enough to obtain proper adjustment, it may be necessary to have the idle circuits reworked (the cam may not be pulling enough idle vacuum). A common reason for lack of adjustability in the idle mixture screws is having the primary butterflies adjusted too far open, which prevents the idle mixture screws from being capable of controlling the idle quality. When setting idle speed, the butterfly openings should be adjusted equally both primary and secondary side. However, the transfer slot should not have more 0.040″ exposed (viewed with the carburetor removed from the engine and turned upside down). Secondary adjustment is set with a screw accessible from the underside of the carburetor.
Remember, the secondary throttles should be just slightly cracked open at idle. Even on carburetors with non-adjustable secondary idle mixture, secondary throttle position at idle may be adjusted. It may be necessary to open or close the throttle adjustment due to the cam design. It’s a trial-and-error proposition to find the setting that gives you the best idle. However, here is a starting guideline. On engines that idle at or above 1000 rpm, start with the primary and secondary butterflies open the same amount. For engines that idle below 1000 rpm, begin with the primary butterflies open about 0.020″ and the secondary butterflies positioned at the bottom of the transfer slot. From these points, begin to adjust the idle adjustment screws until the idle is steady and smooth. button head