|
Weber
Tuning Methodology I’ve gathered the following from several sources, but the basic idea came from information regarding the use of a Weber manufactured/Edelbrock marketed jet block that allowed the use of Weber jets and concepts on Holley carburetors with detachable float bowls. I see no reason that this methodology shouldn’t work just as well for tuning Mukuni/Solex carbs as well as Webers. Please note that I make no recommendations regarding choke sizes and jetting. There are too many possible combinations to do so. As a general rule of thumb smaller chokes will give greater air flow speeds at lower RPM than larger chokes while not flowing as much air at higher RPM. In other words, being realistic in setting up the carbs for your intended use will yield the best results. Terminology The following should assist you in relating the terminology.
Circuit Descriptions: The Weber three phase circuitry is illustrated above. The bottom axis portrays air flow rate from min. to max. Note that the Idle Circuit operates at low air flow rates even up to 3000 - 4000 RPM. This is the upper end of the transition phase (approximately 2500 RPM) and extends through 4500 to maximum RPM. The illustration also indicates which adjustment devices are effective at the various air flow rates. The vertical axis shows the degree (from 0% to 100%) to which each device participates in controlling the air and fuel. The Idle Fuel Jet is the dominant adjustment for idle circuit performance. The Idle Air Corrector Jet does have some effect at idle RPM and becomes more dominant as the air flow increases to the rates where the low speed circuit operates. In the transition phase these two, plus the MFJ, and MACJ all participate. Finally, in the High Speed Circuit (at high air flow) the Main Fuel Jet is initially dominant and then is supplanted by the Main Air Corrector Jet. From this graphic, it can be seen that the tuning procedure should proceed from left-to-right. The idle circuit is tuned first; then the Low Speed Circuit up into the Transition Phase. Finally, the High Speed Circuit including the Transition Phase and the Power Enrichment Circuit are tuned last. Notice that the Transition Phase involves the IFJ, IACJ, MFJ and MACJ. How well all are balanced determines the overall performance, responsiveness and efficiency of the engine. It is important, therefore, that the theory of operation and the tuning procedure be thoroughly understood. Idle Circuit This circuit controls the volume of air/fuel emulsion supplied through the idle discharge port. • Idle Speed Screw - Used to set the idle position of the throttle plates. • Idle Volume Screw - Controls the volume of the pre-mixed air and fuel. It does not control the mixture of air-to-fuel. • Idle Fuel Jet - Controls the amount of fuel supplied to the idle circuit. Changing the idle fuel jet. will richen (if larger) or lean (if smaller) the mixture throughout the idle range. • Idle Air Corrector Jet - This jet controls the amount of air supplied to the idle circuit. A larger size will admit more air resulting in a leaner mixture while a smaller size will admit less air resulting in a richer mixture. Low Speed Circuit This circuit is most often in use during normal driving. Depending on engine carburetor sizes, vehicle weight and engine load, this circuit is in operation from off idle to 4000 RPM. • Idle Fuel Jet - The amount of fuel delivered in the low speed circuit is controlled by this jet. Changing the idle fuel jet will richen (if larger) or lean (if smaller) the mixture throughout the low speed circuit. • Idle Air Corrector Jet - Controls the volume of air supplied to the low speed circuit. A larger size will admit more air resulting in a leaner mixture while a smaller size will admit less air resulting in a richer mixture. The effects of changing this jet will be more noticeable in the upper end of low speed circuit operation. • Idle Passage Plugs- The position of these nylon plugs determines the source of :fuel for transition from the low speed to high speed circuit. The 2 (lower) position duplicates the original Holley circuitry. In this position fuel is drawn from the I emulsion tube well above the main fuel jet. During transition from the low speed high speed circuit the available fuel is shared by both circuits. As the high speed circuit takes over, the fuel available to the low speed circuit decreases until there is none left and the transition is complete. Because of this limited "overlap" the transition is more precise and generally offers better fuel economy. With
the plugs in the #1 (upper) position the low speed circuit receives High Speed Circuit This circuit, with assistance from the power enrichment circuit, supplies the engine's air/fuel needs from the point of transition off the low speed circuit up to its maximum RPM • Emulsion Tube - Its purpose is to mix (emulsify) fuel from the main fuel jet with air from the main air corrector jet to promote optimum vaporization during high speed circuit operation Emulsion tubes are available with different combinations of inside and outside diameters and different quantities and locations of mixing holes. By changing the emulsion tube the fuel curve at less than wide open throttle can be subtly altered. The standard emulsion tubes in the calibration kits are suited for general performance work. • Main Fuel Jet - The main fuel jet controls the volume of fuel to be mixed with air in the emulsion tube well. For a given air flow rate, changing the size of this jet will richen (if larger) or lean (if smaller) the mixture throughout the high speed circuit. • Main Air Corrector Jet - The main air corrector jet controls the volume of air to be mixed with fuel in the emulsion tube well. Changing to a larger size will admit more air. This produces a leaner mixture while changing to a smaller size will admit less air resulting in a richer mixture. The effect of main air corrector jet changes will be more noticeable at the upper end of this circuit's operation. Another important function of this jet is to help control the start of high speed circuit operation. A smaller main air corrector jet will "start" the high speed circuit: at a lower RPM while a larger size will delay its entry. TUNING PROCEDURE: This part of the instructions covers the three circuits, Idle, Low Speed and High Speed. The Idle circuit adjustments are made first. Then the Low Speed circuit adjustments are made. Third, the High Speed circuit adjustments are completed. Remember, from the Theory of Operation, that the adjustments interact. Therefore, if large adjustments are made in the Low Speed, then the Idle adjustments must be rechecked. Likewise, if large adjustments are required in the High Speed circuit then both Idle and Low Speed must be rechecked. The procedure tells you when rechecks must be done. Do the rechecks as requested and the final results will come more quickly and yield a better running engine. If major adjustments are required at any point in the procedure then previous adjustments must be rechecked. Before tuning check the following: • Is your engine sound and in a good state of tune? Problems caused by unrelated components are often blamed an carburetors. • Are the float level and fuel pressure set to standard specifications for the carburetor being used? GENERAL NOTE: Only change one type of component at a time. This will allow you to measure the effect of each change. Multiple component changes will only confuse you. Prior to Starting engine 1. Remove air cleaner 2. Plug off open vacuum lines 3. Connect tachometer/vacuum gauge to engine 4. Idle speed screw should be set at one full turn in (clockwise) from point of initial contact with the throttle lever. 5. Idle volume screws should be set at one full turn out (counter-clockwise) from point of bottoming out (fully closed) Note: Do not over tighten during bottoming out or you may damage the screws. 6. Secondary throttle plates should be closed. 7. Check for fuel leaks. Start Engine and Let Idle • Immediately check for fuel leaks, • If idle speed is too low to keep the engine running, turn the idle speed screw in an additional 1/2 turn. If the engine will still not idle, check for vacuum leaks. • Make sure engine reaches normal operating temperature before proceeding. 1. Idle Circuit Adjustment A. Starting with idle volume screws 1 full turn out from point of fully closed, adjust idle volume screws to obtain fastest and smoothest idle. To do this, slowly :turn each idle volume screw in (clockwise) until the engine speed drops noticeably. From this point, turn each screw out (counter-clockwise) to obtain maximum RPM. B. Once adjusted stop engine and note the position of each idle volume screw. Do this by recording the number of turns in (clockwise) required to fully close each idle volume screw. All screws should require the same approximate number of turns. - Note: Do not over tighten. It is only necessary to seat each screw lightly. C. Compare the number of turns in Step B, above, to the following chart and perform the required action.
When the correct idle fuel jets are installed, final idle volume screw adjustment will be between 1/2 to 1 turns out from fully closed. Idle speed will be sensitive to idle volume screw adjustments within this range. Note: Should idle volume screw adjustment be relatively insensitive check that the secondary throttle plates are not open too far. D. Adjust idle speed screw to achieve desired idle speed. Final adjustment should be within 1/2 to 1-1/2 nuns in from initial contact. If idle speed screw is turned in more than 1 & 1/2 turns the throttle plates will uncover too much of the transition slot and the low speed circuit will begin to operate prematurely. This will cause an off-idle stumble. If you have an off-idle stumble, reset idle speed screw at one full nun in from initial contact and repeat the above procedure from step A. 2. Low Speed Circuit Adjustment Note: Transition RPM values of 2500-2800 are approximate points of reference for circuit transition for medium weight vehicles. These values should be 3000-3300 RPM for lighter vehicles w/larger engines and 2000-2300 RPM for heavier vehicles w/smaller engines. With the vehicle securely blocked to prevent movement and the transmission in neutral, slowly increase engine speed to transition RPM (do this slowly to minimize the effect of the accelerator pump circuit). Use the chart below to determine the action required.
3. High Speed Circuit Adjustment Tuning of the high speed circuit should be done with the engine under a steady-state loaded condition on a chassis dyno or on an open track. An exhaust gas analyzer or plug checks used at the RPM ranges listed below will verify main fuel and air corrector jet sizes. This procedure is laid out in two stages. The first is used to set the overall mixture as determined by the main fuel and main air corrector jets. The second will establish the proper entry point for the high speed circuit. Note: Although these are given as separate procedures they should be done together at the same time. This is necessary because a change of either the main fuel or main air corrector jet size will have some effect on both mixture and entry point. A. Mixture 1. Replace air filter and reconnect all hoses. 2. Using 1st or 2nd gear and moderate acceleration (manifold vacuum above 12.5") run the engine at or near maximum RPM and observe the engine behavior. Refer to the chart below and perform the appropriate action.
B. Entry Point: Using 3rd or top gear and moderate acceleration (manifold vacuum above 12.5") start at 3000 RPM and gradually increase to approximately 4500. Refer to the chart below and perform the appropriate action.
High Altitude As a rule, carburetors tend to run richer at higher altitudes than at sea level. For best performance and economy you may want to decrease the fuel jets by 1 size for every 2000 feet above sea level. For example, at 4000 feet you would decrease the idle and main fuel jets by 2 sizes (a #50 idle jet would be changed to a #40 and a #155 main jet would be changed to a#145. Weber jet sizes are based on flow rates rather than diameter. Drilling any two jets in the field will almost always result in significantly different flow rates. For this reason we strongly recommend against drilling jets. |
