Installation Procedure

The following information is provided as an "installation guide" for your carburetors after rebuilding; "carburetor tuning" after installation is discussed in Periodic Maintenance. 

Preparatory Procedures and Notes

  1. Prior to installing your Webers onto the tops of the intake manifolds the following tasks are recommended to be performed:
    1. The top interface of the of intake manifolds needs to be 100% cleaned of old gasket material.
    2. The top interface should be checked for flatness. Flatness variations of 0.010" (the thickness of three sheets of paper) will distort the throttle body causing throttle shaft binding. The intake manifolds may be corrected by resurfacing; contact Performance Oriented for tech help.
  2. Always use new gaskets between the intake manifolds and the throttle body to insure freedom from intake air leaks and to assure even gasket compression during the torquing process.
  3. Care must be taken during the tightening of the carburetor fastening nuts, over-tightening of these nuts may distort the throttle body resulting in throttle shaft binding. Tighten the nuts in torque stages, working from the center of the throttle body to the outer fasteners; in a crossing pattern. Torque as evenly as possible until a nut torque of eight to ten foot-pounds is achieved.
  4. Before adjusting your Webers the following items must be in order to avoid misdiagnosing a tuning issue as a carburetor problem:
    1. Cam timing is assumed to be correct
    2. Valves have been adjusted
    3. Engine compression is high and even (above 150 psi and cylinder pressure differences within 15 psi)
    4. The points, plugs, distributor cap, ignition wires and air cleaner elements should be in like-new condition and cleaned of oil film and engine grime
    5. Ignition timing has been set and the (mechanical advance) distributor is operating (advancing) smoothly.
      1. Less than optimal ignition advance will require richer air/fuel mixtures.
      2. Total advance will vary with combustion chamber and cylinder head design.
      3. Higher compression or twin spark plug heads will reduce the total advance required.
      4. Safe, basic timing values are:
        1. 5° BTC @ 900±50 RPM
        2. 30° BTC @ 6000 RPM
    6. Fuel delivery pressure at the carburetor inlet has been verified to be 3 1/2 psi, +/-  1/2 psi and measured with the engine running to simulate actual electrical power delivery to the fuel pump while driving.  Note that low cost fuel pressure gauges provide VERY inaccurate pressure readings.
    7. Carburetors and fuel lines require inspection for fuel leakage and all issues require correction before proceeding.
    8. Lubricate the throttle shafts at each journal in the throttle body using a drop of motor oil per bearing. Refresh the grease in the linkage ball joints.
    9. Check to see that the throttle pedal has an adjustable stop and that when the pedal is fully depressed it is the adjustable pedal stop that limits throttle valve opening in the carburetors. If the pedal does not have a positive stop then the carburetors and/or linkages may be damaged from over-stressing.  Also check to see that excessive play has been adjusted out.
  5. Recommended ignition and fuel delivery components are:
    1. For non-restoration cars the MSD-6A or MSD-6AL Ignition Control unit and for restoration vehicles originally equipped with Bosch CDs then a updated and remanufactured unit or a Bosch unit used in conjunction with a hidden MSD unit is recommended.   All distributors would benefit from a tailored remanufacturing and recurving effort.; contact us for referrals.
    2. Pertronix Igniter to replace the traditional points in your distributor although traditional points when used with a CD are very reliable and are VERY long lasting (use point grease and check timing/dwell during routine tuning intervals)
    3. Fuel pump recommendations
      1. For normal applications:
        1. Fuel delivery by these pumps will suffice: the OEM Bosch pump P/N 911.608.107.03, Facet "GOLD-FLO" P/N 40002E, Hardi P/N 901.608.106.00.M225 or Pierburg P/N 901.608.106.00.M244
      2. For performance applications there are two systems to consider, both use a fuel pressure regulator to maintain constant fuel pressure to the carburetors even if the fuel supply from the pump fluctuates a little bit:
        1. Dead-end installation (carburetors are the last item in the fuel delivery system):
          1. Holly two-port, 1-4 psi fuel pressure regulator, #12-804 (Maximum inlet fuel pressure is 16 psi so this unit will not work with higher output pressure pumps as used for CSI systems)  This type of regulator is referred to as a "normally open" regulating valve.
          2. Mallory rotary fuel pump, #4070M (has 6psi output pressure capacity)
          3. Set output pressure from the regulator to the Webers to be 3.5 psi.
          4. Arrange components thusly:
            1. Fuel tank
            2. Fuel strainer (typically inside the fuel tank and is to keep the big chunks out of the fuel system)
            3. Fuel pump (located low and forward in the chassis)
            4. Fuel filter (sized large enough for the fuel pump output flow capacity)
            5. Fuel pressure regulator
            6. Fuel pressure gauge (recommend a removable gauge and install for tune-ups)
            7. Fuel line has tee with each output from the tee going to a carburetor
        2. Return-to-tank circulation system.  This system recirculates fuel from the tank through all fuel lines (no dead-end fuel lines) to keep cool fuel supply at the carburetors at all times, this is particularly good to help control fuel percolation issues within the fuel lines:
          1. Aeromotive X1 Series fuel pressure regulator, Model 13304, 3-15 psi operating range.  This is a flow-through regulator and is referred to as a "normally closed" regulating valve.
          2. Mallory rotary fuel pump, #4070M (has a 6psi output pressure capacity)
          3. Set pressure of fuel delivery system to be 3.5 psi. 
          4. Arrange components thusly:
            1. Fuel tank
            2. Fuel strainer (typically inside the fuel tank and is to keep the big chunks out of the fuel system)
            3. Fuel pump (located low and forward in the chassis)
            4. Fuel filter (sized large enough for the fuel pump output flow capacity)
            5. Fuel pressure gauge (recommend a removable gauge and install for tune-ups)
            6. Fuel line has tee with each output from the tee going to a carburetor fuel rail
            7. Fuel line from the end of each carburetor fuel rail to a tee
            8. Fuel line from tee to fuel pressure regulator
            9. Fuel line from regulator to fuel tank, preferably to the top of the tank
  6. Treat all adjusting needles and brass components with respect (don't over tighten) or they may be damaged. Those to be respected are: idle air correction screws, idle mixture screws, the idle jet holders, the main jet holders and the fuel bowl drain bolts.  If you have brass idle mixture screws then be EXTRA careful when closing them since they are prone to breaking their tips off in the metering hole and can then be devilishly difficult to extract.  Recommendation is to replace them with steel equivalents.
  7. The following spark plugs are recommended:
    • NGK BP5ES for compression ratios up to 9.1:1
    • NGK BP6ES for compression ratios above 9.1:1
    • NGK BP7ES for competition applications
    • Plugs gaps are typically 0.035" when using CD ignition and 0.014" for OEM coil ignition systems; be sure to follow spark plug manufacturer's gap recomendations

    Please note these are recommendations only and final plug selection and plug gap setting are to be determined by the end user for their specific application.

  8. Initial carburetor settings to be adjusted as follows:
    1. Throttle stop screws adjusted until they just touch the throttle arm and then turn 3/4 full turn.
    2. Idle air correction screws are turned in until they lightly seat and then locked with the 8mm hex nut
    3. Idle mixture screws are turned in until they lightly seat and then turned out 1 3/4 full turns.  Be careful!  Seating these screws too tightly, especially those after market ones made of brass (use a magnet to check them) may cause them to seize their tip in the throttle body and then twist it off when backing the screw out leaving the hole VERY plugged.  The stepped-tip mixture screws as used on IDTP Webers do not suffer this issue.
    4. Connect cross bar linkage and drop links to just snap onto the throttle levers without binding, these will be readjusted later but for initial starting it is handy to have these connected.
    5. Check throttle cross bar and drop links for installation alignment, this topic is covered in Advanced Procedures entitled Throttle Cross Bar Adjustment.