Standard Procedures

The topics presented on this web page are those procedures routinely used during the course of performing Periodic Maintenance. 

Adjustment of Fuel Float Levels

The correct setting of fuel levels is quite important since they affect the timing of when the main fuel delivery circuits are activated. If the float levels are even but lower than specified then the main circuits will be activated a bit late since it takes more airflow past the main venturi to draw the fuel into the main circuit. A late initiation creates a lean transition issue during driving.   Float levels not uniformly set result with the initiation of the main circuits at different engine speeds.  Different main circuit initiations will diminish engine power output and create driving issues since some of the cylinders will be lean and others rich for a given power demand.

Note: If fuel delivery pressure changes after adjusting float heights then the float levels WILL be affected; lower fuel delivery pressures will lower the fuel level in the bowl and conversely.

Float levels need to be read after running your engine with the most accurate readings taken when the engine is fully up to operational temperature. The vibration and constant open/close action of the fuel inlet needle valves will settle out to an actual, running fuel level.  Bench setting float levels is fine for initial carburetor setup but this is a static adjustment and prone to errors in actual running float height settings.  Once the fuel level has stabilized, turn off the engine to allow easy reading of the fuel level in the vial.

Fuel float levels require checking at the 6000 mile interval and it is recommended that a check of the needle valves be performed to be sure they are not seeping fuel when they should be securely closed.  Vibration will cause the float levels to self-adjust which is why checking and re-adjusting is important to maintain optimal engine performance.

Determination of fuel level in the float bowl

1. With the engine turned off remove one of the two float bowl drain bolts located on the side of the carburetor.
2. Fuel will begin to spill out of the float bowl so be ready with rags and a catch pan to minimize fuel running onto your engine shroud and other components.  Transfer the retrieved gas to your lawn mower.
3. Attach the external fuel level vial by screwing the knurled bolt into the threads from where the drain bolt was removed.
4. Turn on the ignition and allow the fuel to be replaced in the float bowl, you will see fuel rising in the vial of the float level gauge.
5. Start the engine and allow it to idle for ten seconds to allow stabilization
6. Turn off the engine and note the fuel level in the vial, a flash light is a handy item to help with this observation.
7. Weber carburetors are designed so the fuel level is between the upper two lines found on the typical fuel level gauge, remember that it is the meniscus or bottom of the dish shape the fuel forms in the vial that is the proper indication of fuel level inside the float bowl.

Adjustment of fuel level in the float bowl

1. With engine stopped, remove the 16mm hex cap covering the fuel needle valve to be adjusted.  See NOTES below regarding wrench selection and modification for this task.
2. Use a thin-walled 14mm socket to loosen and remove the needle valve
3. Based upon the fuel level reading add shims to raise the level and conversely.  See NOTES below for handy reference regarding shim thickness vs. float level adjustment.
4. Remember: Adding shims raises fuel level.
5. Take the time to resurface the sealing surface of the hex bushing and the gaskets to help prevent fuel leakage.
6. Repeat these two procedures for the remaining three fuel bowls.

NOTES:

• The 16mm brass hex plug is the easiest item to damage in the whole of Weber service so extra caution is advised.  A good quality, six-point, 16mm or 5/8" wrench is recommended.  Optimization of the wrench for this task is recommended:
  • Grind away one surface of the wrench to remove the radius at the hex opening to provide more engagement with the brass hex feature
  • Heat and straighten the typical 15 degree bend the handle of these boxed-end wrenches typically have
• The distance between the two lines on the fuel level vial equate to approximately a 0.010" thick needle valve shim adjustment
• Floats need to be installed correctly (they may be installed upside down and they need to have correct geometry for proper float action.  See Advanced Procedures for details regarding float blueprinting and installation.

Adjustment of Accelerator Pump Squirt Volume

Remove air cleaners and the intake air horns to gain easy access to the pump jet nozzles. Disconnect throttle linkage drop links so each carburetor may be independently actuated. Turn on the ignition and allow fuel to fill the fuel wells. Actuate the throttle lever using a steady and continuous motion until fuel has filled the accelerator pump and all the fuel delivery galleries. Lower the measuring vial (on a short length of wire) and locate it to catch the fuel squirting out of each pump jet as you actuate the throttle lever. Remove the vial and note the injection amount.  All six pump jets should deliver approximately the same fuel volume.  If volumes vary within one throttle body then look for partial blockage in the pump jets or leaks elsewhere.  If the squirt amount is less than 0.7ml then observe the fuel surface in the float bowl above where the inlet check valve for the accelerator pump is located and actuate the throttle lever arm.  If the surface of the fuel in the bowl is visibly disturbed when actuating the throttles then this indicates the check valve is not sealing properly and needs cleaning, rebuilding or replacement.  An insufficient amount of injected fuel is demonstrated by a stumble during acceleration when the throttles are opened rapidly.

To decrease the injection amount just turn the adjusting nut clockwise on the pump rod, this shortens the effective rod length and decreases the amount of motion imparted to the diaphragm in the top cover of the accelerator pump assembly. The amount of injected fuel required varies by engine size and performance potential but typical injection amount is 0.7 ml per nozzle.

While you are looking down the venturis check to see the stream of injected fuel squirts into the annular  space between the main and secondary venturis. Weber provides different pump jets depending upon the original venturis supplied with the carburetor. Long pump jet nozzle lengths were required to clear the small venturi diameters (27mm diameter for example) while shorter ones were supplied with carburetors having 30mm and larger venturis. Obviously the shorter pump jets disrupt air flow less than the longer ones do which helps in the higher performance applications. What you don't want is short pump jets combined with small venturis; the injected fuel will land on the inner wall of the venturi and effectively eliminate enrichment during throttle depression with an accompanying stumble.

Checking and adjusting the squirt volume injects raw fuel into your engine while it is not running, therefore you should periodically activate the starter to at least clear the fuel away; watch out for back-firing and flame-ups if you don't first disconnect the ignition system! 

This procedure may also be performed on the bench. If the top cover of the carburetor is off you may add fuel in the bowl that feeds the accelerator pump (the one with the brass check valve in the bottom of the bowl). Actuate the throttle lever using a steady and continuous motion until fuel has filled the accelerator pump and all the fuel delivery galleries. Take several readings and don't let the fuel level drop and expose the brass check valve or you will need to re-purge the air that is sucked into the system. Measuring the accelerator pump volume on the bench allows for easy correction of blockages and the other issues mentioned above.

Lean Best Idle Mixture Tuning - Traditional Method

The Lean Best idle mixture adjustment is the process by which the idle mixture screws are adjusted to provide the appropriate fuel to each cylinder during idling operation.  The adjustment process is simple but requires involvement by the tuner along with a goodly dose of patience.  It is surprising how responsive the 911 engine is to individual optimization of idle air flow and mixture adjustments at an individual cylinder.

Although this procedure may be performed on a mildly warmed up engine it is best to confirm the settings on one that has reached full operational temperature which is 175 F.  These adjustments are dynamic and each engine displays its own tuning characteristics.  Individual optimization of idle mixture at one cylinder will result in a speed increase of the engine which upsets air flow balance and mixture adjustments.  If a cylinder does not respond to adjustment then look to fundamentals like idle jet size, idle jet blockage or fuel gallery obstruction.

The following procedure assumes all preparatory steps provided on the Periodic Maintenance web page have been satisfied.

Lean Best Idle Mixture Tuning Procedure - Traditional Method

This is an iterative and inter-active process where each adjustment affects all the others, this means the procedure for adjusting idle mixtures and air flow balances requires repetition until no changes are realized during the course of adjustment.

• Disconnect the adjustable throttle drop links connecting the individual carburetors to the throttle control cross bar.
• Adjust all cylinders to achieve balanced airflow within each throttle body and for side-to-side balance.  Engine RPM may be elevated if necessary to achieve an idling condition from which to work from, this will be adjusted downward once Lean Best is achieved.
• Starting at cylinder #1 turn the idle mixture screw 1/4 turn "in" which is a clockwise rotation.  Turning the mixture screw "in" is a leaning adjustment since the tip of the mixture screw will progressively block off fuel flow into the engine.
• Pause for a count of 5 to allow the carburetor to adjust to the change. 
• If the engine speed increases then continue turning the screw in the same direction, pausing for the "5 count" with each 1/4 turn adjustment.
• If the engine speed decreases then turning the screw "in" is the wrong adjustment so return to the initial adjustment setting and turn the screw 1/4 turn counter-clockwise.
• Continue to turn the mixture screw in by 1/4 turn increments, pausing for the "5 count" after each adjustment and the engine speed will begin to drop.  This is confirmation that the fuel delivery has reached the "Lean Best" adjustment and further adjustments have upset the ideal mixture setting.
• Return that mixture screw to its previous setting, this is the "Lean Best" position of that mixture screw for this iteration of tuning.
• Repeat this procedure for the remaining cylinders.  If the engine idle speed increases (it will) then you will need to re-adjust idle speed stop screws to bring the engine speed back down to idle RPM and check side-to-side STE readings for parity and adjust as necessary.  This resetting of idle speed stop screws WILL affect idle mixture screw adjustments so a repetition of "Lean Best" adjustments WILL be required for all cylinders.
• After adjusting all six mixture screws you will need to revisit all of them but the next round of adjustments will be minor.  Be sure to take your car for a test run to get it up to operational temperature before performing the next set of Lean Best mixture adjustments.

NOTES:

• If a cylinder cannot be made to go lean and stop firing or if you cannot get the cylinder to fire at all then there is an issue that warrants further investigation prior to continuance of the procedure.  Failure to fire may be due to blockage of the idle jet or obstruction within the fuel gallery.  It could also be due to excessive fuel flowing into that cylinder.  This could be due to an improperly seated idle jet (early throttle body using a late-style idle jet holder), overly large idle jet or blocked idle air correction jet.
• The idle mixture is correct for a particular cylinder when a quarter of a turn of the idle mixture screw in either direction results in a drop of engine speed.
• If a barrel is spitting back through the intake then this implies a weak mixture strength.  If the Lean Best adjustment has been performed and the spitting continues then this may be due to uncontrolled air entering the cylinder due to loose throttle plate journals or a worn throttle plate.
• Idle mixture screws will typically be around 1 3/4  to 2 1/2 turns open but if 3 1/2 turns on one cylinder makes your engine happy then that is OK  but if all six are open to that amount then you may need to increase your idle jet size.
• Carburetors that are worn may respond to performing Lean Best idle adjustments at a higher engine speed such as 1200 RPM.
• Lean Best adjustment may cause the engine to suddenly increase as much as 400 RPM which is the result of the mechanical advance mechanism in the distributor activating.  If you cannot return engine speed to 900 RPM with the  idle speed stop screws then perform Lean Best adjustments at an elevated idle speed of 1200 RPM and when finished return the idle speed to 900 RPM and check side-to-side air flow balance before declaring the process completed.
• Those tuning IDTP Webers will be working with mixture screws having a tip with a needle like end followed with a short cylindrical body before the threads start.  All other mixture screws for the Webers have a continuously tapered tip.  The non-IDTP mixture screws are far easier to use since fuel flow incrementally changes with screw adjustment but the IDTP screws provide little finesse in idle mixture adjustment.
• Lean Best adjustment is satisfied when adjusting any mixture screw 1/4 turn either clockwise or counter-clockwise results in a decrease in engine idling RPM

Lean Best Idle Mixture Tuning - Colortune Method

Like the Traditional method of adjusting Lean Best as discussed above the goal of the Colortune Method is to optimize fuel delivery to the engine during idling operation.  The difference between these two methods is the Traditional method relies upon the tuner to "listen" to the engine response to mixture screw adjustments and the Colortune method allows the tuner to "see" the response in each cylinder.  The down side of this method is that it requires shuffling the Colortune tool from cylinder to cylinder before an adjustment may be made.  The upside is that the ability to see the result of mixture change is much more precise than the Traditional method provides.

The Colortune is no substitute for an AFR meter nor is it good for making driving observations of mixtures.  It does, however provide wonderful diagnostic information regarding the health of combustion in a cylinder.  You can see the spark and see the strength of mixture during combustion; both are essential for idling and the Colortune allows for instant verification of each.

With a little practice you can come to correlate audible engine responses to the visual display of combustion efficiency within the cylinder.  This may be of some use in the future to help expedite the Lean Best tuning process using the Traditional method.

The Colortune tool is a temporary replacement spark plug and has a "window" around the center electrode instead of the opaque, white ceramic one.  This window allows the tuner to see the color of combustion within the cylinder while the engine is idling.  In simple terms, a yellow color indicates richness and a blue color is displayed when mixture is completely burned during combustion which is the Lean Best condition being sought.  Also, the Colortune provides supplemental diagnostics but these are left to the owner to investigate.

Lean Best Idle Mixture Tuning Procedure - Colortune Method

This is an iterative and inter-active process where each adjustment affects all the others, this means that the procedure for adjusting idle mixtures and air flow balances will need to be repeated until no changes are realized during the course of adjustment.

• Disconnect the adjustable throttle drop links connecting the individual carburetors to the throttle control cross bar.
• Starting at cylinder #1  remove the sparkplug and install the Colortune tool and attach the high tension lead to the tool and to the plug wire from the distributor.
• Start the engine and allow it to come to steady-state idling condition.
• Adjust all cylinders to achieve balanced airflow within each throttle body and for side-to-side balance.  Engine RPM may be elevated if necessary to achieve an idling condition from which to work from, this will be adjusted downward once Lean Best is achieved.
• Use the mechanic's mirror to see down the passageway so that the window in the Colortune in cylinder #1 is in view.
• Observe the color of combustion within the cylinder and adjust the mixture screw counterclockwise until a definite yellow color is observed.
• Turn the idle mixture screw "in" which is a clockwise rotation using a 1/4 rotation.  Turning the mixture screw "in" is a leaning adjustment since the tip of the mixture screw will progressively block off fuel flow into the engine.
• Pause for a count of 5 to allow the carburetor to adjust to the change. 
• The idea is to adjust from a definitely yellow condition to that of a definitely blue condition and then open the screw so that you are seeing the mix of colors yellow and blue during combustion.
• This is the "Lean Best" position of that mixture screw for this iteration of tuning.
• Repeat this procedure for the remaining cylinders.  If the engine idle speed increases (it will) then you will need to re-adjust idle speed stop screws to bring the engine speed back down to idle RPM and check side-to-side STE readings for parity and adjust as necessary.  This resetting of idle speed stop screws WILL affect idle mixture screw adjustments so a repetition of "Lean Best" adjustments WILL be required for all cylinders.
• After adjusting all six mixture screws you will need to revisit all of them but the next round of adjustments will be minor.  Be sure to take your car for a test run to get it up to operational temperature before performing the next set of Lean Best mixture adjustments.

NOTES:

• If a cylinder cannot be made to go lean and stop firing or if you cannot get the cylinder to fire at all then there is an issue that warrants further investigation prior to continuance of the procedure.  Failure to fire may be due to blockage of the idle jet or obstruction within the fuel gallery.  It could also be due to excessive fuel flowing into that cylinder.  This could be due to an improperly seated idle jet (early throttle body using a late-style idle jet holder), overly large idle jet or blocked idle air correction jet.
• The idle mixture is correct for a particular cylinder when a quarter of a turn of the idle mixture screw in either direction results in a drop of engine speed.
• If a barrel is spitting back through the intake then this implies a weak mixture strength.  If the Lean Best adjustment has been performed and the spitting continues then this may be due to uncontrolled air entering the cylinder due to loose throttle plate journals or a worn throttle plate.
• Idle mixture screws will typically be around 1 3/4  to 2 1/2 turns open but if 3 1/2 turns on one cylinder makes your engine happy then that is OK  but if all six are open to that amount then you may need to decrease your idle jet size.
• Carburetors that are worn may respond to performing Lean Best idle adjustments at a higher engine speed such as 1200 RPM.
• Lean Best adjustment may cause the engine to suddenly increase as much as 400 RPM which is the result of the mechanical advance mechanism in the distributor activating.  If you cannot return engine speed to 900 RPM with the  idle speed stop screws then perform Lean Best adjustments at an elevated idle speed of 1200 RPM and when finished return the idle speed to 900 RPM and check side-to-side air flow balance before declaring the process completed.

Tips for using the Colortune Tool:

• Use a countersink (a short drill used for drilling recesses for the heads of wood screws in cabinets) to bore an internal taper into the end of the tip of the high tension lead that screws onto the threaded stud in the end of the Colortune tool; it really helps to guide it on
• Forget about using the plastic tube and its integral mirror as supplied with the tool; get a mechanic's inspection mirror with a proper mirror on its end.  Note: Be sure there is electrical insulation on the tool since you are subject to electrical shock if the spark intended for the spark plug jumps to the inspection wand handle.
• If the Colortune tool falls out of the spark plug socket you use for installation then use a magnetic tipped mechanic's wand to retrieve it.
• If you have an OEM Porsche spark plug tool then you can install the high tension lead directly onto the Colortune tool so that you can keep it in-place during installation.
• Clean the viewing window of the Colortune tool with acetone or other solvent and a little brush

STE Air Flow Meter Helps Verify Lean Best Idle Mixtures

STE readings between 4.0 and 5.0 indicate Lean Best has been achieved, the smaller number is for 2.0 liter engines and larger air flow readings are for larger engines.  High STE readings such as 7.0 indicate the engine's requirement for sufficient air for combustion of the delivered fuel; excessively rich idle mixtures require larger throttle plate openings to provide the requisite air for combustion which results in high STE readings.  Air flow tuning aids using an adjustable orifice to adjust indicated air flow readings do not provide the constant baseline like the STE does, the adjustable orifice air flow gauges will allow for air flow balancing but will not ferret out excessively opened throttle plates and the associated over-rich idle mixture adjustments they mask.

After achieving Lean Best on your engine record the STE reading as a reference for future tuning efforts.

Adjusting Air Flow Balance at 3000 RPM

This procedure quickly sets the side-to-side, air flow balance at 3000 RPM and at idle.

Assumption: Throttle cross bar blueprinting has been performed; see Advanced Procedures for details. 

1. Adjust cross bar mounting brackets (attached to inner sides of the intake manifolds) so the drop links just slip over the 8mm ball studs on the throttle lever arms and tighten the bracket mounting bolts to snug tightness.  Note: DO NOT adjust the lengths of the drop links; adjust the brackets to get the drop links to just slip over the ball studs WITHOUT upsetting idle speed!
2. Control engine speed using the front-to-rear throttle rod that connects the bell crank to the throttle cross bar; this is important since controlling the throttle position by hand in this fashion replicates how the throttles are actuated while driving.  Alternately get a helper to run the throttles using the throttle pedal inside the car.
3. Hold the engine speed at 3000 RPM and while doing so use your STE air flow meter to determine air flow on cylinders #1 and #4.
4. If the STE readings do not match then use a plastic hammer to advance or retard throttle timing by striking the throttle cross bar mounting plates (snug tight, remember?) so they rotate in a fore and aft direction.
5. Allow the engine to return to idle speed and verify air flow balance for cylinders #1 and #4 are still matched.
6. Tighten the nuts securing the cross bar mounting brackets and recheck for side-to-side balance.

Measuring Fuel Pressure

Weber IDA3C carburetors are designed for fuel delivery pressure of 3.56 psi at the fuel inlet fittings.   It is not adequate to assume OEM or any other fuel pump will actually deliver the specified pressure.  Excessive fuel pressure may flood your carburetors with results such as engine fires or hydro-locked cylinders (flooded with fuel which prevents engine from turning over).  Any fuel delivery pressure from 3.0 to 4.0 psi is acceptable with 3.56 psi being the best pressure to assure quick fuel delivery and float level accuracy.  This fuel pressure should be checked on a warmed-up, running engine since fuel pressure will vary with voltage variation and voltage will be higher on a running engine when the charging system is active.  If you use a performance fuel pump then it is highly recommended that a fuel pressure regulator be installed to assure a constant output pressure regardless of pressure variations of the pump.

It is important to know your pressure indicating gauge is working correctly/accurately when performing these checks to preclude false readings, low cost gauges provide accuracy commensurate with their cost.  The pressure gauge I have come to use is a converted blood pressure gauge or "sphygmomanometer".  These are readily available for low cost on eBay and with a couple of adapter fittings they become an ideal test pressure gauge. 

It is not important to have a fuel gauge permanently installed into your fuel lines but having an easily accessed port is handy when Periodic Maintenance is performed and you want to check fuel pressure.  Also, the constant vibration will damage the mechanical components within the gauge rendering them unreliable for reading accuracy.