Performance Tuning

Overview

This section is provided to help the tuner optimize the performance potential of their 911 when equipped with Weber, Solex, Zenith or PMO triple throat carburetors.  Although the body of the following text will specifically address components as used in the Weber carburetors, all of the processes are applicable to these four types routinely used on Porsche 911 engines.

This section is divided into three parts:

• Idle and Progression circuit

• Main circuit

• Accelerator circuit

The preliminary requirements for performance tuning are those discussed in Periodic Maintenance which assumes:

• The engine is in good condition and valves have been set.

• The ignition system is in good condition, timing has been set, spark plugs have the correct heat range, plug wires and distributor are clean from grease and the distributor is in good operational condition and matched to your engine’s performance.

• Carburetors are in good condition, float levels have been set and are jetted appropriately for your engine.

• Lean Best idle mixture adjustment and air flows for both idling and elevated engine speeds have been adjusted.

Tuning of these three circuits is to be performed during “Open-road” testing which is just as it sounds, you will use public roadways to perform performance runs to optimize timing and jetting for your engine; your engine is different from all others even if it is built to similar specifications of another engine.  Even when your engine, exhaust and ignition are exactly per OEM specifications there will be gains realized by performing optimization of your jetting since the OEM settings were designed to be a “Best Fit” for all possible end users and are therefore a compromise setting with performance improvements waiting to be realized.  Since these tests are conducted on publicly accessible roads it is STRONGLY recommended to select a speed range that allows for testing without exceeding posted speed limits or causing a disruption in traffic flow that would draw attention to your activities.  Roads that are level are good for optimizing the idle/progression circuit and roads with a slight incline are good for optimizing the main circuit. Open areas and good visibility are good bets, the less traffic the better. 

Idle and Progression circuit

The idle and progression circuit (slow speed circuit) is the primary fuel delivery system for 90% of the driving time.  When you are traveling at a constant speed on level roads you are operating solely on this circuit up through 5000 RPM.  Slight accelerations from this speed will demand fuel from the main circuit and more aggressive acceleration demands will be supported with fuel delivery from the accelerator circuit. 

It is possible to test the slow speed circuit for adequacy of mixture strength without activating the main circuit, but this test is best performed on a quiet country road that has a long and level straight section.  Once this circuit has been optimized then re-activating the main circuit will allow clarity on its fuel contribution which helps in tuning it correctly. 

Several methods are presented for testing and optimization of the progression circuit followed by the procedure for optimization of the tuneable idle air correction jets.  These procedures are performed to determine the need for optimizing the idle fuel jet selection and if necessary, modification to allow advanced tuning utilizing tuneable idle air jets.   

Remember that the idle jet and idle air correction jet along with the progression holes in the wall of the throttle body (including the idle mixture screw port) control the mixture strength and delivery during low-speed operation (progression phase) then begin to diminish in their effectiveness during the transition phase when the main circuit operation is initiating.  The progression holes are fixed in quantity, spacing and size but the idle jet, idle air correction jet and idle mixture screw setting are all variables subject to tuning.  (The idle air correction jet is a fixed size that is not typically adjusted but may be modified to provide size adjustments.)  

The tuning procedures offered are listed below:

• Primary procedure

• Subtle-on procedure

• Subtle-off procedure

• Stationary idle circuit testing

• Tunable air correction jet optimization

 primary procedure

• Warm engine to running temp

• Set float levels to be within specified height, check fuel levels while engine is running.  Note that car must be sitting on level surface, not inclined.

• Perform air flow balancing on each bank of carbs with throttle linkage disconnected

• Perform "Lean Best" idle mixture adjustment on all carbs

• Connect linkage & adjust side-to-side airflow balancing at 2500 RPM

• Double check idling airflow balancing

• Seek a low traffic, flat rural road or preferably one with a long and consistent slight incline with a quiet place, off the road to perform your work

• Remove air cleaner and tops of carburetors and remove the auxiliary venturis from the carburetors

• Rotate the auxiliary venturis 180⁰ about their vertical axis and reinstall.  This will block the main circuit from functioning.

CAUTION: If the main circuit is disabled in this fashion and the vehicle is driven on open roads then the driver must be VERY aware that the vehicle has been stripped of main circuit operation and is thereby limited in its performance.

• Perform slow acceleration in 3rd gear from 2000 RPM through 4500 RPM, preferably up a long, slight incline to determine if jetting is good for low-speed circuit & transition onto the main circuit (“slow acceleration” means a minimum of 30 seconds duration run time)

• If slow acceleration is hesitant to increasing throttle application, then this implies a lean progression.  Open mixture screws 1/2 turn and repeat if necessary for a total increase of two 1/2 turns. Repeat slow speed acceleration test for each 1/2 turn adjustment. If the lean response is not resolved, then increase idle fuel jet size by “5” and repeat procedure from Performing “Lean Best” mixture tuning process: Lean Best requires optimization for every jet size change.

  • If the slow speed mixture is troubled by the necessity to use a large idle jet to achieve good transition but the idle jet is too rich for slow speed operation, then tune the idle/progression circuit by modifying the idle air correction jets to be “Tuneable” and follow tuning as per “Tuneable Air Correction Jet Optimization” provided below.

• If slow acceleration is not hesitant to increasing throttle application, then this implies either the mixture delivery is correct or possibly rich.  Close mixture screws 1/2 turn and repeat if necessary for a total decrease of two 1/2 turns. Repeat slow speed acceleration test for each 1/2 turn adjustment.  If a lean response is not experienced, then decrease idle fuel jet size by “5” and repeat procedure from Performing “Lean Best” mixture tuning process: Lean Best requires optimization for every jet size change.

  • If the slow speed mixture is troubled by the necessity to use a small idle jet for idling and off idling operation but is is too rich for operation at mid-range RPM operation, then tune the idle/progression circuit by modifying the idle air correction jets to be “Tuneable” and follow tuning as per “Tuneable Air Correction Jet Optimization” provided below.

• Finalize your idle jet size and mixture screw adjustments and then return auxiliary venturis to their correct orientation and reassemble air cleaners.

NOTES:

• Whatever combination of idle fuel jet, idle air correction jet or mixture screw adjustment makes the idle/progression circuit work better is what you want to do.

• Idle circuit controls fuel mixture up to 5000 RPM when operating on level roads at constant speeds

• If the slow speed mixture is troubled by the necessity to use a large idle jet to achieve good operation in the 2500 to 4500 RPM region of engine speed but the idle jet is too rich for slow speed operation, then tune the idle/progression circuit by modifying the idle air correction jets to be “Tune-able” and follow tuning as per “Tune-able Air Correction Jet Optimization”

Subtle-on procedure

This procedure is intended to be performed as a check to the jetting solution adopted resulting to completion of the Primary Procedure above.

• Drive at a constant speed on a level or slightly inclined road in 2nd or 3rd gear.

• Quickly reduce throttle pedal depression by about 10% and feel how the engine responds.

• If the engine accelerates then the mixture for this engine speed is lean and if the engine does not respond, then mixture may be either correct or rich.

NOTE:  Air is less dense than fuel so when you back off on the throttle, the air flow is diminished very quickly but fuel flow continues due to the inertia of the fuel.  This momentary "transition" will make the fuel mixture richer since less air is delivered for the continued fuel supply.  The engine response to this test lasts about one second so you need to be particularly attentive to the result.

Subtle-on Procedure

This procedure is similar to the Subtle-Off Procedure above but different in method. 

• Drive at a constant speed on a level or slightly inclined road and make very subtle throttle pedal openings, approximately 5%; this is just to be a very minor “blip” in throttle opening and not a sustained opening.  The idea is to increase throttle opening without actuating accelerator quirt.

• Repeat several times without changing velocity of the car, the idea is to evaluate engine response at a particular RPM.

• Evaluate the engine’s response to these minor throttle openings.

  • If engine responds quickly to throttle opening then jetting mixture is good.

  • If engine is a little sluggish or less than instantaneous then jetting may be lean or rich.

  • Repeat engine response for RPM increments of 500 RPM from 2500 to 4500 RPM keeping notes regarding each region of RPM operation.

Tuning action resulting from the above are:

• Idle fuel jet increases mixture strength across low-speed circuit range of operation

• Idle air correction jet increase will lean out progression mixture strength as RPM rises and conversely.

The goal is to tune for leanest mixture for cruise operation (3000 to 4000 RPM) as developed by the Subtle-On Procedure while maintaining instantaneous throttle.

Stationary idle circuit testing

Initial testing of the idle and progression circuits is performed while the car is parked in lieu of having convenient access to a suitable roadway for testing.  Even if the tuner has access to a chassis or engine dynamometer for wide-open-throttle testing, the idle and progression circuit will require tuning for partial throttle operation.  

Note: Stationary testing places demands on your engine that may cause it to heat to levels beyond those realized during typical idling operation.  While it is VERY important that finalization of testing be performed when the engine is thoroughly warmed up (175 degrees F) it is also important to monitor the engine temperature during driveway testing to avoid excessively high temperatures.

Idle air jet selection procedure:

This procedure evaluates the progression circuit for fuel mixture strength.  It is assumed that the idle fuel jet selection has been achieved per the previous procedure and that Lean Best adjustments have been satisfied with mixture screws nominally 1 3/4 turn open.

Initial test:

• Adjust idle mixture screws for "Lean Best" running

• With the engine warm, disconnect drop links and set the idle speed as slow as possible (750 RPM) using the idle speed stop screws and maintaining air flow balance

• Adjust idle speed stop screws clockwise in 1/8 turn increments to increase engine speed.  Note that each adjustment of these screws requires a side-to-side air flow balance of the carburetors to be maintained.

• Allow engine to settle for five seconds after each adjustment has been completed.

• Note engine smoothness after the adjustment.  When all is right there should be no change in how the engine responds to the increased RPM.

  • If there is a change in engine smoothness or a lack of RPM increase for a screw adjustment then note the response and the associated engine RPM.

• Continue to make 1/8 turn adjustments while maintaining side-to-side air flow balance and observations regarding engine smoothness until engine speed reaches 3000 RPM 

Second test:

• If the "Initial test" resulted with the engine responding by stumbling or "sneezing" up through the intakes (indicating a lean mixture) then open all mixture screws 1/2 turn and repeat the above procedure as described in "Initial test".

• If the engine continues to stumble or "sniff" then decrease the idle air jet size by one size (a 110 jet size would be replaced by a 100 size) and repeat the "Initial test" with the mixture screws set to "Lean Best" condition.  This jetting change requires carburetor modification to allow use of tuneable idle air correction jets.

• Continue with the process until the lean response is corrected.  Of course, if the progression is rich then the idle air correction jet size should be increased until it generates a lean response where it would then be replaced with the previous, smaller idle air correction jet size.

NOTE: Changing the idle air bleed jet requires re-evaluation of the idle fuel jet selection and readjusting "Lean Best" idle mixture screw setting.

After the selection of Idle Air Jet has been determined then it is time for a road test.  Perform road tests using the Subtle-on and or the Subtle-off procedures as provided above.

Tunable air correction jet optimization 

The tune-able idle air correction modification of the triple throat Weber carburetor provides a powerful aid for tuning the slow speed fuel delivery circuit which is referred to as the idle and progression region of operation and for the transition onto the main circuit region of operation.  Tuning optimization tests of the slow speed circuit will reveal if the fuel delivery curve is well matched to your engine’s needs or if adjustment of the idle fuel jet and of the idle air correction jet are warranted.  The idle fuel jet supplies fuel delivery across the slow speed circuit via the progression holes in the throttle bore and through the mixture screw hole.  If it is determined that the idle fuel jet provides a good fuel delivery for a portion of the slow speed circuit but is either lean or rich in another portion of the circuit, then an adjustment of the idle air correction jet may be used to tailor the fuel delivery curve to satisfy fuel strength across the slow speed operational range.

 Optimization is achieved by testing the adequacy of the idle fuel jet and idle air correction jet as provided in the previous sections of progression circuit tuning.  In summary of these testing procedures:

 Perform a slow speed acceleration in 3rd gear noting throttle response using the Primary Procedure, preferably with the main circuit deactivated by reversing orientation of the auxiliary venturi.  By trying different air correction jets, it will become clear which combination of idle fuel jet and idle air correction jet the engine prefers. Remember that any change in either jet requires a Lean Best idle mixture adjustment before making a test run.

 Specifically: If the engine runs well up to 2500 RPM and then hesitates to accelerate smoothly from there up through 3500 RPM then the idle fuel jet is good for the slow speed running but does not deliver enough fuel for transition.  In this case, DECREASE size of idle air correction jets by "10".

• Repeat Lean Best idle mixture adjustments and then repeat driving test

• The idea is to MINIMIZE idle fuel jet size while maintaining good slow speed driving performance throughout progression and transition range of engine speed.

main circuit

Once the idle and progression circuit have been optimized it is then time to work on the main circuit.  The working assumptions for tuning the main circuit are the selections of venturi, main fuel jet, main air correction jet and emulsion tube are appropriate for your engine’s performance.

Jetting the main circuit is verified either by dynamometer runs on a rolling road or by performing open road testing.  Rolling road testing can become expensive and has the disadvantage of only checking for wide open throttle (WOT) performance from 3000 RPM through redline.  While WOT performance is valuable to know what jetting will provide a safe fuel delivery curve it does not evaluate driveability and partial throttle operation.  This is where open road testing becomes valuable.

Open road testing is just as it sounds, you will use public roadways to perform performance runs to optimize timing and jetting for your engine; your engine is different from all others even if it is built to similar specifications of another engine.  Even when your engine, exhaust and ignition are exactly per OEM specifications there will be gains realized by performing optimization of your jetting since the OEM settings were designed to be a “Best Fit” for all possible end users and are therefore a compromise setting with performance improvements waiting to be realized.  Since these tests are conducted on publicly accessible roads it is STRONGLY recommended to select a speed range that allows for testing without exceeding posted speed limits or causing a disruption in traffic flow that would draw attention to your activities.  Roads that are level (or better yet; slightly inclined for a long run) with open areas and good visibility are good bets, the less traffic the better.  Also, it wouldn’t hurt to place a big sign inside the rear window saying “EMISSIONS TESTING” to help lend plausible validity to your efforts; failing that it would behoove you to have a notebook with your test data open and available for review if proof of your efforts is to have credence.

NOTES:

·         The main fuel jet controls fuel mixture strength across the operational range of the main circuit

·         The main air correction jet adjusts to some extent the initiation of the main circuit but mostly affects the mixture during upper RPM operation

·         The emulsion tube provides air for emulsification of the fuel delivery across the range of main circuit operation but has a great effect upon WHEN the main circuit is initiated

Main Venturi selection

See: Throttle Body and Main Venturi Selection

Main fuel jet selection

A simplified main jet selection is based upon venturi size and emulsion tube selection. Discussion for main jet optimization follows.

Generalized main jet sizing provided for typical venturi selections. Subtract “5” for altitudes of 4500 ft and above. These selections are from verified customer dyno developed engines and include a variety of cam and exhaust selections.:

• 27mm:

  • 110 with F1 and F2

• 30mm:

  • 125 with F26 and F1

  • 120 with F3

• 32mm:

  • 135 with F26

  • 130 with F3

  • 130 with F24

• 34mm:

  • 150 with F26

  • 145 with F3

• 36mm:

  • 155 with F26

  • 150 with F3

• 38mm:

  • 165 with F26

  • 160 with F3 and F7

  • 170 with F24

• 40mm:

  • 170 to 200 with F24

  • 185 with F7

• 42mm:

  • 170 with F24

  • 160 with F5

Tuning the main circuit is rather straightforward and may be accomplished with a selection of jets and a series of road tests.  In the process to select a main jet it is useful to use an idle jet that is slightly undersized (too lean) as this will help assure the main jet size supplies adequate fuel to help the transition speed range and helps to define when the main circuit becomes effective.  If the engine runs “OK” above 3000 RPM but acts sluggish then it is probably too rich.  Try dropping down on your main jet size and try driving again.  It is good to optimize by repeating the decrease in main jet size until a definite lean situation has developed indicated by a subtle lean surge under part throttle while cruising at a steady 4000 RPM in 4th; increase the main jet size by "5" which should resolve the lean symptoms. 

A more technical approach would use timed runs, preferably in 3rd gear and up a long steady incline.  Using a throttle stop to positively set throttle position to 80% of WOT is recommended for repetitive results.  The procedure would be something like applying WOT at 2500 RPM and start timing when 3000 RPM is reached.  (Opening the throttle at 2500 allows accelerator squirt to be completed when 3000 RPM is reached.)  Stop timing at 6500 RPM.  You want to minimize run time as a function of jetting changes so keeping a notebook handy is essential for recording your results.  This process is also useful for setting distributor timing. Adjust main jet size up by “5” and repeat timed run.  Repeat again for main jet size decrease by “5” from initial jet selection.  The minimum run time for the selected main jet will be the optimal solution. 

NOTES:

• The use of high-octane race fuel is good insurance against detonation during full-power, high-speed testing.  This fuel will not affect jetting results but may help prevent other damage before final jetting has been determined or if you are near the limit of lean running.

• Use fresh sparkplugs for testing and carry several reserve sets; the use of plugs with a colder heat range than used for street driving is another good insurance plan against detonation. 

• Buy a set of jet gauges and jet reamers to modify jets during testing to avoid buying jets individually until final selections are determined

• Have a selection of jets available for tuning comparisons.

• Changes in temperature, vehicle weight, amount of fuel in the gas tank, tire pressure, crosswinds, etc. will affect your data from one day to the next so for each new test day it is important to run a base-line data set to determine the current standards of performance.

• A larger main fuel jet will enrich fuel delivery for the mid to high RPM range for throttle openings larger than 25%, assuming the air correction jet is kept constant – since most cruise driving is performed at throttle openings of 10% or less; the 25% throttle position assumes the idle/progression circuit is mostly inactive.

  • So, if your engine makes good power up through 5000 RPM and then falls off or “stutters” then try a smaller air correction jet (increments of 10 in size) but if your engine has poor power throughout mid-range RPM operation through high RPM then the main jet should be increased (increments of 5 in size.)

• Main air correction jet is tested from 6000 RPM through redline.  Adjust pedal stop to allow for WOT running and run engine to redline.  Increase main air correction jet size in increments of “10” until the engine stutters which indicates a lean condition and then reduce main air correction jet size by “10” or “15”. 

  
  
  
  

the Subtle-Off method for the main circuit jetting:

With the engine fully warmed up, cruise at 3000 RPM in fourth (allows a little more time to evaluate the test response).  Apply full throttle and allow engine to reach 6000 RPM (time enough to settle-in and for accelerator squirt to dissipate).  Quickly Roll-Off the throttle to a 80% throttle open position.  This will momentarily enrich the fuel mixture.  If the engine responds by gaining power and accelerating, then this indicates a lean jetting configuration.  If the engine goes flat, stutters, or runs rough then the mixture is rich.  Remember that the fuel delivery on the main circuit is the summation of fuel supplied by the main jet, the emulsion tube and the main air correction jet. 

• The Subtle-Off test provides an indication of jetting during a particular region of operation, other RPM fuel mixtures may be evaluated but enough time between cruise RPM and test RPM must be allowed to dissipate accelerator pump contribution.

• Don’t let the accelerator pump function fool you.  Even a little movement of the throttle (when almost closed) and add 10X the idle fuel.  You can disable the pump if you want.

Emulsion tube selection

Emulsion tube selection is considered an enigmatic process.  This is due to its subtle effect on the main circuit occurs during operation; without performing dynamometer runs it is very difficult to quantify effectiveness for fuel mixture at given RPM regions of operation.  The emulsion tube is very effective in determining when the main circuit is activated but as peak RPM operation is approached, the emulsion tube becomes “all in” regarding its progressive adjustment of fuel mixture.  The selection of main jet and air correction jet sizes is conducted almost regardless of emulsion tube selection.  It is possible to achieve basic jetting success with most emulsion tubes but the best engine performance on the main circuit is achieved by optimization of the emulsion tubes.   

There are a small number of emulsion tubes used in the triple throat Webers. These being:

• F1 and F2:

  • Used in emission era, 2.0 liter engines of lower high RPM capacity, specifically the 911T of 1969 and the 914/6.

• F26:

  • The most commonly used, OEM emulsion tube. Good, all-around E-tube for Solex and S-cammed engines.

• F3:

  • Developed especially for the 2.0 liter 911S of 1967 and 1968 for use in the IDS Webers which utilized a High Speed Enrichment circuit. These emulsion tubes used a smaller main jet than the F26 E-tubes but the High Speed Enrichment feature of the IDS Webers made up for their smaller main jet size at elevated RPM.

  • These provide earlier main circuit activation than the F26 E-tubes which helps bridge a lean transition from the idle/progression circuit when large venturis are selected.

• F7:

  • Same design as F3 E-tubes but have a richer top end fuel delivery due to elimination of the bottom holes in the E-tube compared to the F3 tubes.

• F24:

  • Typically used in engines developed for road racing.

Generalized comments regarding emulsion tubes:

• The vertical location of the bleeds entering the main well influences the fuel flow in the following ways:

  • Holes above the fuel level in the float bowl will delay main circuit activation.

  • If the uppermost holes are below the fuel level, then the main circuit will be activated more quickly.

  • Holes at the bottom of the tube affect mixture strength at high RPM.

• The emulsion tube’s greatest effect is during low fuel demands and the main air correction jet has most influence at high RPM.

• At wide-open-throttle the fuel inside the emulsion tube itself is depleted and the main jet is directly supplying all fuel for the demands of the engine, this fuel is emulsified by all the holes in the emulsion tube injecting air as it flows through the annulus.

• Emulsion tubes with small outside diameters (F24 E-tubes) are selected for engines requiring strong acceleration up through high RPM.

• Emulsion tubes with large outside diameters will lean-out the fuel mixture more quickly than those with small diameters, they require larger main jets to help make up for its lean characteristics compared to that of a smaller diameter tube.  The reason they are "leaner" than smaller diameter tubes is that during increasing fuel flow, the fuel level in the annulus between the emulsion tube well and the outside diameter of the E-tube will drop more quickly than for a smaller tube and will therefore expose more holes for air flow through the tube into the delivered fuel.

• A change of emulsion tubes will usually be accompanied with main and air correction jet changes

Main air correction jet selection

Once the main jets have been selected for engine speeds from 3000 to 5000 RPM the air correction jets are then selected for upper RPM usage.  The air correction jets meter air, not fuel so larger means leaner fuel mixtures.  The selection procedure is like that used for the main jets only at engine speeds above 5000 RPM.  Test WOT operation through redline and at a steady cruise at 6000 RPM and check for stuttering or for going “flat”.  When a lean stutter occurs then decrease the air corrector jet size until it disappears.  Once you have selected your air correction jet it is prudent to select one that is one size smaller (by “10”) to protect your engine at sustained high speed. 

After selection of the main jet and main air correction jet sizes from the above methods it is good to try other methods for verification of jetting suitability, remember that carburetors are tuned to be a "Best Fit" of jetting to various driving conditions.  With any change of any jetting feature of your Webers there will be associated effects upon other regions of carburetor operation and jetting.  Carburetor jetting is an iterative process that requires patience and understanding of the various components and their interactions to achieve the optimum jetting package for any engine and application.

Typically 170 or 180 main air correction jets are used for street driven cars using F1, F2, F3 and F26 emulsion tubes. The 170 main air correction jet is used to help assure a slightly richer top end than 180 jets provide which is good due to today’s fuels requiring a jetting providing a richer mixture than used 50 years ago. F7 emulsion tubes have an enhanced, high RPM richness compared to the other E-tubes.

Engines developed for racing typically use a F24 emulsion tube. The main air correction jet for these applications will be close to the same size as the main fuel jet.

accelerator circuit

The Acceleration Circuit provides fuel delivery during rapid throttle openings to maintain a combustible mixture.  When the throttles are opened rapidly, airflow into the engine increases almost instantly but the fuel delivery lags due to the fuel having more mass than air thereby taking longer to be drawn into the airflow.  The accelerator circuit injects raw fuel into the air stream to bridge the fuel mixture with the increased airflow until the main circuit fuel flow “catches up”.  This injected fuel would not be needed if the throttle operation was incremental but sudden demands such as a passing maneuver require the squirt to provide an appropriate fuel mixture. 

Testing is performed by establishing a cruising speed at 3000 RPM and then quickly depressing the throttle to fully open. If the response is to initially slow down then increased injection amount from the squirter nozzles is warranted. The maximum amount of squirt is achieved when the roller cam-follower on the lever arm in the top cover of the accelerator pump is in the crook in the cam lever arm mounted on the throttle body. Adjustable length pump rods (from 1968 year model to current date) allow for this adjustment.

If the engine response is initially delayed then it is possible to advance the squirt delivery by upgrading to an IDTP style cam lever arm.

Beyond this, the next course of action would to modify your cam lever arm to provide a greater total lift. This requires either making new cam lever arms or building-up your existing lever arm to have more lift.

Ignition Timing

The Importance of Ignition Timing

Timing is the single most common issue with a badly-running Weber system, bar none. Set the total advance at the recommended amount for your engine and forget about "intial" timing; whatever it is, that's what it is. Initial timing means nothing; total advance is EVERYTHING. If it's too retarded, you'll have a lazy, rich, engine with farting and popping carburetors, and way too much heat in the heads, which can make the carburetors boil the fuel in the bowls...and drip. It will cause all sorts of issues...none to do with the carburetors...but they'll get the blame most of the time.

Procedure for Timed Runs (Ignition advance testing)

The Timed Runs procedure for main jet selection is also perfect for optimizing ignition advance.  Ignition timing Perform the Timed Run for various amounts of advance.  You will be able to determine the best advance angle that corresponds to a minimum elapsed time.  Prudence will have the advance decreased a bit to accommodate for fuel quality variations.  Be especially careful of too much advance if you performed Timed Run testing with race fuel and plan to use pump gas in the engine.