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Remanufacturing processes

Remanufacturing vs. Rebuilding

The 40 IDA-3C, three barrel Weber carburetor has been in service for decades (since conception for use with the Lancia Aurelia in the mid 1950's) providing exceptional performance for six cylinder Porsche engines, Fiat Dinos, Lamborghinis, Ferraris and for many other engine applications. No longer manufactured, these carburetors are typically at the end of their service life and are scarce to find in decent condition. Fortunately they may be resurrected by careful remanufacturing and tuning to provide performance and reliability equal to or exceeding that of new carburetors.

"Remanufacturing" is the term Performance Oriented chooses to use to clarify the many tasks it performs in the return-to-service of your aged Weber carburetors. "Rebuilding" is a term describing the process where old gaskets are replaced with new ones and is a sub-task included with Level 2 and Level 3 Remanufacturing services as with the Restored, Rebuilt & Tuned service.

Services provided by Performance Oriented during its various Remanufacturing efforts include the following:

• Machining and installation of new, oil impregnated bronze bushings for 100% of throttle shaft bearings
• Replacement of the throttle shafts (as required)
• Resizing of the main throttle bores and installation of new throttle valves
• Re-surfacing gasketed interfaces to assure reliable fuel and air sealing
• Corrosion control of carburetor body and refinishing of components
• Careful inspection of all components for wear, distortion and irregularities
• Fuel gallery cleaning (requires lead plug removal)
• Blueprinting of sub-components (see Blueprinting discussion below)
• Replacement of standard hardware with OEM quality equivalents
• On-engine testing & tuning for all projects (Level 1 projects are bench tested only)

Throttle shaft rebushing

The IDA-3C (or 3 Choke) carburetor is a development from the two choke IDA Weber carburetor. One of the differences between these two designs is the deletion of ball bearings used to support the throttle shaft as utilized in the design of the two barrel IDA carburetor. This detail design change has the IDA-3C throttle shafts in direct contact with the throttle body; wearing both the shafts and the throttle body bearings. Over time this wearing creates clearances allowing uncontrolled air into the fuel/air mixture delivered to your engine. Erratic idling, 'sniffing' through the intakes, ticking noises heard at idle or popping from the exhaust are the results.

The deletion of the ball bearings also allows the throttle valves to edge wear due to uncontrolled axial movement of the throttle shafts. To a lesser extent there are more clearance issues resultant to the relaxation of the throttle body over time with associated distortions causing air leakage.

The picture shows carbon build-up indicating air leakage past throttle valves and throttle shafts. The throttle valve in the middle bore is not closing due to a twist about the axis of the long throttle shaft. All three throttle valves display carbon build-up due to edge wear from axial movement and from throttle shaft clearance in the throttle bodies.

The idle air adjustment screws provide some compensation for the throttle valve to bore clearance problem but will not adjust for the uncontrolled air introduced through worn throttle shaft to bore interfaces. Over time, these two wear related problems interact to cause tuning issues which are corrected by remanufacturing.

Carbon deposits indicate wear issues

The earliest Webers used plain bearings to support the throttle shafts but changed the design in 1967 to incorporate nylon "bearings" which are shown as Item #78 on the Weber Parts web page. The bad news is that the nylon easily displaces under running loads and allows the throttle shaft to rapidly wear with the remaining support area of the throttle body; the good news is that the throttle shaft under the nylon bearing is routinely serviceable "as-is" during a remanufacturing process. (The picture shows the localized area of accelerated wear and the area mostly unaffected where the nylon bearing was located.)

Worn throttle shafts with nylon "bearing"

Throttle Valve Replacement

Edge wear of the throttle valves renders them unusable. This wear occurs due to uncontrolled axial location of the throttle shafts allowing the throttle valves to rub with the bores of the throttle bodies. Performance Oriented preloads the shaft coupling in the course of all remanufacturing processes to control future wear issues. Since the throttle valves contact the throttle bores at nearly closed throttle conditions (most driving is done using the progression circuit) the wear is transferred to a small area to the throttle bodies. For this reason and due to age related distortions from natural stress relief of the original parts stock throttle valves are not used. The original throttle bodies are resized and custom machined throttle plates are fitted. (The picture displays a worn throttle valve in the area of the throttle shaft.)

Edge wear of throttle valve

Blueprinting

• Fuel Float

  I have yet to find any float (new or from a customer's carburetor) that has factory geometry! I do find, however many carburetors with one float shim (Item #75 on the Weber Parts web page) installed. Shimming tolerance on the fuel level is equivalent to a 0.010" range of shim thicknesses. Since the main fuel circuit is activated upon vacuum generated by throttle opening it then should be obvious that with four fuel floats you will have four different main circuit activation times, not all at once as would be desired. It is for this reason the floats are carefully set to have factory geometry and fuel levels carefully set on a running engine.

  The tab shown in the photograph has a divot from where the fuel float valve contacts the tab on the float. This divot is in a unique location based upon how the parts are assembled and changing float valves may result in interfaces that cause sticking and erratic float operation or worse yet - flooding. Performance Oriented resurfaces this tab to provide a fresh interface with the float valves and also corrects the tab geometry with the hinge axis to provide an orthogonal surface for the valve. Of course other checks and adjustments are performed (adjusting hinge fit with fulcrum pin, checking for leaks, straightening the float body with the hinge arm and setting float height relative to hinge axis and tab location) during a remanufacturing process but those highlighted are the most important.

  
  
  Float divot

  
  
  

• Accelerator Pump

  Most projects display fuel weeping from the accelerator pump body. They also display indications that the nuts used to tighten the cover onto the body have been torqued to the point of mashing the stud holes closed around the studs. The only good way to correct for this is to resurface the top cover so it provides a uniformly flat surface to seal the gaskets without undue torque on the fasteners. (The picture shows the high spots on the sealing surface of an accelerator pump cover before resurfacing and the resulting flattened surface afterward.)

  
  
  Warped and resurfaced accelerator pump covers

   

  Reassembly of the pump cover allows the fulcrum pin of the lever arm to be reinstalled and rotated to restore crisp pump action without lost motion. (The picture shows a worn pin which when reinstalled provides a fresh interface with the lever.)

  
  Worn accelerator pump fulcrum pin

   

  This doesn't finish the attention with the accelerator pump system; the injection amount for each Pump Jet (Item #70) is set to factory specifications and adjusted as necessary. Each Pump Jet is also checked and adjusted as necessary to insure they spray into the annulus between the auxiliary venturi and the main venturi and not onto the venturi wall. (The picture shows two types of Pump Jets, the longer one is used for small main venturis (27mm) while the shorter one is used with larger venturis.) Additional checks are performed on the six accelerator circuit Delivery Valves (Item #71) and two Intake Valves (Item #72) to verify proper operation.

  
  
  Different Pump Jets for different applications

   

  Another area of rapid wear is the accelerator pump rod (Items #34 & #42 on the Weber Parts web page) where it connects with the lever arm (Item #41 on the Weber Parts web page) on the throttle shaft. Nothing to do here except replace both the pump rod and the lever arm with new ones. (Picture below.)

  
  
  Recently retired Accelerator Pump Rod and Lever Arm

  
  
  

  Sometimes a project mixes up parts or a customer is buying used Webers and isn't sure of their vintage. The following pictures help identify accelerator pump components. The photo of the pump covers and lever arms shows a later style (note the reinforced perimeter and the plastic roller for the pump cover on the left, vintage 1969 or newer) while the photo of the rocker arms shows (in clockwise order from the top-left) IDS, IDA/IDAP and IDTP versions. Discrepancies in carburetor configurations are noted during the disassembly and pricing proposal phase so the customer may make intelligent decisions about their project.

  Accelerator Pump Covers		Rocker Arms

  
  
  
  

• Auxiliary Venturi Interfaces

  The main circuit delivers fuel into the airflow through the throttle body via the inside bore through the auxiliary venturi. The emulsified fuel is drawn by vacuum from the well housing the emulsion tubes and past the interface between the throttle body and the auxiliary venturi. This interface has no seal other than that of an intimate fit. If this fit is not tight then unwanted air will be drawn into emulsified fuel mixture (weakening it in the process) and it will also weep into the intakes without atomization.

  Performance Oriented takes care in resurfacing these two interfaces to assure the intimate interface per Weber design requirements is maintained. Holding this interface in tight compliance is the spring set into the opposite ear of the auxiliary venturi. These springs are removed, inspected, refinished and re-tensioned to provide the requisite fit-up at the fuel delivery side. (Picture shows the tall Auxiliary venturi as used with 46mm Webers on the 906 and 911R engines next to a short one delivered as stock on production 911 engines.)

  
  
  Racing and standard auxiliary venturis

  
  
  

• Throttle Valve Replacement and Alignment

  Throttle shafts must be straight to provide freely operational throttle operation and the long throttle shaft must be checked for twist to assure both throttle valves close completely and simultaneously. Twist of the short shaft is not important since the throttle valve it locates may be rotated into closure independently of those mounted in the long throttle shaft and fixed there using the throttle shaft coupling. This does not complete the task of throttle valve alignment. The bottom edges of the throttle valves are designed to align with the bottom edges of the first holes in the transition (progression) circuit. This is to keep these fuel delivery ports closed during idling operation so that fuel at idle is delivered only by the idle mixture adjusting screw.

  Since these holes were drilled with small bits there is some minor deviations with the alignment of the three throttle plates mounted on the throttle shaft. By observing the variations in alignment of these closed throttle valves with the hole positions in the progression circuit minor adjustments (local edge relief filing per Weber approved technique) may be made to the throttle valves to bring all three into perfect alignment with the progression circuit.

• Fuel Gallery Cleaning

  Those projects having a cosmetic restoration (Level 3 and Restored, Rebuilt & Tuned) have the lead plugs removed from the fuel delivery galleries to assure thorough cleaning of debris and of the cleaning agents utilized during restoration. New lead plugs are cast by Performance Oriented and peened into the original openings to complete this task. These openings were required to provide machining access of the fuel delivery galleries during original manufacture of the carburetor bodies. It might be noted here that Weber throttle bodies differed based upon type application. The IDS bodies always had extra fuel galleries to provide fuel to the "Enrichment Circuit" unique to these carburetors. Other throttle bodies did not use these galleries but sometimes the IDS throttle body casting was used for standard IDA or IDTP applications and the drilling of the IDS specific galleries was omitted. (The picture shows the external differences between an IDS and an IDA throttle body, the IDS is the one on top, of course.)

  
  IDS and IDA throttle bodies

  
  

• Resurfacing Sealing Interfaces and Thread Repair

  The bottom flanges of the throttle bodies are resurfaced since it has been demonstrated that as little as 0.010 inches of out-of-flatness between the throttle body and the top of the intake manifolds is sufficient to bind the throttle shaft to the point of sticking. Not a good thing to diagnose! Additional re-machined surfaces are those used for delivery of the fuel into the top cover of the carburetor. Many brass fittings have been severely torqued and knarled when the wrench slipped off in the attempt to stop fuel weeping past these sealing points. (The pictures show surfacing a couple of these interfaces.)

  Machining Fuel Float Valve Well Hex Cap surface and bottom mounting flanges

  
  

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Weber Model Variations

Porsche used the IDA type carburetors on production 911's and 914/6's; the following is a listing of those used by type and model application.

40IDT3C and 40IDT3C1

• 1968 911T (non-USA)

40IDA3C and 40IDA3C1

• 1966 - 67 911
• 1968 911L (non-USA)

40IDS3C and 40IDS3C1

• 1967 - 68 911S

40IDAP3C and 40IDAP3C1

• 1968 911 & 911L (USA)

40IDTP3C and 40IDTP3C1

• 1969 911T
• 1971 only 2.2 911T (non-USA)
• 1970 - 71 914/6

Although Webers were not used on production Porsches after 1971 they were available as an after-market replacement for mechanical fuel injected, Solex and Zenith carbureted engines. These were frequently provided for years either from Italy or from the USA in later years. All the after-market Webers (not sold through Porsche service centers) were identified as type 40 IDA 3C or 3C1 carburetors.

The following pictures highlight some of the more common variations of the Weber 40 IDA-3C carburetor components:

Three short intake air horns and a tall one on the right:
The left and right horns are nickel plated;
the second from the left is zinc plated with yellow chromate finish
and the third from the left is plastic.

 

Idle jet holders: The upper one is the later version used with an o-ring to help seal against air leakage
and the lower one is an early style used without an o-ring.

 

Throttle Lever Arms: The left one is from an IDTP and
the other is used on IDA, IDAP and IDS carburetors.
Note the different throttle stop screws.

 

Idle mixture screws: The left one is not common but is seen occasionally;
the middle one is typical for IDA, IDAP and IDS bodies
and the right one is from an IDTP.

 

Throttle shaft couplings: The upper two are the old design, folded-spring style (note variations)
and the newer, better type is the cylindrical one on the right.
The cylindrical coupling uses set screws that positively grip the throttle shafts
where the folded-spring style will allow shaft rotations causing throttle valves to become unsynchronized.

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Progression Circuit Operation

The progression circuit is designed to provide an air/fuel delivery to the engine for operation from idle to the main circuit and a little beyond. The IDA 3C's have been used on Porsche six cylinder engines for quite some time, either as OEM or aftermarket replacements for other fuel delivery systems (MFI, Solex, Zenith, etc.) Their adaptability for usage on 2.0 liter through 3.0 engines has made them popular for enthusiasts for many years. My interest in the progression circuit operation came from my research of several tuning texts which describe the idealized setup for proper operation of this circuit.

According to the Weber operation manual, proper throttle valve position at idle should block the first progression hole with the bottom edge of the throttle plate aligned to be level with the bottom of the first progression hole. On a new 40mm Weber this would correspond to an idle stop screw adjustment of about 1/2 turn from throttle plates completely closed upon the ID of the 40mm bores in the throttle bodies. This is a physical setting that is not to be adjusted. Most tuning guides have this throttle stop screw adjusted to achieve side-to-side balancing of the carburetors and to adjust the desired idle speed.

As my curiosity increased I became aware of the air flow demands of larger displacement engines. To many, the selection of the 40mm IDA's to engine displacement and performance characteristics has been one of calculating the cylinder displacement and determining at which operational speed (RPM) the engine makes its peak horsepower; from there a handy Weber chart suggests an appropriate venturi size. This is fine for the main venturi, but the idle circuit has been left behind. Racers don't worry much about progression circuit operation but those who want street performance and fuel efficiency may wish to consider the next few lines. (The ideal throttle bore size is proportional to the venturi size and this is discussed in Selection criteria for modified throttle bore diameters.)

Since the progression circuit is (by design) inactive at idle, the only fuel delivered to the engine is from the bypass fuel circuit (adjusted by the idle mixture screw) and the only air delivered is from the idle air bleed screw plus the area between the throttle body bores and by the barely opened throttle valves. Engines of different displacements (and of various cam lift/timing and valve diameter selections) require different volumes of air at idle to allow the engine to run (a 3.0 would require 1 1/2 times as much air as a 2.0 would in a simplified comparison.) To keep a larger displacement engine running means that the air correction screws would need to be opened enough to allow 50% more idle air than a 2.0 would need while maintaining the throttle valve alignment as specified earlier. The idle air screws are not adequate to achieve this additional air demand along with the need to adjust for cylinder-to-cylinder air flow variances. Also, they tend to whistle a bit when they are asked to deliver this additional air. The result is that one is left with adjusting the idle speed stop screws to provide additional air at idle.

The problem with all this is that once you have uncovered the first port in the progression circuit you have effectively bypassed the design intent of the idle circuit being separate (but not isolated) from the progression circuit. You have now established fuel delivery from two ports (the idle mixture port and the first progression circuit port) and adjusting idle mixture with the idle mixture screw upsets the idle jet selection resulting in a leaner idle jet than is really needed for progression. (You will find the idle mixture screw is unresponsive since it will only affect one of the two fuel delivery holes.) The upper RPM operation of the progression circuit is affected by the air correction jet for this circuit, a brass jet that is pressed into the throttle body and is therefore not easily replaceable. The air correction jet for the three choke IDA throttle bodies is the same size except for the larger jet size used in the IDT and IDTP throttle bodies. Since a smaller jet richens the progression circuit fuel delivery as RPM's rise, the IDT and IDTP carburetors will lean-out more than the other types at the transition to the main circuit. This is a cause of the dreaded "Flat Spot" of Weber carbureted 911 engines. (Of course there are other issues to consider such as proper ignition advance, valve size, port size, exhaust configuration, camshaft selection, engine condition, etc. which I am passing over to those more experienced in this area.)

Further investigations of the progression circuit in the various throttle bodies revealed the IDT and IDTP carburetors having unique geometry from the others, again providing a leaner idle mixture with a larger orifice for the upper RPM transition to the mains.

The result of all this is:

• Large displacement engines using 40mm throttle bores demand more air to idle than is available while keeping the progression circuit inactivated. Smaller idle jets are selected to lean out the idle operation which results in a flat spot at 3000 RPM.
• Air correction jets for the progression circuit are not easily replaced therefore tuning is difficult.
• IDT and IDTP throttle bodies were delivered as an economy and not a performance carburetor and therefore display limitations for tuners.

Performance Oriented has addressed these issues with the following offerings:

• Replaceable air correction jets may be installed into your throttle bodies to aid in tuning your progression circuit operation. (See the picture below showing the replaceable air correction jets.)
• Modified throttle bores may be selected to match the carburetor to the engine application to avoid using a too small bore size for your larger displacement engine, see Selection criteria for modified throttle bore diameters

Measuring progression circuit port locations

 

46mm IDA'S		40mm IDA'S/IDAP'S/IDS'S

40mm IDTP'S

Progression circuit port arrangements for various Weber throttle bodies

 

Tune-able idle air jets, fuel well baffles and relieved mains (under the baffles)

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Sources of Debris in Float Wells

There are three basic sources of debris in the float bowl:

1. Dirt in the fuel getting past the fuel filter or internally decaying fuel lines downstream of the fuel filter.
2. Dirt bypassing the air seals on the air cleaner (or rust in the troughs where the air seals are installed) and entering through the holes and vent pipes in the top cover of the carburetor body.
3. Rusting of the inside diameter and internal bits of the vent pipes in the top cover. This is typically missed as a source of debris. Wire brushing and neutralizing the corrosion is recommended by Performance Oriented for Level 1 customers. For customers selecting Level 2, Level 3 and Restored, Rebuilt & Tuned the restoration of the zinc plated finish of these pipes is included.

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Installation and Basic Tuning Guidelines

The following information is provided to guide the installer/tuner during the basic installation and initial tuning of a set of Weber 40 IDA 3C carburetors. These have been generated by reviewing the many procedures created for this purpose and edited to create this version. I have added those items I believe pertinent, in particular I wish to highlight the variance from common practice in setting idle air corrections. Commonly idle air corrections are performed by minimizing the idle air correction settings and adjusting idle speed solely with the idle speed stop screws. This practice is in difference with the Weber tuning procedure and produces progression circuit tuning errors. More discussion on this is provided in Progression Circuit Operation above.

Preparatory Procedures and Notes

1. Prior to installing your Webers onto the tops of the intake manifolds the following checks must be made:
   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 removed and resurfaced (contact Performance Oriented for tech help) or carefully matched gaskets may be used to rectify the issue.
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. Similar to an out-of-flat intake manifold interface you may distort the throttle body during improper nut torquing 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 the carbs the following items must be in order since many times tuning troubles are ascribed to carburetor problems when actually they have been misrelated issues:
   1. Cam timing is assumed to be correct
   2. Valves have been adjusted (See Valve Adjusting Tech below)
   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. Typical timing values are:
         1. 5° BTC @ 900±50 RPM
         2. 30° BTC @ 6000 RPM
   6. Fuel delivery pressure has been set at the carburetor inlet to be 2 1/2 to 3 1/2 psi; measured with the engine running to simulate actual electrical power delivery to the fuel pump while driving.
   7. Carburetors and fuel lines require inspection for fuel leakage and all issues require correction before proceeding.
   8. Lubricate the throttle shafts using a drop of 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 overstressing. Also check to see that excessive play has been adjusted out.
5. Recommended ignition and fuel delivery components are:
   1. Perma-Tune Electronics Model 911E902 Capacitive Discharge ignition mated with a good coil with internal ballast resistor such as a Bosch #0 221 122 008
   2. Pertronix Igniter to replace the traditional points in your distributor
   3. Holly two-port, 1-4 psi fuel pressure regulator, #12-804
   4. Mallory rotary fuel pump, #4070M
6. Treat the needle seats (in the throttle body) with respect (don't over tighten) because they can't be replaced. Those to be respected are: idle air correction needles; idle mixture screws and the idle jet holder which presses the idle jet into the throttle body.
7. For non-competition usage the following spark plugs are recommended: NGK BP 5ES for compression ratios to 9.1 and NGK BP 6ES for ratios above 9.1. Plugs gaps set to be .028" - .030".
8. Initial carburetor settings to be adjusted as follows:
   1. Throttle stop screws adjusted all the way out and turned back in until they just touch the throttle arm and then turn in 1/2 full turn. Alternately: Remove the plugs covering the transition holes (Item #55 on the Weber Parts web page) and adjust the throttle valves until the bottom edges of the valves just begin to uncover the lowest progression circuit holes.
   2. Idle air correction screws are turned in until they lightly bottom out and then turned 2 full turns out and locked in place with the 8mm lock nut
   3. Idle mixture screws are turned in until they just bottom out and then turned out 2 full turns
   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.

Valve Adjusting Tech

The following valve clearance procedure came from a series of discussions on the 911 Technical forum. This method verifies the 0.004" valve clearance specified between the elephant's foot adjuster screw and the top of the valve stem by measuring the clearance between the cam and the slipper-foot end of the rocker arm. The benefits of using this method are twofold:

1. It's easier to insert the feeler gauges
2. You utilize a "Go - No Go" method that verifies proper clearances have been achieved.

The method is:

1. Adjust valve clearance as normal at the tip of the valve stem
2. Verify the gap at the interface between the rocker arm and the cam
3. If the 0.0030" feeler gauge won't pass through the rocker slipper-foot/camshaft gap then the gap at the valve stem is less than 0.0042" meaning the clearance is not too loose.
4. If the 0.0025" feeler gauge will pass through the rocker slipper-foot/camshaft gap then the gap at the valve stem is greater than 0.0035" meaning the clearance is not too tight.
5. Correctly set, the 0.0025" will slip into the rocker arm/cam gap and the 0.0030" will not.
6. Hint: If you rotate the crank in 240 degree increments you will perform all the valve adjustments on one side of the engine before you jump to the other side

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Basic Tuning Procedure

1. Preparatory Procedures and Notes in the above section have been satisfied, Right?

2. Accelerator Pump 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 easily and independently actuated. Turn on the ignition and allow fuel pressure to rise to full value (2 1/2 to 3 1/2 psi) which assures fuel bowls are filled. 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 as you actuate the throttle lever. Remove the vial and note the injection amount. Each pump jet should deliver close to the same amount on each carb. It is good to check the amount injected on all three using a short time interval between data points and then retake the readings after waiting 30 seconds between samplings. This tests the sealing of the check valve in the securing bolt holding each pump jet. If the amount of fuel is significantly different than for the first set of data then the check valves may be corroded or otherwise damaged and not sealing effectively. The loss of injectible fuel due to poor check valve operation would contribute to a lack of fuel delivery during acceleration with an accompanying stumble.

   To decrease the injection amount just tighten the adjusting nut on the pump rod (shortens the effective rod length.) The amount varies by engine size and performance potential but typical injection amounts range from .4 to 1.0 cc per cylinder for the early 2.0 cars.

   While you are looking down the venturis check to see the stream of injected fuel squirts into the annular air space between the main and secondary venturis. Weber provides different pump jets depending upon the original venturis supplied with the carburetor. Long pump jets were required to clear the small venturi diameters (27mm diameter for example) while shorter ones were supplied with carburetors having 32mm 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.

   Since you will be dumping raw fuel into your engine while it is not running 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 fuel level drop and expose the brass check valve or you will need to re-purge the air. Measuring the accelerator jump volume on the bench allows for easy correction of blockages and the other issues mentioned above.

3. Set Fuel Float Levels

   The setting of fuel levels is quite important since they affect the timing of when the main circuit is activated. So, if the float levels are even but low then the main circuit will be activated a bit late since it takes more airflow past the main venturi to draw the fuel into the main circuit. Uneven setting of fuel levels will result in different cylinders transitioning to the main circuit at different times than other cylinders.

   I set float levels using a fuel delivery pressure of 2 3/4 psi: if a different fuel supply pressure is used for your vehicle then float settings WILL be affected; lower fuel delivery pressures will lower the fuel level in the bowl.

   You will need to check them at your 6000 mile tune-up interva; especially after a Performance Oriented remanufacturing since the needle valves will indent the tabs on the floats where they come into contact with them. I removed these divots during my work and the float valves will divot them again. The divoting (and to a little extent the settling in of the needle valves themselves) will cause the fuel level to rise a bit.

   Set float levels with engine is running; the vibration keeps the float needle valves from sticking and results in a better reading.

   I sometimes use a short rod inserted down the vent pipes to just 'tickle' the float to allow it to seek its happy place! Another reason to have the engine running is to assure the fuel delivery pressure is constant and not low due the battery not supplying the same power as the electrical system would deliver with a running engine.

   The engine should be brought to full operating temperature to achieve the most accurate setting possible. You may make an initial adjustment to get your engine so it may be driven but as in all the tuning processes the best job is achieved with a warmed up engine, 175° F or better.

   With the fuel pump off remove one of the two float bowl plugs on one of the carbs. Catch the fuel that drains out of the hole with a little jar and return it to the tank or fuel your lawn mower with it. Screw the float level gauge onto the side of the throttle body being sure there is a good seal (good o-rings will seal quite nicely without much torque being applied.) Start the engine and fuel will fill the float chamber of the carburetor and the float gauge as well. You want to have the bottom of the fuel meniscus settle between the two top lines on the gauge.

   Adjust float level by:

   1. Stop the engine and remove the hex cap covering the needle valve well. I use a six-point Proto wrench modified by grinding off the radiused edges where the wrench interfaces with the flange of the hex cap and the handle was straightened to prevent it from slipping off these pesky to tighten/loosen hex caps.)
   2. Remove needle valve
   3. Use your dial calipers to measure the shim stack thickness
   4. Adjust shim thickness using this Handy Guideline: The distance between the two lines at the top of the float gauge is approximately equal to about 0.010" of shim thickness beneath the fuel needle valve.
   5. You may use sand paper (180 grit or finer) to reduce the thickness of a shim to achieve adjustments finer than that achievable with the standard copper washer shim pack. The copper washers may be reused for many shimming/tightening operations.
   6. Adding shims raises fuel level. If you are not able to shim the float it may be due to the float geometry is not per Weber specifications. The top of the float should be 30.8mm above the pivot pin axis and the tab that the needle valve contacts should be level and coplanar with the pin axis.
   7. Reinstall needle valve and that pesky hex cap. I resurface the hex cap and its sealing interface with the needle valve well as this is a difficult seal to maintain. I also recommend aluminum sealing washers in lieu of the fiber ones which don't seem to seal as dependably as their aluminum counterparts do.
   8. Repeat the float level measurement procedure until the meniscus requirement has been achieved.
   9. Remove the float level gauge and set the other three float levels using the same procedure.

   

   A Handy Tool:
   Modify your float gauge like this to help keep the fuel loss to a minimum,
   the tubing attaches to a brake bleed valve that has been tapped into the head of the thumb screw
   while the jar is a specimen jar from your local medical supply house

   
   
   

4. Adjust Idle Air Correction Screws

   The following procedure flies in the face of conventional tuning procedures so please review Progression Circuit Operation for clarifications.

   Reminder: The initial setting for the idle air adjusting screws is two full turns open from the closed position.

   Start the engine and once it is capable of running without you nursing the throttle disconnect the drop links connecting the cross bar to the throttle arms so the idling is controlled only by the carbs themselves. Adjust the two idle speed adjusting screws (previously set to just touch the throttle arms and then turned in 1/2 turn) using 1/8 turn increments to just keep the engine running.

   Use your synchrometer to measure air flow through the six air intake trumpets. Find the average reading for each bank of cylinders and adjust the high and low readings to match the average reading using the air correction screws. Then use the two idle speed adjusting screws to adjust both banks to have the same reading while adjusting idle speed to be 900 RPM.

5. Adjust Idle Mixture Screws

   I present two methods of adjusting the idle mixture screws, the first is the classical "Best Lean" procedure and the second is the method I have used for as long as I have owned my 1967 911S (since 1979) and on every carbureted engine I've tuned since then. Either method works well but I like the second one better since I am able to visually correlate actual combustion richness with the audible clues given in the first method.

   These adjustments are dynamic and each engine displays its own tuning characteristics. Only attempt final tweeking of the idle settings on a really warmed up/hot engine, this is especially important since fuel atomization is best on a warm engine and the mechanical advance mechanism may be a bit lethargic when not fully warmed up and result in erratic idle settings.

   Lean Best Method

   This method assumes the initial settings of the idle mixture screws results in a rich condition at each cylinder.

   Select a cylinder to start with and slowly turn the idle mixture screw in using 1/4 turn increments followed by 5 second pauses allowing the system to adjust to the changes. The first indication of leaning (in that cylinder) is a dull popping sound from the exhaust or by a light sniffing sound coming through the intake trumpet. Continue turning the screw in and the engine RPMs will begin to drop. Back that mixture screw out until idle speed comes back up which should be about 1/8 of a turn. If the dull popping continues in the muffler you may continue the adjustment to make that just go away. Now note the position of this screw and open it another 1/4 turn. This is the initial setting for that cylinder which may be repeated for all cylinders before continuing with the adjustments. (Carefully screw the mixture screw for the first cylinder in until it bottoms out, counting the number of full turns and partial turns to determine how it was finally set.)

   If during any of the mixture adjusting process the idle speed changes then it will need to be readjusted to 900 RPM. If this is not kept at a constant RPM then the mixture settings will be set at different air flows.

   After adjusting all six mixture screws the amount of tweeking them will be minor but tweek them you must. Take a test drive to warm the engine up fully before making your final adjustments. Check the idle air adjustments with your synchrometer and idle RPM and repeat the Lean Best adjusting procedure until all popping/sniffing is gone. If 2 1/2 turns out makes your engine smooth out then that is OK but if you need 3 1/2 turns out then your idle jets are probably too small.

   Using the Colortune Tool

   This method assumes nothing about the initial settings of the idle mixture screws since you will see the quality of mixture when you use the Gunson Colortune tool. Colortune is used to replace the spark plug in the cylinder being adjusted for idle mixture. A 'window' in the base of the tool allows viewing the combustion taking place within the cylinder while the engine is running. An electrode adapter and a mirror attachment provide the spark and the ability to see down the hole where the plug resides. Start the engine and look through the window, what you will see is the spark which ignites the fuel within the cylinder and the resulting combustion color. Just as you may see the color of a gas pilot flame in your water heater you may also see the combustion color of the fuel mixture. Yellow/orange is rich, Bunsen blue is lean and black is no fuel in the cylinder. No spark is also obvious, so if you aren't getting a cylinder to fire you may easily determine which rabbit you should chase, the ignition one or the fuel delivery one.

   Adjust the mixture screw to change the yellow to Bunsen blue while listening to the engine and compare the Best Lean tuning signals to what you see happening within the cylinder. Move the Colortune from cylinder to cylinder to set all the same. Be sure to maintain idle RPM at 900 during the procedure.

6. Adjust Air Flow Balance at 3000 RPM

   This procedure quickly sets the balance at both 3000 RPM and at idle

   1. Adjust drop links to be same length while at your work bench. You can set two 8mm (or 5/16") bolts in a vice so they stick up and allow one of your drop links to just fit over the ends of the bolts. This is now a distance gauge to use in setting the other drop link the same length.
   2. Put the cross bar on your work bench so the bar is parallel to the bench (use two large hex nuts of the same hex size to support the cross bar) and adjust the 8mm ball studs until they are equal distance from your bench making the axis through the balls parallel to the cross bar (twist the stamped arms to do this)
   3. Adjust and lubricate the big ball studs that locate the cross bar so the cross bar is securely supported and located
   4. Adjust cross bar mounting brackets (attached to inner sides of intake manifolds) so drop links just slip over 8mm ball studs on the Weber throttle arms and tighten bracket mounting bolts to snug tightness. NOTE: DO NOT adjust lengths of drop link; adjust brackets to get the drop links to just slip over the 8mm throttle arm balls.
   5. Control throttle position with the front-to-rear throttle rod that runs from the bell crank on the front of the driver's side intake manifold to the throttle cross bar (this is important to do in this fashion as other methods of operating the throttle do not match how the engine is controlled while driving.) Alternately get a helper to run the throttle from the driver's seat.
   6. Monitor airflow in each bore on each set of carbs @ 3k RPM and adjust one of the two cross bar mounting bracket (by tapping with a plastic hammer after loosening the fixing nuts to a snug-tight condition) to change the carburetor's air flow balance
   7. Tighten the bracket mounting nuts and re-check for balance
   8. Return to idle and check that 900 RPM is achieved and drop links are free

7. Test Drive

   Tuning is best accomplished after the engine is warmed to working temperatures (175 degrees or more) to ensure efficient fuel atomization, combustion temperatures are up fuel level in the float bowls are set under operation. Take the car for a test ride. Is there any surging between 2000 and 3000 RPM? Is it popping on deceleration? Is it transitioning seamlessly between idle and main circuits? That is, no flat spots, right?

   Return to home base. Make any final adjustments that you feel are necessary to the mixture screws. In other words, if it popped occasionally or surged, turn all mixture screws out an extra 1/8. Repeat test ride. Make any further adjustments that are necessary.

   If the mixture screws are turned out more than 3 1/2 turns from full in, the idle jets are too small. If the engine stumbles on acceleration or is slow to return to idle, the idle jets may be too small. See the section on Performance Tuning for help in diagnosing tuning and jetting issues.

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Performance Tuning

Information in this section is provided to help in making adjustments to the jetting and tuning of your Webers. More information will be added over time (timed runs and plug cuts) but the following trouble shooting guide provides the basics:

Black smoke from the exhaust (sustained operation, not during acceleration):

• Mixture screws set too rich (low speed operation)
• Float level too high
• Fuel pressure too high
• Fuel valve leaking or sticking
• Fuel float(s) rubbing on wall of fuel well
• Fuel floats crushed or leaking
• Idle jets too large or progression air correction jets too small (low speed operation)
• Main jets too large or main air correction jets too small (high speed operation)
• Blocked progression air correction jet (causes fuel to siphon into cylinder)

Sneezing up through intakes (idle or under moderate driving conditions):

• One or more cylinders too lean (open mixture screws)
• Timing not advanced enough or is not advancing smoothly
• Idle jets too small
• Intake air leak (gaskets not sealing, uncontrolled air leakage past throttle shafts, cracked manifold, open vacuum tap on manifold or carburetor, etc.)

Dull popping from exhaust (at idle):

• One or more cylinders too lean (open mixture screws)

Backfiring from exhaust during acceleration:

• Fouled spark plug

Flat spot in acceleration (up through 3500 RPM):

• Idle jets too small
• Progression air correction jets too large
• Floats set too low
• Wrong emulsion tubes
• Try adjusting mixtures at 1800 to 2000 RPM
• Exhaust header too big
• Valve clearance set too tight (advances timing)
• Ignition over-advanced
• Weak spark

Flat spot in acceleration (near redline):

• Main jets too small
• Main air correction jets too large (Find largest air correction jet that just causes a high RPM misfire and decrease air jet size 10 to 20)
• Floats set too low
• Wrong emulsion tubes
• Exhaust header too big
• Valve clearance set too tight (advances timing)
• Ignition over-advanced
• Weak spark

Poor power throughout driving range:

• Timing not advanced enough
• Main jets too small

Exhaust popping on deceleration:

• Exhaust leak
• Intake air leaks

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Selection criteria for modified throttle bore diameters

Although a 40mm bore throttle body may be used adequately for engine sizes from 2.0 liters through 3.0 liters and larger it is preferable to match the throttle body bore to the main venturi size which is selected based upon two criteria:

• Cylinder displacement
• RPM at which the peak horsepower is generated.

After the main venturi size has been identified the throttle body bore is selected on a proportional scale

The 40mm Weber throttle body may be re-machined to provide larger bore sizes up to 46mm and Performance Oriented has identified two over-bore diameters which provide balanced carburetor systems tailored to the engine size and application. The table below provides recommendations for various applications:

Throttle body bore, mm	Venturi size, mm	Displacement Range
		Street performance (Touring cams)	Track performance (Sport cams)	Race only (Race cams)
40	32	2.0-2.3	2.0-2.2	N/A
40	34	2.3-2.6	2.2-2.4	2.0-2.2
42	36	2.6-2.9	2.4-2.7	2.2-2.4
42	38	2.9-3.2	2.7-3.0	2.4-2.6
46	40	3.2-3.5	3.0-3.3	2.6-2.8
46	42	3.5+	3.3+	2.8-3.0

The 42mm overbore conversion includes the following tasks:

• Boring the throttle bodies to 42mm diameter and fitting new throttle valves
• Tapering the inside of the main venturis to match the new 42mm throttle body bore

The 46mm overbore conversion includes the following tasks:

• Boring the throttle bodies to 46mm diameter and fitting new throttle valves
• Re-machining the throttle shafts for the larger throttle valves
• Modify the throttle body for the four-hole progression circuit
• Tapering the inside of the main venturis to match the new 46mm throttle body bore (requires 46mm specification venturis or billet venturis to allow for machining)
• Supply six new grub screws for retaining the new venturis

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Considerations when buying used Webers

After review of the topics regarding Weber model variations and the types of services offered by Performance Oriented to return worn-out Webers to serviceable units you probably know that buying used Webers advertised to be in good condition may be a bit misleading regarding the knowledge level of the advertiser. Moreover the seller may have found a set of Webers on his uncle's shelf and is copying the words from a previous advertisement to sell the current set. The following is a list of topics to be explored in order to help assess those used carbs you are considering for purchase:

What are the type designations of the carbs and are they matching?

• Are they paired? (3C for the passenger side and 3C1 for the driver's side in a 911)
• Do they have "Made in Italy" on the body? If not from Italy they were made in the USA and they may suffer from fuel leakage which is why they have been sitting on a shelf. Many of the non-Italian carbs are fine but they should be taken from a running car which would indicate they are not "leakers."
• IDA'S, IDAP's and IDS's may be mixed into a good pair but don't mix them with an IDT or an IDTP throttle body.

What condition are they in?

• Better to be from a running car than sold from storage. Sold from storage may mean they have become corroded (due to poor storage conditions) and this will almost render them unusable.
• In the course of preparing for sale the carburetors may have been sand or bead blasted to clean them up which unless performed VERY carefully will destroy zinc finishes and fill air/fuel galleries with blasting media.
• Compressed air will crush the fuel floats (not visible) and render them junk. (See picture below.)
• Throttle shafts that are stated to be "freely rotating with no play" may be a matter of opinion. A few thousandths of radial shaft clearance WILL cause tuning issues.
• Sometimes a "rebuilt" set of carbs will have the nylon shaft bearings replaced but this is a temporary fix and probably the nylon bearings were not replaced on both ends of the throttle body.
• Are there broken features on the throttle body? Items such as: bosses for the jet and metering screws; safety wire tabs for the float fulcrum screws and the housings for the torsion springs at each end of the throttle body are common items to be damaged.
• Are the jet holders seized in the throttle body? How about the metering screws?

What is the jetting?

• Most likely you will want to replace the jets and venturis with those correct for your application but it would be nice to get an inventory. Of course there is the risk of jets having been drilled to a larger size than their marking would indicate. Items to ask about include:
  • Main venturi size
  • Main jet
  • Idle jet
  • Air correction jet
  • Emulsion tubes
  • Accelerator pump jet (long or short style)
• Common issues with the jet holders (mains and idles) and the fuel bowl drain plugs are they have been over-torqued resulting in their becoming twisted or worse yet, cracked; rendering them iffy for use.

Plan to replace the accelerator pump rods (they wear quickly and are probably worn out) and to buy a standard rebuild kit that services both carburetors.

 

Floats crushed with high-pressure air