Firstly, my car failed on me, it's a 260000 mile Audi A6, blown turbo pipe which has caused the turbo to fail..
Hopefully I'll sort something out in December, maybe persuade the troops to buy me the bits for that exhaust I've built in my head.
weber tuning guide
Postby gen mk 1rs2000 » Fri Oct 28, 2011 9:02 pm
i had this sent to me
some usefull reading ??
Subject: Weber carburettor tuning and formula required correct jetting for Ford Escort 2.0 Pinto engines etc.
Selection and tuning of Weber DCOE carburettors
By D V Andrews – UK
A very popular modification for RH7 owners is the fitment of twin Weber DCOE or DCO/SP
carburettors; these not only deliver the goods but also look very good. A good deal of mystique
surrounds Webers, specifically Weber jetting and tuning. However Weber DCO series carbs are
not as complicated as you might imagine, and whereas there is no substitute for a good rolling
road session to tune them, there is much you can do to tune them yourself, by selecting the
correct choke sizes and initial jet settings according to a fairly simple set of rules. This should get
the engine running to a reasonable standard in preparation for the rolling road.
Arriving at the correct carb/venturi size
When selecting Webers, the most commonly asked question is "Should I have 40s or 45s"
coupled with "Surely the 45s will give more power". This shows a basic misunderstanding of the
construction and principles of operation of the DCO series. It is not the barrel size (40 or 45) which
determines the airflow and therefore potential horsepower; it is the size of the main venturi or
choke. Selection of the correct main venturi size is the first step in selecting the carburettor.
It is easy to make the assumption that biggest is best when selecting a main venturi size, but the
purpose of the main venturi is to increase the vacuum acting on the main jet in order to draw in
and effectively atomise the fuel mixture. The smaller the main venturi, the more effective this
action is, but a smaller venturi will inhibit flow. A large venturi may give more power right at the top
end of the power band, but will give this at the expense of lower RPM tractability. Only a circuit
racer will benefit from this sort of compromise, on a road car, driveability is much more important.
95 percent of the time, a road engine is nowhere near its peak power, but is near its peak torque
for 75 percent of the time. It is much more important therefore to select the main venturi for best
driveability, once the venturi size has been selected, then the appropriate carburettor size can be
arrived at.
Here is a small chart showing the available Main Venturi size for Common DCO series carbs
Size Available Venturi sizes
40 24-36mm
42 24-34mm
45 28-40mm
48 40-42mm
48/50SP 42-46mm
55SP 46-48mm
Below is a chart that will allow the correct selection of main venturi size for engines given the
engines capacity and the RPM at which peak power is realistically expected to be achieved, for
road engines peak power is usually between 5250 and 6500, depending on the cam selection.
After the correct venturi size has been arrived at it is a simple matter to determine whether 40/45
or 48 DCOs are required, take the venturi size and multiply by 1.25, the result is then the ideal
barrel size which will accommodate the venturi size selected.
Chart Showing Main Venturi Sizes for Various Engine sizes and RPM ranges
(For four cylinder cars)
Carburettor Barrel size calculation
Venturi/choke size * 1.25
For example: a two litre engine giving its maximum power at 6000RPM will require a venturi size
of 36mm, and therefore an ideal barrel size of 45mm (36 * 1.25). For this application 45 DCOE is
the ideal solution, however a 40 DCOE will accommodate a 36mm choke, so if funds are limited
and the engine is not going to be tuned further then 40 DCOEs will do the job.
If you have bought your Webers second-hand, it is important to understand that it is unlikely that
they will be 'ready jetted'. However if you do not want the expense of changing the main venturis,
you will still need to know their size, this is normally embossed on the venturi itself, so look
carefully down through the main barrel of the carb from the air cleaner side.
Diagram of Main Jet assembly
Main Jet and Air Corrector Size Selection
A useful formula for the calculation of main jet size when the main venturi size is known is to
multiply the main venturi size by 4. This will give a starting point for the main jet size which should
be 'safe', again as a starting point the emulsion tubes can be selected from the table shown below,
although for Pinto F9 or F16 will generally be OK. If your carbs are already equipped with these,
then that will save you some money. Air corrector jet initial settings should be around 50 higher
than the main jet.
Main jet size Venturi size * 4
Air corrector Main jet size + 50
Using these formulae, a venturi size of 36mm will require a main jet of 145 and an air corrector of
around 190.
Emulsion tube Selection
Below is a table showing suggested emulsion tube type, for a given single cylinder capacity.
Cylinder capacity Suggested tube
250-325 F11
275-400 F15
350-475 F9, F16
450-575 F2
Using the above formulae, the ideal settings for a 2000cc Pinto with power peaking at 6000RPM
(290 degree cam or above) are as follows
36mm chokes
F16 or F2 Emulsion tubes
145 Main jet
190 Air corrector
The 2000cc Pinto in just on the cusp of change for emulsion tube type between F16 and F2, if you
already have F16 tubes, use them it is not worth the expense of change, they will just cause the
main circuit to start marginally earlier. A 2.1 or 2.2 Pinto should however be using F2s although
F16s will do the job acceptably well.
Diagram of Idle Jet Assembly
Idle Jet selection
Idle jets cause a lot of confusion; although their name suggests that they govern the idle mixture,
this is incorrect. It is true that the fuel consumed at idle is drawn through the idle jet, but the idle
mixture is metered not by these jets, but by the idle volume screws mounted on top of each barrel.
The idle jets control the critical off-idle progression between closed throttle and the main jet circuit,
it is this part throttle operation which is so important to smooth progression between closed throttle
and acceleration and for part throttle driving. If this circuit is too weak then the engine will stutter or
nosedive when opening the throttle, too rich and the engine will hunt and surge especially when
hot. The technique for establishing the correct idle jet size is detailed later, but as a starting point
40/45f9 idle jets for a 1600 engine 45/50 f9 for an 1800 and 50/55f9 for a 2000 will get you out of
jail free.
Below is a chart showing approximate idle jet sizes for given engine sizes, this assumes one carb
barrel per inlet port E.G. two DCOEs.
Engine size Idle jet size
1600cc 40/45
1800cc 45/50
2000cc 50/55
2100cc 55/60
Establishing the correct idle jet for a given engine is not easy but usually an approximation will
make the car acceptably driveable. If the progression is weak then the engine will nosedive when
moving the accelerator from smaller to larger throttle openings. A certain amount of change
(richer/weaker) to progression can be achieved by varying the air jet size on the idle jet; this alters
the amount of air that is emulsified with the fuel drawn through the idle jet. If this does not richen
the progression sufficiently then the next jet size up, with the same air bleed should be tried.
Below is a small chart showing the most commonly used air size designations, running from weak
to rich. Generally speaking start your selection with an F9 air bleed.
Weaker Normal Rich
F3 , F1 , F7 , F5 ,F2-F4 ,F13 ,F8-F11-F14,F9 , F12 , F6
The ones in normal use are F2,F8,F9 and F6.
Diagram of DCO type carburettor
Setting the Idle and slow running
Rough running and idle is normally down to the idle mixture and balance settings being incorrect,
below is a technique to establish a clean idle and progression. Before adjusting the carbs in this
manner you must make sure that the following conditions are met.
i) The engine is at normal operating temperature
ii) That the throttle return spring/mechanism is working OK
iii) That the engine has sufficient advance at the idle speed (between 12 and 16 degrees)
iv) That an accurate rev counter is connected.
v) That there are no air leaks or electrical faults.
A reasonable idle speed for a modified engine on Webers is between 900 and 1100 RPM.
If you are adjusting the idle for a set of carbs already fitted then progress to the second stage, if
the carbs are being fitted for the first time, screw all of the idle mixture adjustment screws fully
home and then out 2.5 turns. If you are using DCO/SP carbs then start at one turn out. Start the
engine and let it reach normal operating temperature. This may mean adjusting the idle speed as
the engine warms up. Spitting back through the back of the carburettor normally indicates that the
mixture is too weak, or the timing is hopelessly retarded. If this happens when the engine is warm
and you know that the timing is OK, then the mixture will need trimming richer on that cylinder. Set
the idle as near as you can to 900RPM.
Using an airflow meter or carb synchroniser adjust the balance mechanism between the carbs to
balance the airflow between them, if the rearmost carb is drawing less air than the front, turn the
balance screw in a clockwise direction to correct this. If it is drawing more air, then turn the
balance screw anti-clockwise. If the Idle speed varies at this point, adjust it back to 900 RPM, to
decrease idle speed screw in an anti-clockwise direction, to increase, screw in a clockwise
direction.
When you are sure that the carbs are drawing the same volume of air, visit each idle mixture
screw, turn the screw counter clockwise (richening) in small increments (quarter of a turn),
allowing a good 5 - 10 seconds for the engine to settle after each adjustment. Note whether
engine speed increases or decreases, if it increases continue turning in that direction and
checking for engine speed, then the moment that engine speed starts to fall, back off a quarter of
a turn. If the engine speed goes well over 1000RPM, then trim it down using the idle speed screw,
and re-adjust the idle mixture screw. If engine speed decreases then turn the mixture screw
clockwise (weakening) in small increments, again if engine speed continues to rise, continue in
that direction, then the moment it starts to fall, back off a quarter a turn. The mixture is correct
when a quarter of a turn in either direction causes the engine speed to fall. If that barrel is spitting
back then the mixture is too weak, so start turning in an anti-clockwise direction to richen. During
this procedure, the idle speed may become unacceptably high, so re-adjust it and repeat the
procedure for each carb barrel.
After all the mixture screws have been set, the idle should be fairly even with no discernible
'rocking' of the engine, if the engine is pulsing, spitting or hunting then the mixture screws will need
further adjustment. If the engine is rocking or shaking then the balance is out, so revisit with the
airflow meter/ carb synchroniser. No amount of adjustment will give a good idle if the throttle
spindles are bent or leaking air or the linkages are loose on the spindles!
That’s all there is to it.
Starting technique for Weber equipped engines (engine cold)
Some Webers have a cold start circuit (choke), others don't, in my experience, it is very easy to
flood the engine and wet plugs using the cold start mechanism, as it very crude in operation. The
accepted technique for cold starting is as follows:-
Allow the float chambers to fill if you have an electric pump, this should take about 5-10 seconds,
fully depress the accelerator rapidly four times, then on a light throttle, turn the engine over, if it
does not start immediately, repeat the procedure three times. The engine should fire, but may
need 'nursing' for a minute or two before it will idle, gentle prodding of the accelerator should keep
it alive long enough for it to warm up. If the engine does not fire within three attempts, then try five
or six pumps. If this does not work, depress the accelerator fully and hold it open while turning the
engine over for 5 to 15 seconds, then close the accelerator and try again.
Buying second-hand
When buying Webers second-hand ensure that they are a matched pair. Look carefully at the
serial numbers on the top of the carbs, these should be the same, or very similar. If they are not
then they are not a matched pair and may well give problems when trying to jet them, as the
progression drillings could be different. Inspect the carbs very carefully before parting with your
cash, check their general condition, check for fire/heat damage, check that the butterflies open
and close smoothly and that the linkages are smooth in operation and the carbs don't stick open. A
common problem with Webers is the attachment of the throttle quadrant to the spindle, these can
wear and will give an erratic idle and progression which no amount of tuning will cure. It is
important to note that Webers are very rarely 'ready jetted' so factor the cost of jets etc. when
deciding on your purchase. Check the throttle spindles for wear, excessive wear here will bleed air
into the engine and again will affect setting up dramatically. Servicing kits for Webers are relatively
cheap so a neglected pair, provided that the above checks are carried out, can be restored to very
good condition by a thorough clean and service, the servicing is not difficult but has to be done in
a clean environment, using a methodical approach.
Example Jetting from real applications
Jetting for standard 2000/1800/1600 Pinto on 40s
34mm Chokes
135 main jets
F11 emulsions
190 air correctors
35 pump jets
40/f9 idle jets,45/f9 for 1800/2000
4.5 aux vents
Jetting for modified 1600 Pinto on 40s
34mm chokes
140 main jets
F16 emulsions
190 air correctors
40 pump jets
40/f9 idle jets
4.5 aux vents
Jetting for modified 1800 Pinto on 45s
36mm chokes
140 main jets
F16 emulsions
170 air correctors
40 pump jets
45 f11 idle jets
4.5 aux vents
Jetting for modified 2000/2100 Pinto on 45s
38mm chokes
145 main jets
F16 emulsions
180 air correctors
40 pump jets
50f9 idle jets
4.5 aux vents
Dave Andrews
Triple-Weber DCOE for Triumph TR6/TR250
Webers are often maligned. This is more often due to poor installation by inexperienced and impatient mechanics. Webers are not rocket science, nor do they require that one be an experienced mechanic. What is required is a good deal of common sense, a methodical and patient approach, and some understanding of the air-fuel system and its combustion.
For every owner I have spoken with who have expressed their nightmare with Webers, there are 2-3 owners who have glowing remarks. The only real complaint that can be leveled at Webers is their consumption of fuel (which may be symptomatic of incorrect jetting/valving). Expect 15-18mpg in a TR6 that is driven at a more-than pedestrian pace.
Introduction
Read TerriAnn Wakeman's page on DCOEs for TR2/3.
Terri's article is highly recommended.
Task: To provide an electronic archive of accumulated knowledge of fitting a set of Weber 40 DCOE Webers to Triumph TR250/TR6. Sources include books, email, interviews and web-sites.
Note: All DCOE carbs are not equal. They will often differ in the length of the accelerator pump stroke and pattern of progression holes. The designator is stamped on the casting. Weber recommend the 40 DCOE-18 for Triumph 3/4/250/TR6/GT6, whereas Haynes Techbook suggests that the 40 DCOE-2 is best for TR250/6/GT6 and 42 DCOE-8 for TR2-4. I attempt to explain the reason for this discrepancy in the article below. This little fact is almost completely ignored by most so-called Weber experts. Get this wrong, and you may always have a flat-spot.
Note: The TR6 heads for 1969-73 differ from 1974-75 in intake port size and location. Before you purchase a set of Webers, make sure the intake manifold will match your heads.
40 DCOE jettings and settings for TR250/6
D = choke size
E = auxiliary venturi
F = main jet, controls fuel mixture in the emulsion tube as cruise circuit is activated
G = emulsion tube
H = air corrector jet, affects only performance at high RPM
I = idle jet, affects idle and progression circuit
J = accelerator pump jet
K = accelerator pump inlet valve with exhaust orifice (also called a discharge bleed).
L = needle valve, affects fuel flow into the float bowl
Reference D E F G H I J K L-----------------------------------------------------------Weber (0) 30 4.5 120 F11 160 50/F11 40 50 ? Haynes (0) 27 4.5 130 F2 160 50/F11 45 0 2.00Fitzgerald (0) 27 4.5 120 F11 160 49/F9 60 ? 1.75TriumphTune (0) 28 4.5 115 F16 175 50/F9 35 ? -TriumphTune (0) 30 4.5 125 F16 180 50/F9 35 ? -Phillips 30 4.5 155 F11 200 45/F9 50 ? -C. Kantarjiev 28 4.5 110 F2 160 45/F9 - ? -A. Nugent (1) 28 4.5 120 F16 175 50/F9 35 ? -A. Nugent (2) 30 4.5 120 F16 180 50/F9 35 ? -A. Nugent (3) 33 4.5 125 F16 185 50/F9 40 ? -L. Bickel (4) 28 - 120 F11 160 -/F9 - ? -B. Mains (5) 30 - 120 F11 160 60/F9 50 ? - R. Lang (6) 28 4.5 125 F16 160 55/F12 45 ? -R. Lang (7) 28 4.5 120 F2 165 55/F12 45 ? -S. Ingate (8) 30 - 120 F11 - 50/F11 40 ? -40 DCOE-151 (9) 30 4.5 115 F11 200 45/F9 40 100 1.75S. Ingate (10) 45 C. Arnold (11) 140 F11 160 55F9 (0) See reference list at end of page.
(1,2,3) (Allen Nugent at unsw dot edu dot au, Aug 98)
(4) "... and most important- choke tubes (venturis)- 28mm Disconnect the choke cable- it isn't needed. Make sure that the float level is correct. Also I run 26 degrees total advance on the distributor (10 + 16)." (Larry Bickel, ljbtvr at aol dot com, Apr, 97).
(5) "The cam specs: Intake-Open/Close-36' BTDC/ 69' ABDC Exhaust-72' BBDC/33' ATDC Lift 405 Dur 285..... The multiple spark feature of the MSD 6AL Ignition module made a BIG difference for the fuel richness/plug fouling, .055 spark plug gap for NGK BPR6ES, using standard points/gap and a Lucas Sport coil. Timing is about 28' advanced.......Roller rockers are 1.50:1. Stock exhaust headers and freeflow, dual mufflers. Compression-9.5:1. We tried the 45F9 and 60F8 idle jets but didn't work. Also tried 180 air, F16 emulsion and 125 main jets. Initially had 40 accel pump jets. Again no luck." (Bob Mains, President, Buckeye Triumphs, bob dot mains at ode dot state dot oh dot us, Sept 00) .
(6) Race car, 11:1 compression, Piper 270 symmetrical cam, Monza header, mild porting, stock rockers, steel valve guides. Mallory dual-point dizzy, no spark box. Guesstimate is 150 HP. (Bob Lang, lang at isis dot mit dot edu, Oct 02)
(7) Street car. Had a lot of trouble in the progression circuit, rich at idle. Probably needed better spark control. F2 emulsion tubes better for street driving, and F11 for running Street Prepared (with 125/165 main/air corr.). Guesstimate is 130 HP. (op. cit)
(8) Mild street car. DCOE-18 on car for 17 years, one owner, who was pleased with the way they ran. No other details available (author, Apr 98).
(9) The current model 40 DCOE is the 151 series (made in Spain), and I give its calibration as a means of reference. The 151 series seems to be appearing in "TR6 kits" offered by the big parts vendors and found on the web. Additional calibration material are: progression holes=120/100/100, throttle-plate angle=78 degrees. I'm not sure how well this unit would work "out of the box"; probably not!
(10) Goodparts GP3 cam, 9.5:1 compression, Crane ignition, Grp 44 headers. Stumbled at 1800 rpm, ran rich across the spectrum (author, Jan 03)
(11) Runs a D9 cam, heavily ported head and tuned on a dyno. With a 270 degree cam, ran 120 mains, 160 air (Chuck Arnold, chuck dot arnold at oracle dot com, May 03)
Tuning Techniques
Clearly space precludes and exhaustive treatise. But to summarize, the only thing that a tuner is testing is the mixture strength. One can do this by the "plug cut" method (checking color of the spark plug electrode), or analysing exhaust-gas. Use of an O2 sensor plugged into the exhaust system will certainly help the tuner (K&N make a good system), the best technique has always been the plug cut, conducted either on a dyno, or correctly on the road.
Plug-cut technique
Generally quite simple once a suitable time and place are chosen. Ensure that the correct plugs are being used, and are clean (i.e. new, or near new), with a white insulator.
Hold the RPM at the necessary level in as high a gear as possible for 10 or so seconds
Then, simultaneously, foot off the gas, disengage clutch, ignition off.
Inspect the plug(s), usually one plug for each of the carburettors.
If the plugs are sooty black, then the mixture is too rich. If the color is that of milk chocolate, it is a little to lean. If the color of dark chocolate, then a little too rich. A good reading will have the insulator showing almost white, with just a suspicion of grey, and a plug body on the dark side, just enough to mark a piece of white paper with soot. Further interpretation of plug color can be found elsewhere on the web, but the color to avoid is an even battleship grey on the whole plug, electrode, insulator and body - a sure sign of detonation or pre-ignition (ignition too advanced).
Low- and Mid-RPM Hesitation
This may be the domain of the accelerator pump. As noted above, the 40 DCOE comes in different series, each with a variety of accelerator pump stroke lengths. The series 2, 4, 24, 27, 28, 32, 33 have 14mm stroke lengths, the 18, 22/23, 29/30 have 10mm stroke lengths, the 31, 34/35, 44/45, 76/77 have 16mm stroke lengths, and 72/73, 80/81 have 18mm stroke lengths. The pump stroke rod (which governs stroke length) is interchangeable between series, but must be chosen carefully when combining with the various combinations of accelerator pump jet and pump bleed valve. Most owners have suggested using a blank bleed, which is consistent with the Hatnes recommendations, but this is inconsistent with the Weber recommendation.
The importance of the accelerator pump occurs during rapid transitions to full throttle. When the throttle is opened quickly, the sudden rush of air cuases an increase in pressure in the inlet tract which causes atomized gas particles to condense and fall like rain to the floor of the duct (which will remain there unless the manifold is heated; this lake of fuel is often the reason for Weber fires). All this results in a massive flat spot, and the purpose of the accelerator pump is to fill this hiatus until the main jet can ctach up with the demand. The perceptive reader can now grasp that the long-stroke pumps discharge more gas for a longer duration than the short-stroke pumps. On the other hand, the short-stroke pumps deliver a shorter, more intense "squirt". If the accelerator pump jet is made larger, the duration of delivery will be less. If the spring that controls the r
We are thus we are playing a game of balancing three variables; pump stroke, jest size and bleed size (4 variables if you wish to include pump spring rate), all of which regulate the volume and duration of gas delivery between throttle application and when the main jet can catch up. And of course, we would like the accelerator pump system to discharge no gas when the throttle is kept constant or opened slowly.
Ultimately, the solution to these variables will depend on engine modifications, style of driving, and rpm. In general, the short-stroke pumps are well-suited to engines having siamesed inlet ports or one carb feeding multiple cylinders. This could then explain the Haynes recommendation of using the 40 DCOE-2 series with its 14mm long stroke. However it seems that to compensate for the long discharge duration, Haynes recommend that the bleed be closed.
Blanking off the pump discharge bleed ensures a rapid response, but this also means that the total gas quantity pumped by the piston differs little whether the accelerator pedal is moved slowly or rapidly, meaning that the mixture will be overly rich on gentle acceleration. Thus, among other things, a closed bleed will greatly increase gas consumption. IMHO, the dedicated owner could well improve any flat spots by experimenting with open discharge bleeds.
This may well be Weber's thinking when they suggest the 10mm stroke pumps with 50 bleed. Haynes were probably thinking of the Federalized TR6 motor with its 104hp. I suspect that Weber's recommendation is for Euro-spec motors, likely the original 150hp PI, or modified TR250/6.
Progression Ciruit
It seems that many Weber owners blame mid-range fade on the progression circuit. The different DCOE series do place the progression holes in in different places, but I have not confirmed if they differ between the DCOE-2 and DCOE-18 series. I need to do more research on the subject, but I can definitely state that the sizes of venturis, nozzles and emulsion tubes are not relevant to progression.
Air Horns (or Trumpets or Velocity Stacks)
This is an area often ignored by owners. Weber advises that the distance from the mouth of the carburator to the nearest obstruction must be at least equal to the bore diameter of the carburetor intake. This means the bore of the air horn , not the diameter of the entry flare. Many triple-Weber kits are supplied with 35mm high horns which may look impressive, but most TR6 owners remove them because they are too long for the 1.75" air cleaners that must be used in order to clear the inner fender arch. This is a BIG mistake. Never run your DCOEs without air horns that insert into the bore. Do not use air horns that simply bolt-on and are readily available and cheaper. An important function of the air horn is to hold the auxiliary venturi in place. Without the air horn, the aux. venturi will not fall out, but it will rattle around in the breeze. Low profile (12 or 13mm) air horns are available which will allow you to use air cleaners. This is probably one reason why TR6 owners become frustrated with their DCOEs.
However, there is a down side to running short air horns, relating to smoothness of torque delivery and the amplitude of the pressure-wave or "standoff" which appears as a spray of fuel mist during over-run. Remember, the one choke per cylinder of the DCOE acts like an organ pipe, so the length of the tube from the center of the intake valve to the entry flare of the air horn. Passini suggests that this length be about 4 times as long as the piston stroke. I've not made the mesurements, but I'm sure that S&H, Cannon and TWM who cast the intake manifolds were aware of this and chose the 35mm air horns as providing the optimum length for the TR6. I'll confirm this in due course.
So, if you have the space, use the 35mm high horns with 3.5" K&Ns. If not, run with the 12mm horns and the 1.75" K&Ns.
Air filter socks: If you are running horns and wish to run an air sock, ITG make socks for TR6s running DCOEs on TWM manifolds. Use a ITG JC100 series filter with a 36-JC-100 backplate (presumably for 36 DCOE). This snippet from TerriAnne Wakeman gleaned from an old TWM catalog.
Fuel pressure
Webers are very sensitive to variations in fuel pressure and quality. Install the biggest and best fuel filter you can buy (the FRAM HP-1 is recommended). If your Webers seem to lose their edge with time, it is probably because of dirt in the jets. Remove each jet and blow it clean. Only 10 minutes work, but you will be rewarded with pristine performance again.
The standard Triumph fuel pump is totally inadequate. Webers need lots of fuel flow but can't stand pressures over about 5psi (3-4psi is a good number). The shiny-toy pressure regulators sold in discount stores cannot flow enough fuel to keep three big float bowls full at high engine speed, unless you use one per carb. An unregulated stock fuel pump will put out up to 8psi at hi-rpm which will sink the floats, causing other problems.
Cams
Discussions with people who have successfully implemented multi-Weber installations, it seems that the key issue to resolving the Weber's lack of popularity amongst amateur tuners is the cam. Matching the cam to the Webers appears to be of prime importance, because then nominal jetting can be used.
Also see my article on performance cams.
What constitutes a good cam? It appears that Webers prefer cams with long duration. This makes sense because there is little vacume provided in the DCOE/IDA design, and the long duration is needed as a compromise.
References (in no particular order)
"Weber DCOE Series Manual", Weber Part # 95.0022.35
"Weber Carburator Manual", Haynes Techbook, 1995
"Performance Manual", TriumphTune/Moss Europe Ltd, 1992
John Passini, "Weber Carburettors, Theory, Tuning and Maintenance", MRP Speedsport, 1992
Pat Braden, "Weber Carburetors", HP Books, 1988
TerriAnn Wakeman, DCOEs for TR2/3
Paul Tegler, DCOEs for Spitfires
John P. Fitzgerald, "Triumph TR6 Performance: Parts, Sources & Procedures for the Enthusiast", 1996