Weber DCOE Set up 101

Discussion in 'Technical Q&A' started by miketuason, Feb 26, 2020.

  1. miketuason

    miketuason F1 World Champ
    Silver Subscribed Owner

    Feb 24, 2006
    Cerritos, CA.
    Full Name:
    Selection and tuning of Weber DCOE

    A very popular modification for kit car 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

    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

    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
    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.

    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
    F9, F16

    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.

    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

    Below is a chart showing approximate idle jet sizes for given engine
    sizes, this assumes one carb barrel per inlet port E.G. two DCOE

    Engine size Idle jet size

    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.

    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

    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

    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
    16 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
    jmaienza likes this.
  2. MerlinTech

    MerlinTech Formula Junior

    Mar 5, 2010
    Full Name:
    Wade Williams
    Nice write up. Thank you for the information.
    miketuason likes this.
  3. Sledge4.2

    Sledge4.2 F1 Rookie

    Oct 19, 2007
    Full Name:
    What do you mean by “kit car owners” as it relates to fcars?
  4. MerlinTech

    MerlinTech Formula Junior

    Mar 5, 2010
    Full Name:
    Wade Williams
    This Weber is very popular with kit car builds that use the older inline four cylinder Ford engine. I just tuned a Super 7 kit that had these carbs.
    This article, while very helpful in understanding Webers, was not written for Ferraris. That is why the reference is to Pinto engines. Very popular engine in small kit cars. The formulas still are accurate if you are modifying your Ferrari. With the fuel of today, you will need to richen the fuel circuits to get the best performance from them, in my experience anyway. The DCOE Weber on the 308s usually responds very well to minor adjustments to the idle jets and air correctors. The alcohol content makes the fuel lean compared to late 70s fuel that these engines were tuned to.
    JCR and miketuason like this.
  5. miketuason

    miketuason F1 World Champ
    Silver Subscribed Owner

    Feb 24, 2006
    Cerritos, CA.
    Full Name:
    Back in the days DCOE Weber carbs are so famous on Ford Escort with the Cosworth engine.
  6. Tojo

    Tojo Formula Junior

    Apr 12, 2002
    Full Name:
    Just a point of clarification and I’m happy to stand corrected but I’m pretty sure the 308s ran DCNF downdraft Weber’s. DCOE Webers are the horizontal ones on 365s 400s etc

    Sent from my iPad using FerrariChat
    Rifledriver likes this.
  7. MerlinTech

    MerlinTech Formula Junior

    Mar 5, 2010
    Full Name:
    Wade Williams
    You are correct, i clearly typed that wrong.
  8. Tojo

    Tojo Formula Junior

    Apr 12, 2002
    Full Name:
    All good, I've sure made my share of typo's in my day!

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