Here is my starting points
Introduction
A very popular upgrade for a wide range of engines is the fitment of twin Weber DCOE carburetors (Figure). There exists a great deal of mystique and confusion with regard to setting up Weber DCOE carburetors, and in particular the correct starting point for jetting. However, Weber DCOE carburetors are not as complicated as many fear, and while fine-tuning is best performed using a rolling road dynamometer (chassis dyno) an excellent first guess can be obtained based upon the engine size and power band desired. The following provides the calculations that are required to achieve an excellent initial set-up, irrespective of the application.
The Weber 40 DCOE sidedraught carburetor.
Weber is an Italian company producing carburetors, currently owned by Magneti Marelli Powertrain, in turn part of the Fiat Group. The company was established as Fabbrica Italiana Carburatori Weber in 1923 by Edoardo Weber (1889–1945). Weber carburetors were fitted to standard production cars and factory racing applications on automotive marques such as Abarth, Alfa Romeo, Aston Martin, BMW, Caterham, Ferrari, Fiat, Ford, Lamborghini, Lancia, Lotus, Maserati, Porsche, Renault, Triumph and VW. Weber carburetors were produced in Bologna, Italy up until around 1990 when production was transferred to Madrid, Spain, where they continue to be produced today.
The prefix number on the DCOE, e.g., 40 DCOE, is the diameter of the throttle plate (the throttle bore) in mm; DC means doppio corpo (double throat); O means orizzontale (horizontal); E means it is a die cast carburetor; and the number or number and letter suffix is the variation type (e.g., 40 DCOE151). An example of a 40 DCOE is shown in Figure, while a parts diagram is shown in Figure with the parts description given in Table.
A parts diagram for a Weber 45 DCOE carburetors. The number key for selected parts is given in Table.
Selected parts key to Figure.
Part Number in Figure
Filter 3
Jet inspection cover 4
Needle valve 8
Float 9
Emulsion tube holder 10
Air corrector jet 11
Idle jet holder 12
Emulsion tube 13
Main jet 15
Idle jet 16
Auxiliary venturi 17
Air horn 18
Main venturi 22
Air bypass screw 26
Throttle plate 33
Idle mixture screw 56
Pump jet 57
Starter air jet 74
Determination of the correct venturi size
The most common issue with badly tuned Weber DCOE series carburetors is the choice of the correct carburetor. It is commonly (and incorrectly) assumed that 45s will give more power than 40s because of the larger carburetor barrel. However, it is not the barrel size (i.e., 40 or 45) that determines the airflow and therefore potential horsepower, it is the size of the main venturi or choke (22 in Figure and Figure). Selection of the correct main venturi size is the first step prior to selecting the carburetor. The size of the venturi is embossed on the inside lip (see Figure).
A pair of DCOE venturis/chokes.
The purpose of the main venturi is to increase the vacuum acting on the main jet (15 in Figure) in order to draw in and atomize the fuel mixture in the most effective manner. 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 tractability at lower engine speeds (rpm). Race cars will benefit from this latter compromise, but on a road car drivability is much more important.
Figure shows a chart that allows for the correct selection of main venturi size for engines given the engines capacity and the rpm at which it is expected to achieve peak power. The rpm value primarily depends on the choice of cam; however, it is necessary to ensure that the rest of the engine is built to meet the needs of that engine speed. For example, the use of double springs on a pushrod engine or solid (rather than pneumatic) lifters in an overhead cam engine.
Chart showing main venturi sizes for various engine sizes and peak rpm ranges. The red line is for a Formula Vauxhall Lotus, while the blue line is for a Ford crossflow powered Lotus Seven S3.
Calculation of the carburetor barrel size
Once the correct venturi size has been determined from Figure it is a simple matter to determine which carburetor is required. The ideal barrel size that will accommodate the venturi size selected is calculated according to [link]. Table shows a list of the main venturi size available for common DCOE series carburetors.
The main venturi size available for common DCOE series carburetors.
DCOE carburetor Available venturi sizes (mm)
40 24 - 36
42 24 - 34
45 28 - 40
48 40 - 42
48/50SP 42 - 46
55SP 46 - 48
Example 1: Using Figure a 2000 cc Vauxhall/Opel engine giving its maximum power at 7000 rpm will require a venturi size of 38 mm, and therefore an ideal barrel size of 47.5 mm (i.e., 38 x 1.25). For this application 45 DCOE is the solution, since 38 mm chokes are not available for 40s or even larger carburetors (see Table).
What venturi size will a 1600 cc Ford crossflow engine require if its maximum power is delivered at 6500 rpm?
Main jet and air corrector size selection
Once the choice of venturi is made, the appropriate sizes of the main jet and air corrector can be made. The main jet (Figure) and air corrector (Figure) are positioned either end of the emulsion tube (Figure), which is located beneath the jet inspection cover (4 in Figure). Both main jets and air correctors are sized in increments of 5, and the sizes are embossed on the outside of both (e.g., Figure).
A pair of main jets.
A pair of air correctors.
Diagram of the main jet assembly for Weber DCOE carburetors.
The main jet has an effect over the whole rev range, whereas changing the air correction jet has more effect at higher revs. Increasing the size of the main jet will enrich the fuel mixture and visa versa. In contrast, increasing the size of the air correction jet will lean out the mixture. A summary of the results of changes in the main and air correction jets is given in Figure.
The relationship between jet size and fuel mixture.
The formula for the calculation of main jet size when the main venturi size is known is Equation. This will give a 'safe' starting point for the main jet size. The air corrector jet initial settings should be about 50 higher than the main jet, Equation.
Using the results from Example for the 2000 cc Vauxhall/Opel engine, a venturi size of 38 mm will calculate a main jet size of 152. Since main jets are sized in increments of 5, so a main jet of 150 would be suitable, while the appropriate air corrector would be 200. However, a main jet of 155 and air corrector of 205 could also be tried.
What main jet and air corrector sizes will be needed for Ford 1600 cc crossflow engine with a venturi size of 30 mm? What if the venturi was increased to 32 mm?
Emulsion tube selection
The emulsion tube (Figure and Figure) holds the main jet and the air corrector, and is located (13 in Figure) beneath the jet inspection cover (4 in Figure). The size of the emulsion tube is defined by the cylinder capacity. Table shows suggested emulsion tube types for a given single cylinder capacity.
An emulsion tube for a DCOE carburetor. The main jet fits into the bottom while the air corrector fits in the top.
Suggested emulsion tube type for a given single cylinder capacity.
Cylinder capacity (cc) Suggested emulsion tube
250 – 325 F11
275 – 400 F15
350 – 475 F9, F16
450 – 575 F2
For a 2000 cc Vauxhall/Opel engine each cylinder capacity is 500 cc and a F2 emulsion tube would be appropriate. However, a 2000 cc engine 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.
What emulsion tube would be used for a 1600 cc Ford crossflow engine?
Idle Jet selection
Idle jets (Figure and Figure) cause a lot of confusion; although their name suggests that they govern the idle mixture, this is not true. The idle mixture is actually metered by the idle volume screws (56 in Figure) mounted on top of each barrel. The function of the idle jet is to control the progression between closed throttle and the main jet circuit. As such it is 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.
An example of an idle jet for a DCOE carburetor.
Diagram of idle jet assembly for a Weber DCOE carburetor.
Idle jets have two numbers; the first is the size of the fuel orifice (Figure), while the second ‘f’ number, is the air bleed (also known as the air drilling, see Figure). As with the emulsion tube, the idle jet is chosen based upon the cylinder volume. [link] shows the approximate idle jet sizes for given engine sizes; this assumes one carburetor barrel per inlet port, i.e., two DCOEs per 4 cylinder engine.
The idle jet sizes appropriate for a given engine size.
Engine size (cc) Idle jet size
1600 40/45
1800 45/50
2000 50/55
2100 55/60
For each size of idle jet there are a range of air bleed alternatives available. The ones in normal use are F2, F8, F9 and F6. Generally speaking start your selection with an F9 air bleed. A full list of the various ‘f’ numbers as it relates the rich to lean running is shown in Figure.
The most commonly used air size designations, running from weak to rich. Those in most normal use are shown in bold.
Setting the idle and slow running
Rough running at idle is normally due to the idle mixture and balance settings between multiple carburetors being incorrect. Before adjusting the carburetors it is important to make sure that the following have been checked:
The engine is at normal operating temperature.
The throttle return spring/mechanism is working properly.
The engine has sufficient advance at the idle speed (between 12 and 16°). As a starting point the idle speed for a modified engine on Webers is between 900 and 1100 rpm.
An accurate rev counter is used.
There are no air leaks or electrical faults.
The following represents a step-wise approach to the correct setting of the idle. Reference to Figure and Figure for the position of the appropriate screw positions.
Diagram of Weber DCO type carburetor.
If the carburetors are being fitted for the first time, screw all of the idle mixture adjustment screws (Figure and 56 in Figure) fully in and then out 2.5 turns.
Start the engine and let it reach normal operating temperature. This may mean adjusting the idle speed as the engine warms up. Set the idle as near as you can to 900 rpm.
Spitting back through the back of the carburetor 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.
Using an airflow meter or carburetor synchronizer (Figure) adjust the balance mechanism between the carburetors such that the flow of air is the same for each carburetor. If the rearmost carburetor (i.e., cylinders 3 and 4) is drawing less air than the front (i.e., cylinders 1 and 2), 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, adjust it back to 900 rpm, to decrease idle speed screw in an anti-clockwise direction, to increase, screw in a clockwise direction.
Once the carburetors have the same airflow, turn the idle mixture screw (Figure and 56 in Figure) for the number 1 cylinder anti-clockwise (which will make it richer) in small increments (a quarter of a turn is sufficient). Allow 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 during this process the engine speed goes well over 1000 rpm, then trim it down using the idle speed screw, and re-adjust the idle mixture screw. If on the first turn, the engine speed decreases then turn the mixture screw clockwise (which will make it weaker) 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.
Repeat this process for the idle mixture screws for each cylinder on each carburetor.
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/carburetor synchronizer.
A typical carburetor synchronizer tool/air flow meter.
Bibliography
P. Braden, Weber Carburetors, Penguin Putnam (1988).
D. Hammill, How to Build and Power Tune Weber and Dellorto DCOE and DHLA Carburettors, Veloce Publishing (2006).
A. K. Legg, Weber Carburettor Manual, Haynes Manuals (1996).
J. Passini, Weber Carburettors Tuning Tips and Techniques, Brooklands Books (2008).
Resources
Carbs Unlimited, Inc., 727 22nd St NE, Auburn WA 98002,
www.carburetion.com.
Pegasus Auto Racing Supplies, Inc., 2475 S 179th Street, New Berlin WI 53146,
www.pegasusautoracing.com.
Webcon UK Ltd., Dolphin Road, Sunbury, Middlesex TW16 7HE,
www.webcon.co.uk.
