Selection and Tuning of Weber Dcoe Carburetors 2

OpenStax-CNX module: m37431 1

Selection and Tuning of Weber

DCOE Carburetors

Andrew R. Barron

This work is produced by OpenStax-CNX and licensed under the

Creative Commons Attribution License 3.0

1 Introduction

A very popular upgrade for a wide range of engines is the tment of twin Weber DCOE carburetors (Figure 1).

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 ne-tuning is best performed using a rolling road dynamometer (chassis

dyno) an excellent rst 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.

Figure 1: The Weber 40 DCOE sidedraught carburetor.

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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 (18891945). Weber carburetors were tted 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 prex 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 sux is the variation type (e.g., 40 DCOE151).

An example of a 40 DCOE is shown in Figure 1, while a parts diagram is shown in Figure 2 with the parts

description given in Table 1.

Figure 2: A parts diagram for a Weber 45 DCOE carburetors. The number key for selected parts is

given in Table 1.



Part Number in Figure 2

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

Table 1: Selected parts key to Figure 2.

2 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 airow

and therefore potential horsepower, it is the size of the main venturi or choke (22 in Figure 2 and Figure 3).

Selection of the correct main venturi size is the rst step prior to selecting the carburetor. The size of the

venturi is embossed on the inside lip (see Figure 3).



Figure 3: 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 2) in

order to draw in and atomize the fuel mixture in the most eective manner. The smaller the main venturi,

the more eective this action is, but a smaller venturi will inhibit ow. 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 benet from this latter compromise, but on a road car drivability is much more

important.

Figure 4 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.



Figure 4: 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 crossow powered Lotus Seven S3.

3 Calculation of the carburetor barrel size

Once the correct venturi size has been determined from Figure 4 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 (1). Table 2 shows a list of the main venturi size available for common DCOE series carburetors.

(1)

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

Table 2: The main venturi size available for common DCOE series carburetors.



Example 1

Example 1: Using Figure 4 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 2).

Exercise 1 (Solution on p. 14.)

What venturi size will a 1600 cc Ford crossow engine require if its maximum power is delivered

at 6500 rpm?

4 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 5) and air corrector (Figure 6) are positioned either end of the emulsion tube (Figure 7),

which is located beneath the jet inspection cover (4 in Figure 2). 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 5).

Figure 5: A pair of main jets.



Figure 6: A pair of air correctors.

Figure 7: Diagram of the main jet assembly for Weber DCOE carburetors.

The main jet has an eect over the whole rev range, whereas changing the air correction jet has more

eect 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 8.



Figure 8: 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 (2). 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, (3).

(2)

(3)

Example 2

Using the results from Example 1 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 crossow engine with a

venturi size of 30 mm? What if the venturi was increased to 32 mm?

5 Emulsion tube selection

The emulsion tube (Figure 7 and Figure 9) holds the main jet and the air corrector, and is located (13 in

Figure 2) beneath the jet inspection cover (4 in Figure 2). The size of the emulsion tube is dened by the

cylinder capacity. Table 3 shows suggested emulsion tube types for a given single cylinder capacity.



Figure 9: An emulsion tube for a DCOE carburetor. The main jet ts into the bottom while the air

corrector ts in the top.

Cylinder capacity (cc) Suggested emulsion tube

250 325 F11

275 400 F15

350 475 F9, F16

450 575 F2

Table 3: Suggested emulsion tube type for a given single cylinder capacity.

Example 3

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 crossow engine?

6 Idle Jet selection

Idle jets (Figure 10 and Figure 11) 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 2) 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.



Figure 10: An example of an idle jet for a DCOE carburetor.

Figure 11: Diagram of idle jet assembly for a Weber DCOE carburetor.

Idle jets have two numbers; the rst is the size of the fuel orice (Figure 11), while the second `f' number,

is the air bleed (also known as the air drilling, see Figure 11). As with the emulsion tube, the idle jet is

chosen based upon the cylinder volume. Table 4 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.

Engine size (cc) Idle jet size

1600 40/45

1800 45/50

2000 50/55

2100 55/60

Table 4: The idle jet sizes appropriate for a given engine size.



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 12.

Figure 12: The most commonly used air size designations, running from weak to rich. Those in most

normal use are shown in bold.

7 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 sucient advance at the idle speed (between 12 and 16 ). As a starting point the idlespeed for a modied 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 2 and

Figure 13 for the position of the appropriate screw positions.



Figure 13: Diagram of Weber DCO type carburetor.

Step 1. If the carburetors are being tted for the rst time, screw all of the idle mixture adjustment screws

(Figure 13 and 56 in Figure 2) fully in and then out 2.5 turns.

Step 2. 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.

Step 3. 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.

Step 4. Using an airow meter or carburetor synchronizer (Figure 14) adjust the balance mechanism between

the carburetors such that the ow 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.

Step 5. Once the carburetors have the same airow, turn the idle mixture screw (Figure 13 and 56 in Figure 2)

for the number 1 cylinder anti-clockwise (which will make it richer) in small increments (a quarter of

a turn is sucient). 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 o 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 rst 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 o 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.

Step 6. Repeat this process for the idle mixture screws for each cylinder on each carburetor.

Step 7. 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 airow

meter/carburetor synchronizer.

Figure 14: A typical carburetor synchronizer tool/air ow meter.

8 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).

9 Resources

Carbs Unlimited, Inc., 727 22nd St NE, Auburn WA 98002, www.carburetion.com1 . Pegasus Auto Racing Supplies, Inc., 2475 S 179th Street, New Berlin WI 53146, www.pegasusautoracing.com2.

Webcon UK Ltd., Dolphin Road, Sunbury, Middlesex TW16 7HE, www.webcon.co.uk3 .

1

http://www.carburetion.com/

2

http://www.pegasusautoracing.com/

3

http://www.webcon.co.uk/



Solutions to Exercises in this Module

Solution to Exercise (p. 6)

Using Figure 4 a 1600 cc Ford crossow engine giving its maximum power at 6500 rpm will require a venturi

size of 31 mm. However, since come in even numbered sizes, then either 30 mm or 32 mm venturi would

be chosen. From this choice an ideal barrel size of 37.5 40 mm (i.e., 30 x 1.25 and 32 x 1.25) would be

calculated and hence 40 DCOE is the ideal solution, even though 30 mm and 32 mm venturis are available

for 45 DCOE.


A Ford 1600 cc crossow engine with a venturi size of 30 mm will use a calculated main jet size of 120 with

an air corrector of 170. The alternative 32 mm venturi would require a main jet of either 125 or 130, with

either 175 or 180 air correctors, respectively.


For the 1600 cc Ford crossow engine each cylinder capacity is 400 cc and the emulsion tube could be F9 or

F16, or marginally F15. Generally, F16 is a good, `safe', choice in most applications of this size of engine.


Documents

Selection and Tuning of Weber Dcoe Carburetors 2