selection and tuning of weber dcoe carburetors2.pdf/... · selection and tuning of weber dcoe...

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 ascomplicated as many fear, and while �ne-tuning is best performed using a rolling road dynamometer (chassisdyno) an excellent �rst guess can be obtained based upon the engine size and power band desired. Thefollowing provides the calculations that are required to achieve an excellent initial set-up, irrespective of theapplication.

Figure 1: The Weber 40 DCOE sidedraught carburetor.

*Version 1.2: Aug 27, 2011 3:41 pm -0500�http://creativecommons.org/licenses/by/3.0/

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Weber is an Italian company producing carburetors, currently owned by Magneti Marelli Powertrain, inturn part of the Fiat Group. The company was established as Fabbrica Italiana Carburatori Weber in 1923 byEdoardo Weber (1889�1945). Weber carburetors were �tted to standard production cars and factory racingapplications 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 carburetorswere produced in Bologna, Italy up until around 1990 when production was transferred to Madrid, Spain,where they continue to be produced today.

The pre�x number on the DCOE, e.g., 40 DCOE, is the diameter of the throttle plate (the throttlebore) in mm; DC means doppio corpo (double throat); O means orizzontale (horizontal); E means it is adie cast carburetor; and the number or number and letter su�x 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 partsdescription given in Table 1.

Figure 2: A parts diagram for a Weber 45 DCOE carburetors. The number key for selected parts isgiven 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 correctcarburetor. It is commonly (and incorrectly) assumed that 45s will give more power than 40s because ofthe larger carburetor barrel. However, it is not the barrel size (i.e., 40 or 45) that determines the air�owand 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 theventuri 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) inorder to draw in and atomize the fuel mixture in the most e�ective manner. The smaller the main venturi,the more e�ective this action is, but a smaller venturi will inhibit �ow. A large venturi may give more powerright at the top end of the power band, but will give this at the expense of tractability at lower enginespeeds (rpm). Race cars will bene�t from this latter compromise, but on a road car drivability is much moreimportant.

Figure 4 shows a chart that allows for the correct selection of main venturi size for engines given theengines capacity and the rpm at which it is expected to achieve peak power. The rpm value primarilydepends on the choice of cam; however, it is necessary to ensure that the rest of the engine is built to meetthe needs of that engine speed. For example, the use of double springs on a pushrod engine or solid (ratherthan pneumatic) lifters in an overhead cam engine.



Figure 4: Chart showing main venturi sizes for various engine sizes and peak rpm ranges. The red lineis for a Formula Vauxhall Lotus, while the blue line is for a Ford cross�ow 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 whichcarburetor is required. The ideal barrel size that will accommodate the venturi size selected is calculatedaccording to (4). Table 2 shows a list of the main venturi size available for common DCOE series carburetors.

(4)

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 1Example 1: Using Figure 4 a 2000 cc Vauxhall/Opel engine giving its maximum power at 7000rpm will require a venturi size of 38 mm, and therefore an ideal barrel size of 47.5 mm (i.e., 38 x1.25). For this application 45 DCOE is the solution, since 38 mm chokes are not available for 40sor even larger carburetors (see Table 2).

Exercise 1 (Solution on p. 14.)

What venturi size will a 1600 cc Ford cross�ow engine require if its maximum power is deliveredat 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. Themain 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 sizedin 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 e�ect over the whole rev range, whereas changing the air correction jet has moree�ect at higher revs. Increasing the size of the main jet will enrich the fuel mixture and visa versa. Incontrast, increasing the size of the air correction jet will lean out the mixture. A summary of the results ofchanges 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 (8). This will givea 'safe' starting point for the main jet size. The air corrector jet initial settings should be about 50 higherthan the main jet, (8).

(8)

(8)

Example 2Using the results from Example 1 for the 2000 cc Vauxhall/Opel engine, a venturi size of 38 mmwill calculate a main jet size of 152. Since main jets are sized in increments of 5, so a main jet of150 would be suitable, while the appropriate air corrector would be 200. However, a main jet of155 and air corrector of 205 could also be tried.


What main jet and air corrector sizes will be needed for Ford 1600 cc cross�ow engine with aventuri 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 inFigure 2) beneath the jet inspection cover (4 in Figure 2). The size of the emulsion tube is de�ned by thecylinder 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 aircorrector �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 3For a 2000 cc Vauxhall/Opel engine each cylinder capacity is 500 cc and a F2 emulsion tube wouldbe appropriate. However, a 2000 cc engine in just on the cusp of change for emulsion tube typebetween 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 cross�ow engine?

6 Idle Jet selection

Idle jets (Figure 10 and Figure 11) cause a lot of confusion; although their name suggests that they governthe idle mixture, this is not true. The idle mixture is actually metered by the idle volume screws (56 inFigure 2) mounted on top of each barrel. The function of the idle jet is to control the progression betweenclosed throttle and the main jet circuit. As such it is important to smooth progression between closedthrottle and acceleration and for part throttle driving. If this circuit is too weak then the engine will stutteror 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 ori�ce (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 ischosen 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 areF2, 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 mostnormal 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 carburetorsbeing incorrect. Before adjusting the carburetors it is important to make sure that the following have beenchecked:

• The engine is at normal operating temperature.• The throttle return spring/mechanism is working properly.• The engine has su�cient advance at the idle speed (between 12 and 16 ◦). As a starting point the idle

speed for a modi�ed 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 andFigure 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 idlespeed 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, orthe timing is hopelessly retarded. If this happens when the engine is warm and you know that thetiming is OK, then the mixture will need trimming richer on that cylinder.

Step 4. Using an air�ow meter or carburetor synchronizer (Figure 14) adjust the balance mechanism betweenthe 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 balancescrew in a clockwise direction to correct this. If it is drawing more air, then turn the balance screwanti-clockwise. If the idle speed varies, adjust it back to 900 rpm, to decrease idle speed screw in ananti-clockwise direction, to increase, screw in a clockwise direction.

Step 5. Once the carburetors have the same air�ow, 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 ofa turn is su�cient). Allow 5 - 10 seconds for the engine to settle after each adjustment. Note whetherengine speed increases or decreases. If it increases continue turning in that direction and checkingfor engine speed, then the moment that engine speed starts to fall, back o� a quarter of a turn. Ifduring this process the engine speed goes well over 1000 rpm, then trim it down using the idle speedscrew, and re-adjust the idle mixture screw. If on the �rst turn, the engine speed decreases then turnthe mixture screw clockwise (which will make it weaker) in small increments, again if engine speedcontinues to rise, continue in that direction, then the moment it starts to fall, back o� a quarter aturn. 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-clockwisedirection 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 furtheradjustment. If the engine is rocking or shaking then the balance is out, so revisit with the air�owmeter/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 .

1http://www.carburetion.com/2http://www.pegasusautoracing.com/3http://www.webcon.co.uk/



Solutions to Exercises in this Module

Solution to Exercise (p. 6)Using Figure 4 a 1600 cc Ford cross�ow engine giving its maximum power at 6500 rpm will require a venturisize of 31 mm. However, since come in even numbered sizes, then either 30 mm or 32 mm venturi wouldbe 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 becalculated and hence 40 DCOE is the ideal solution, even though 30 mm and 32 mm venturis are availablefor 45 DCOE.Solution to Exercise (p. 8)A Ford 1600 cc cross�ow engine with a venturi size of 30 mm will use a calculated main jet size of 120 withan air corrector of 170. The alternative 32 mm venturi would require a main jet of either 125 or 130, witheither 175 or 180 air correctors, respectively.Solution to Exercise (p. 9)For the 1600 cc Ford cross�ow engine each cylinder capacity is 400 cc and the emulsion tube could be F9 orF16, or marginally F15. Generally, F16 is a good, `safe', choice in most applications of this size of engine.


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