Selection and Tuning of Weber DCOE Carburetors



Selection and Tuning of Weber DCOE Carburetors
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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
Figure 1: The Weber 40 DCOE sidedraught carburetor.
∗ Version
1.2: Aug 27, 2011 3:41 pm -0500
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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.
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Number in Figure 2
Jet inspection cover
Needle valve
Emulsion tube holder
Air corrector jet
Idle jet holder
Emulsion tube
Main jet
Idle jet
Auxiliary venturi
Air horn
Main venturi
Air bypass screw
Throttle plate
Idle mixture screw
Pump jet
Starter air jet
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).
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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
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.
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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.
DCOE carburetor
Available venturi sizes (mm)
24 - 36
24 - 34
28 - 40
40 - 42
42 - 46
46 - 48
Table 2: The main venturi size available for common DCOE series carburetors.
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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.
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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.
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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).
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.
Exercise 2
(Solution on p. 14.)
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.
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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
275 400
350 475
F9, F16
450 575
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.
Exercise 3
What emulsion tube would be used for a 1600 cc Ford crossow engine?
(Solution on p. 14.)
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.
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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
Table 4: The idle jet sizes appropriate for a given engine size.
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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
• 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 idle
speed 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.
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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
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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
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, .
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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.
Solution to Exercise (p. 8)
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.
Solution to Exercise (p. 9)
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.