Selection
and tuning of Weber DCOE carburettors
A
very popular modification for RH7 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
40 24-36mm
42 24-34mm
45 28-40mm
48 40-42mm
48/50SP 42-46mm
55SP 46-48mm
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 DCOE 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.
Diagram
of Main Jet assembly
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
250-325 F11
275-400 F15
350-475 F9, F16
450-575 F2
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.
Diagram of Idle Jet Assembly
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 free.
Below
is a chart showing approximate idle jet sizes for given engine sizes, this
assumes one carb
barrel
per inlet port E.G. two DCOEs.
Engine size Idle jet size
1600cc 40/45
1800cc 45/50
2000cc 50/55
2100cc 55/60
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.
Diagram of DCO type carburettor
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 cold)
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 applications
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
F16
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
Dave
Andrews