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Difference from prior major
revision.
minor diff author diff hide diff=== Selection and tuning of Weber DCOE carburettors ===
A very popular modification for kit car 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
}}}
carbs01-70.gif Here 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.
==== Carburettor Barrel size calculation ====
Venturi<br>
<nowiki>----------</nowiki> * 1.25<br>
choke size
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.
==== Main Jet and Air Corrector Size Selection ==== carbs02-70.gif
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:<br>
36mm chokes<br>
F16 or F2 Emulsion tubes<br>
145 Main jet<br>
190 Air corrector<br>
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.
==== Idle Jet selection ==== carbs03-70.gif
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.
==== Setting the Idle and slow running ==== carbs04-70.gif
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
||**Application**||**Chokes**||**Main Jets**||**Emulsion Tubes**||**Air
Correctors**||**Pump Jets**||**Idle Jets**||**Aux Vents**||
||Standard 2000/1800/1600 Pinto on 40s||34mm||135||F11||190||35||40/f9
<br>45/f9 for 1800/2000||4.5||
||Modified 1600 Pinto on 40s||34mm||140||F16||190||40||40/f9||4.5||
||Modified 1800 Pinto on 45s||36mm||140||F16||170||40||45/f11||4.5||
||Modified 2000/2100 Pinto on 45s||38mm||145||F16||180||40||50/f9||4.5||
**The above article was kindly donated to the Lotus7 Club FAQ by Dave Andrews**
**Dave's website can be found here....** http://www.dvapower.com/