Weber Carburettor

Selection and tuning of Weber DCOE carburettors

A very popular modification for MG and Spridget owners is the fitment of a twin choke Weber DCOE carburettors; this not only delivers the goods but also looks 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 a 40 or 45" coupled with "Surely the 45 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 7000, some road engines and race engines can easily be 8000 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 DCO is 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 1380 engine giving its maximum power at 7000RPM will require a venturi size of 33mm so lets say a 34mm, and therefore an ideal barrel size of 42.5mm (34 * 1.25). For this application a 45 DCOE is the ideal solution, however a 40 DCOE will accommodate a 34mm choke, so if funds are limited and the engine is not going to be tuned further then a 40 DCOE would do the job.

If you have bought your Weber second-hand, it is important to understand that it is unlikely that it 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 to see the size stamp.

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 us with 1275/1380 F15 will generally be OK. If your carb is 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 34mm will require a main jet of (136) 140 and an air corrector of around 185/190.

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 1380cc A Series with power peaking at 7000RPM (285 degree cam or above) are as follows

34mm chokes
F15 Emulsion tubes
140 Main jet
185/190 Air corrector

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 1300/1380 engine, although I have seen a few engines that could easily accommodate a 50f9 so try the 45 and see what happens.

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 a Weber is between 900 and 1100 RPM depending on cam.

If you are adjusting the idle for a carb already fitted then progress to the second stage, if the carb is being fitted for the first time, screw the idle mixture adjustment screws fully home and then out 2.5 turns. 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. Set the idle as near as you can to 900RPM, this may be above 1000 with a hotter (286) cam.

Now adjust 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 barrel.

After both 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. 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.