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 substitutefor 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 selectingthe 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 powerright 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 mainventuri 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 40 42 45 48 48/50SP 55SP Available Venturi sizes 24-36mm 24-34mm 28-40mm 40-42mm 42-46mm 46-48mm
Below is a chart that will allow the correct selection of main venturi size forengines 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 idealbarrel 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 isthe 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 knowtheir 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...