The idle mixture screw should be between 1 & 2 turns from closed. (if I remember correctly) If its less the 1 turn open the idle jet is to large and if its more than 2 turns open its too small. The idle jet size is a function of cylinder size and the aircorrector part is to ensure smooth progression from idle to the main jet system. 50F9 is a good starting point for a 1275.
The accelerator pump height is set by the length of the pump rod. There are at least 3 different lengths and this determines the initial volume of fuel available in the accelerator pump fuel well. The pump bleed jet is a fine tuning device for the pump volume. The rate at which the accelerator pump pushes fuel into the engine is determined by the accerator pump spring and there are a number of different strength springs. There is no positive linkage to the accelerator pump as in most other types of carby. Webers are different. When the throttle is closed it holds the accelerator pump rod in the highest position and when you snap open the throttle the spring attempts to force the pump piston down and this is resisted by the amount of fuel bled off via the pump bleed jet and by the size of the somewhat misnamed pump jet. Opening of the throttle does not directly operate the accelerator pump. The so called pump jet is a dual purpose jet and its primary purpose is to act as a wide open throttle power jet. It just happens to also be used to discharge fuel from the accelerator pump well. The size of the pump jet should be determined by the wide open throttle mixture strength required. The volume of fuel admitted and the time over which it occurs during acceleration should be adjusted by the accelerator rod length, the pump bleed jet and the strength of the pump spring. This is not easy or cheap. The cheap way is to fiddle with the pump/ full power jet. You need to remember that the Weber was designed for one cylinder per choke not two and it can be difficult to get the accelerator pump mixture right.
A guide to main jet sizes is to start with a jet approx 4 times the size of the main venturi. eg for a 34mm venturi use a 135 main. If you use too large a main venturi you may never be operating on the main jet system except at high speed. For road use it is always best to err on the side of smaller rather than larger venturis. There are charts in the various Weber catalogues and after market books which give some guidelines for selecting venturi size.
Emulsion tubes should probably be F2 for 1275's and maybe F16 for smaller engines. The emulsion tubes determine the amount of capillary action in the fuel well as they come in different diameters and the air corrector holes vary in position and number. The air corrector jet is a sort of fine tuning device for the emulsion tube and effects the mixture more at higher RPM and the main jet effects the mixture more at lower rpm. If you change one you may well have to change the other.
Don't know if these ramblings will help or confuse but it should help to get you to a starting point. But when its all said and done its what works in your car that matters no matter what anybody says.
Hope this helps.
RonR
