Setting up twin carbs - theory meets practice

Well,

First time I ever really played with webers with a view to tuning and blueprinting them very specifically to a particular engine.

So I thought I’d write up the carb theory and then see how it works out in practice!

OK, for dual throat twin carbs (IDFs, IDAs, Dellortos etc etc).

Carb size is given by the formula (1)

Carb size = Square root (cc per cylinder x maximum rpm) divided by 40.

So, for example, for a 2200cc motor (550 cc per cylinder) which will pull 5500 rpm

Carb size = Square root (550 x 5500) divided by 40 = 43.48mm …

You should start with a 44mm carb (44 IDF or whatever…)

Choke diameter is given by the formula (2)

Choke (main venturi) diameter = carb size x 40 divided by 50, so for a 44mm carb

Choke (main venturi) diameter = 44 x 40 divided by 50 = 35.2 mm, so a 36mm choke is good

Main jet size is given by the formula (3)

Main jet = choke (main venturi) diameter x 4, so for a 36mm choke

Main jet = 36 x 4 = 144, so a 150 main is good.

Now the interesting thing about webers is that if you get the smallest choke (main venturi) for a given carb type you can in fact machine them out to any diameter up to the maximum you can get for the carb. Thus you can ‘blueprint’ the venturi to whatever diameter you desire, rather than the ‘stock’ 4mm steps. The casting is a little grainy but they do machine OK. You can of course also blueprint the jets by reaming to whatever size you desire (rather than going in .05 mm steps….).

I wonder how the theory will hold up?

RobK

[Edited on 2-12-2004 by splitbusaustralia]