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]
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