Source: Lotus West
Author: Jim Gallagher
Date: 1973
LW# 00FU055

There have been volumes and volumes of material written on Weber carburetors and, though much of it is theory that doesn't concern even the serious enthusiast, a lot of it is really of a practical nature and necessary if we are to tear into the little beauties with modifications in mind. Included in the Lotus/West tech articles are a detailed tune up procedure and an excellent setting-up article. BAP/GEON’s “A Guide to Tuning” and Weber's "Technical Introduction" to their master catalog are also very good. Unfortunately, there is so much material that the beginning tuner spends six hours of reading for every hour on the carbs. This article is designed to be an ultra-condensed version of all of this material and is intended to give basic ballpark settings for the most commonly changed internal components of the Weber. It should work for the person converting the Elite to Weber as well as the Elan owner who just wants to go up one choke size, but isn’t sure what it will do to disturb his jetting. A $4.95 accelerometer from Lotus Central was an invaluable aid to tuning for this author after the chassis dyno people said the Lotus 7 was too low for their roller. Anyway, here it is for the folks who don't want to be burdened with the facts, but just want the number. Bear in mind that enlarging anything that passes air through the Weber will lean the mixture and enlarging anything that passes fuel richens the mixture.

IDLE JET (Idle mixture)
cc 	size
1000 	40
2000 	55
3000 	70
(One size richer for Siamese ports)
Idle screw should be correct with idle screw ¼-1 turn out.

IDLE AIR CORRECTOR (Idle transition)
Raise idle 300 RPM above normal and correct mixture with mixture screw to determine whether to richer or lean out idle air jet.


cc	size
1000	28
1600	33 ±2
2000 	35
(Siamese ports +4)
MAIN JET (Mixture strength)

Basically 4x choke size of for choke changes 5x choke size change

AIR CORRECTOR JET (Top end mixture)

Main jets size +60
Racing – same as main jet size (One size main jet change is equivalent to three sizes air corrector change i.e. +5 main jet = -15 air corrector jet)

EMULSION TUBE (Mixture curve)

Lean low end – F2, 3, 11, 14, 15, 16
Rich low end – F7, 8