You're still correct.
The SU Carb on the Z car feeds 400cc of cylinder.
The SU Carb on the 2000 feeds 500cc of a cylinder.
On the U20, we have a firing order of 1-3-4-2. So if
you notice you get 3 drawing in a/f, then 4 drawing
air fuel. Ditto on the front carbs, with 2 drawing in
a/f then #1.
The catch is any fuel mixture is atomized in the manifold
will stay in the manifold, until it is drawn into the
cylinder. So #2 & #3 cylinders are "first feeders" and
tend to run a bit richer than #1 & #4 cylinders.
One solution would to for one carb to feed #1 & #4...
the other #2 & #3. Manifold would not be so simple.
NOTE: Wieland did make a single weber manifold that did
just this, which really helped running a single two
barrel side draft carb (manifold was sold for the
Now go back and look on the Z car for the firing order.
There is a little more overlap between "feeding" the cylinders.
(Every 120 engine degrees).
Other trivia: the 68-70 roadster were tuned to run RICH at
idle. The air injection from the air pump would help burn
any excessive fuel in the exhaust pipe. Also the retarded
timing at idle would keep combustion temperatures down,
and allow for burning to occur in the exhaust. So with the
Z vs. Roadster needles... it is possible that swapping
may be of benifit to BOTH. We really need leaner mixture
at idle, the Z may benifit from richer mixture at idle
As someone once told me "There is no magic bullet". Seems
true of software and roadster tuning!
P.S. For those who have ESPN. Motorola Marathon will
take place on Sunday - Feb 14th. If you catch the lead vehicle,
an electric vehicle, you'll spot me, Emerson (8), and Liam (6)
in the Truck. No power steering, nor power brakes... makes the
roadster feel like power assisted! ;-)