Just to add to the database, I've noticed the same thing when using
a Unisyn on my Maserati. Putting the Uni-syn on the center carb would
make the engine really want to stall. Putting the Uni-syn on either of
the other two carburettors would have less of a tendency to do this.
Putting the uni-syn on either throat of any given carb had about
the same effect. From this last piece of data I would assume that
this might indicate a mis-set float level. If it was anything else,
two throats on the same carb could very well have different effects,
and I didn't see that.
One other possibility (never overlook the obvious) - on my Lotus Europa
only the front carb had a "choke" (fuel-enrichment circuit, for cold
starting/running), so if your engine is not thoroughly warmed up and in
a steady-state, then blocking different carbs with the Uni-Syn could
have drastically different results. Or if the "choke" was not properly
disengaging after warm up, affecting idle but not cruising performance...
I would tend to suspect bad float level, mis-adjustment, leaking gasket,
etc. however...
There is also a neat gadget called "Carb-Stix" which consists of four
(six available?) mercury-filled glass tubes. Connect each tube to a
tap on the intake manifold past each carb, and *POOF* instant vacuum
check of how each manifold is doing. Not very good for cross-connected
carbs (like Lotus/Strombergs), but great for Webers (one throat per
cylinder, or even cylinder pair, etc, as long as the carbs are *NOT*
interconnected other than at the gas line). Once the vacuum level is
adjusted the same in all carb circuits, then you adjust the mixture
to be the same everywhere (on motorcyles - every cylinder has its own
carb! - one trick was just to pull the plug wire and see how many rpm
you lose, adjust carb so that each cylinder loses the same rpm when
you disable it). Or just run for awhile, then check the condition of
the spark plugs... The Carb-Stix were really neat, you could see the
mercury columns "bouncing" at idle, in response to the constantly-
changing vacuum in each intake track as the engine runs. And trying to
get all four input tracks to oscillate within the same 1cm of mercury
vacuum was hard - it takes very little adjustment to make a significant
change in vacuum!
Another thing to consider is that idle-circuit and cruising circuits may
be different - syncing the carbs at idle may have virtually no effect
on highway-speed synchronization (consider the slack/play in the throttle
mechanism between the carbs). So check your carbs at 2000, 3000, 4000,
etc. rpm as well as idle. You may need earmuffs . . .
-RDH
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