"Peter:
Like Jeff said, the
"Peter:
Like Jeff said, the idle mixture screws won't make any difference once you open the throttles enough to expose the transfer slots.
sadly, a few GPH difference between engines, at the same RPM, especially counter-rotaters, is not uncommon on factory tuned, carbed setups. What you can do is to use the vacuum gauges and record VAC & GPH vs RPM, for both engines (ignition advance is also good to record). You should find the GPH varies inversly with VAC; vac goes down, GPH goes up. Getting identical VAC readings on both engines across the RPM range is very tough due to tolerances and wear.
Once you have the baseline measurements, it isn't too hard to figure where to start tweaking to get things where you want them. It does take lots of time and most people don't want to invest it.
Unless your boat is super heavy or geared wrong, you should have a reasonable amount of VAC in the intake manifold at cruise. Everything you can do to increase that cruise VAC reading will improve your fuel economy."