Nice looking rig!
Just an observation, but if you have the motor jacked too high on the transom, both exhaust and cooling intake might suffer. It's not a thru-hub exhaust, but you likely won't clean out the leg if you aren't deep enough. Likewise if you have any airflow under the anti vent plate, the intake on the front of the snorkel won't operate as it should. Finally, if mounting too high, you may be kicking up nasties -- might be what you are seeing.
Think I might try lowering it a bit and see if you get cleaner running -- maybe better cooling too. For what it's worth, my '57 Fastwin runs clean using 24:1 and Champion J4's.
