I do not think your mechanic is well versed on inline 6 engines. Not all his fault as those motors are usually older than the mechanic. Il6 motors do not have slip rings. They have labyrinth seals, which seal the crankcases individually, as required by a 2 cycle engine. These fail, but do so rarely.
Your OEM fuel pump is what you want to use. An electric one invites danger, as it will still keep pumping if the motor stalls. Rebuild your OEM fuel pump with all new gaskets. New check valves and gaskets are a good idea as well. Make sure the transfer covers when the fuel pump bolts on are not leaking.
When you cleaned the carb, did you remove and clean the idle restrictor tube? It is Merc's term for idle jet. There are pinholes in the carb throats that are the idle circuit. These need to be clean as well. Did you rebuild the carbs with new gaskets, inlet needle and seat and floats? If not, you should do that. These are std maintenance items. Set the carb float levers to be parallel with the carb bowl cover, when inverted.
Set the idle mixture to 1-1/2 turns open. Adjust on the water, in gear for smooth acceleration. You will likely need to richen the mixture about 1/2 turn to achieve that. The lower carb will naturally run a bit richer, so not need that much adjustment.
Check the engine wiring for bad insulation. If compression is good, and insulation is bad, replace the wiring harness. Do you have spark on all 6 cylinders? Have you checked the Link and Synch on that motor? If not, it needs to be done.
If spark plugs look real clean, you could be getting water into the cylinders. if lower cylinders only are affected, lower crankshaft seal is likely culprit. Upper cylinders getting water indicates warped of holed baffle or inner water jacket cover. Hopefully that is not your issue?
Fresh water motor means water infiltration much less likely.
An OEM service manual will help tremendously.
