Thoughts on going from 7.4 MPI Gen VI to Carb Gen IV

[PlayDate]

AS EFI systems age they become problematic. The MPI's don't tolerate wear in throttle shafts well and what that play does to that fragile calibration. Their ECM's are bulletproof though.
Half the battle in Mercruiser EFI is finding a mechanic that knows what they are doing not only to diagnose, but repair and calibrate. It seems by reading posts here that "mechanics" throw snot on the wall hoping something sticks and consequently the massive cost associated with their work. On the other hand getting a carburetor to calibrate for use isn't plug and play either and finding someone do do that is getting rare. Sure they run bolted on but they still require jetting, idle mixture setting, power valves (Holley), throttle blade set for fuel transition, etc. I'm with @PlayDate on the Holley fuel injection especially if you are going to hang onto the boat. But, here we are again needing expertise getting the system set up correctly even though it is so much more sophisticated in the data on common laptop computers and the ability to get it tuned and trimmed.

Assuming you're not a sick motorhead (like me), I'd stay with the Mercruiser system but get a qualified mechanic to get it right.

Remember when we used to "read" sparkplugs for carb engines? Anyway things change.......I installed a AFR gauge and a wideband O2 sensor on each engine to dial in the carb. The primaries were okay at 72 but lean....so going to 74 was better. On the back side Holley shipped the carbs with 92 on the back plate which turned out to be about 10 steps larger than it needed to be. So 74/82 keeps the AFR at 13.5 across the throttle range.

The Holley EFI systems are the only thing I would change the carbs for. Simple installation, learning mode and the ability to tweak the parameters. The only downside is having to use the Mercruiser fuel pump.

 

[Scott06]

I thought the newer blocks don't have the fuel pump boss on the block. I would stick with efi . Easy to diagnose and find parts for. Don't like thunder volt at all. Lots of issues I've heard

Later carbed motors use an electric fuel pump (low pressure)

 

[Scott215]

I figured that but didn't know the plan

 

[Scott215]

Then you could go Holley Terminator 2 depending on what you want to spend. They make awesome fuel injection and you would never have an issue. I think they have marine in throttle body and mpi

 

[Stee6043]

Later carbed motors use an electric fuel pump (low pressure)

Some older MPI motors have mechanical fuel pumps as well. It's the wild west out there.

 

[ttmott]

Then you could go Holley Terminator 2 depending on what you want to spend. They make awesome fuel injection and you would never have an issue. I think they have marine in throttle body and mpi

You could even go Holley Dominator EFI but like the Terminator why spend the money on the additional features these systems have that would never be used. Who would need the 42 IO ports the Dominator provides? I guess you could flush the head.....
All of the sensors and input devices on the Mercury systems are essentially GM type and the voltages and impedance are GM. Holley uses the GM style sensors on their systems. So, for the most part a Holley system can be plug and play (calibration required) on a Mercury engine. But why? There is nothing wrong with the Mercury EFI systems provided they are in working order.
What @PlayDate is suggesting, as the Mercury system is wearing out (injectors not flowing correctly, can't keep the TPS consistant, throttle shafts are wearing, sensors are not returning good data, etc) it may be a path of least resistance and least cost to install an aftermarket intake manifold and Holley Sniper TBI.