I notice that you keep mentioning that you experience the flat spot, "then it clears up as you push through to WOT." In other words, you are having a part throttle flat spot that clears up once you get the throttles wide open. If you are opening the throttle(changing the TPS %), you are changing the operating parameters, thus making diagnosis more difficult.

What happens if you go to WOT immediately instead of slowly opening the throttle? Or what happens if you open the throttles to say 75% and leave them there?

Obviously the A/F ratio will need to be richer while at WOT than at part throttle. PE(Power Enrichment) mode adds extra fuel when the engine is needs extra fuel during acceleration.

Sounds like the Marine TBI systems use the high pressure regulator similar to the 1994-95 TBI BBC trucks use.

peace
PAuly