Okay, so difference was:
gas tank, pick ups, and feed line to engine ?

engine compartment vs dyno room

Anything else ? Mufflers in boat ? Or etc ?

Have you checked the gas tank vents?

This may have been covered before, but have you tried disconnecting the knock sensor/s? Thinking maybe some type of vibration from outdrive/gimble bearing could be triggering the knock sensor??
Can you datalog the ign timing to see if it's getting pulled back? Does ign system have plenty of voltage (12+ volts) at high RPM?

As mentioned prev by SB, any chance of connecting to a prop dyno??

Power supply is another difference.


Ryan

Thats pretty much it. No mufflers or anything else. We've tried running with the engine hatch opened a bit to eliminate a possible lack of air supply too.

i have. We can blow right through the vents into the tanks with no restrictions

yes knock sensors have been disconnected with no change. We've also checked the timing and we di have 12+ volts at rpm.

Im assuming your fuel tank pickups are the same size as the ones on the dyno? youve monitored fuel pressure while under load? How are you running your fuel system in the boat versus on the dyno? same pump etc?

yes pickups are the same size as the dyno. We actually just ran a 3/8" hose right into a fuel can as opposed to using the fuel supply off the dyno so all the same pumps, etc. I'm starting to think its definitely a fuel supply issue now. There seems to be little to no throttle response above 3800 rpm. Fuel pressure never changes so I'm thinking it more of a lack of volume. I'm going to pull the tanks pickups and see if there's restrictions somewhere

Was alternator used when running on dyno?
If not, maybe rectifier is going bad and causing elec noise??

Are batt switches set to "1 on 1" and "2 on 2" or are they both set to "both"?? Reason I ask is I recently lost a MSD box on my port eng (#2). Tach started jumping around, then a few stumbles, then dead. While idling back in on one eng, I switched both batt switches to "both". Tach on good eng started jumping around. Set both batt switches to "1" and tach behaved the remaining 2.5 hrs idling back in.