I'm betting your fuel psi regulator is bad. It should max at 43psi.

X-2 what griff said also the erratic guage indicates a bad regulator

How do I diagnose which regulator to replace? The one on the fuel rail or the cool fuel regulator?

Before you start throwing money towards parts, remove the plastic cover from the cool fuel exchanger assembly. Remove the two screws that's holding the fuel pressure regulator[ controls excess fuel return to the water separator] on the copper exchanger and discard the s/s filter that sits under the regulator.What it seems in your youtube video ,your pressure spikes up [ hence the black smoke ] too high when throttle applied, dumping excess fuel through one or more injectors. If you have twins, you can start swapping the regulators at the top of intake first before diving into the cool fuel system below.

Makes sense, I will give it a try. What does discarding the filter do?

You must disconnect the vacuum line to the regulator to get an accurate reading. The gauge is fluctuating because of the varying vacuum as you rev the engine. The regulator is passing the most fuel at idle. If the regulator was the issue it would be most prevalent at idle. Also, the regulator on the cool fuel is what controls pressure. The one on the fuel rail just acts as a damper.

Bob Lloyd
Full Throttle Marine

Thanks for the info Bob, do I need to pull both vacuum lines or is the cool fuel vacuum enough to get an accurate reading? I didn't record it, but when I pulled the cool fuel vacuum line the fuel pressure didn't change. Anything else I should be trouble shooting?

I had the same problem with my port engine as you've described. Black discharge through the exhaust with SmartCraft warning message; 'Throttle back' over 3500rpm. My Rinda scanner had the same message when I checked. Found that the tiny s/s filter [ about 1/4 " in dia ] got contaminated, blocking fuel return and higher than normal [ 70 psi on the Schrader valve] pressure readings over 3500 rpm. After tossing the filter, problem solved. Merc has a redundant system I guess. There's a filter before the low pressure pump, delivering fuel through the water separator/filter, another small s/s filter in the high pressure pump inlet as well. Got single tank feeding both engines. Idle was smooth at 600 rpm for both at all time even when I had the issue. Like Bob mentioned ,the regulator on the cool system controlling the fuel pressure between 39-43 PSI. It's a b,,,tch to get to them with twin engines cramped into the engine bay,especially on the starboard one.

http://www3.telus.net/spectrasonly/Twin%20377's

What is the serial number on your engine? Some of the earlier 6.2L ran different regulators and lower fuel pressure. Maybe someone replaced it with the wrong one. Compare the fuel pressure reading with your other engine. Is your fuel pump under the engine or the module on the side? You need to unplug the hose running to the regulator at the fuel pump.

She is 0w063672. What is the spec for that serial?

I am not actually sure where the fuel pump is, I need to look around a little more to find that out. Sounds like I got another good list of troubleshooting to do this weekend.