Volvo 7.4 won't run past idle--no power

[nuclearfishin]

I'm having an engine problem that I know somebody here is likely to know the answer to. 2000 Volvo 7.4 fuel injected. I took the boat out yesterday and it idled fine, no problems idling through the no wake zone. When I got to open water, I gave it some gas and the boat coughed, sputtered, idled up/down, and would never get power to go on plane. It gave the symptoms of running really rich and not being able to combust properly. I thought I might have bad gas, but I was just out on the boat last week and it ran great, no problems. I haven't put any new gas in since so I'm thinking it shouldn't be a gas issue, although it's behaving like it has bad gas. It even back-fired a little when trying to get up to speed. When I go to idle, it runs smooth with rpm's right where they should be. When I try to rev it a bit (while prop not engaged) it gives same symptoms of coughing, no power, etc. There is a little black smoke typical of running rich. The only other symptom is that sometimes (not always) there is a sound that sounds like high pressure air or a vacuum sound coming from the flame arrestor (or right by it). I took it off and the arrestor is totally clear. I used some Sea Foam just in case it was bad gas, but it ran fine last week on the exact same fuel.

Does anybody have any idea what might be the problem before I have the mechanic haul it out of the water to look at it? It's the Volvo 7.4L engine. Any help would be greatly appreciated.

Thank you!