Dirty exhaust

I have a 2000 Ski Nautique with the 5.0 Gm throttle body fuel injection system.
I get a lot of dark soot on my transom after skiing. I run 91 octane fuel and put a bottle of sea foam in every couple tanks. Every 50 hrs or so I have to clean my plugs because they get fairly fouled by then. I do a new cap and plugs every season, or about every 100 hrs.
The perfect pass says 800 hrs (300 by me) but it might be more. My gauges failed so I replaced them when I got the boat 4 years ago.
I am not an gear head but I’m handy. What does my engine need, or is it heading to the train station ( Yellow Stone reference).
If it needs replaced how much, if any of the parts can be re used on a 5.7 if I re power it.
It still runs really well, pulls nice through the course and holds speed it’s just a bit of a dog out of the hole and tops out at 41mph
Sorry for being long winded.
Mike

Could also be dirty or leaking injector.  

You only need 89 octane fuel...not 91.  It is a myth that "premium" fuel makes your engine run better.  That MAY contribute a little to your soot problem as higher octane fuel burns slower than lower octane fuels.

Mike:  bad injector(s) is the most likely culprit.  In my old 1994 SN, I had the throttle-body Electronic Fuel Injection and one of the injectors was dribbling quite badly. That produced a lot of dark gray to black smoke that wound up on my transom after every trip.  The engine would load up on too much fuel at lower RPMs.  High RPM was OK. 

Pull out an injector and look it up.  I use www.RockAuto.com.  It shouldn’t be marine specific.  I would change them all out at the same time.  Don’t go cheap and try to save a few buck$ — put in good quality injectors.

I replaced both of my injectors. The service manager that I talk to said another problem might be my O2 sensor or oxygen sensor. He thinks those may cause it to run to Rich as if it were at altitude. I haven’t run it yet but I wonder if I should look into that as well

I’m going to water test today. Ran great on the hose, and looked like it was spraying well.
Service manager was thinking 94 Chevy truck, since that’s how we found the part number for my injectors. I say boat and I get a Deere in the headlight look or they tell me they can’t help me at all

It’s always “I can’t look up a boat! We don’t carry any boat parts. Have you tried the marina?”
My marina services jet skis and pontoons. Everything else is guess work

Looking at a different option on injectors. My GM 305 tbi replacement injectors were not functional.
Thinking about refurbished injectors from rock auto or a complete replacement like this.
https://www.ebay.com/itm/Motor-Man-5235279-TBI-Fuel-Injector-Kit-Regulator-GMC-Chevrolet-5-0L-305/292963979289?epid=74260124&hash=item4436039019:g:xloAAOSwFlNcZteD
Sorry I made this ridiculous link. Can’t figure out how to shorten it

Glad you got it figured out.