I've also read where some people have changed spark plugs and went to the AC-DELCO RAPID FIERE #1'S and have had a lot of success with these plugs. Easier starting when cold and cleaner transoms from the fuel/air fuel mixture being burnt. Plus, it would be the cheapest route to solve your problem too!

You can do a search for some info on this. Type in the subject to search as "502 spark plugs". I have not tried these plugs yet, but I plan on installing a set of these in my 502 EFI in the spring.

I would agree with other comments, you need a good 87 octance. I would assume their is a Shell station you could get to?You should see an improvement on the back of your transom instantly!

Those are all I run,,, the heat up faster to help in not fouling up. That on top of headers and the Kand N filter almost completelty cleaned up my transom.

madbouyz, I would check the fuel pressure before installing the Maxflow regulator, just as a base point. Also do you have a VST (vapor seperator tank) in your fuel system? I'm not sure, but I think the fuel pressure is different for the VST vers. the fuel cooler version. I have the VST tank and the book says the fuel pressure should be 30-32 which I believe is lower than the fuel cooler version (maybe 34-36 but not sure). Someone here should be able to let us know what it is supposed to be. Everything that everyone is telling you here will help the soot issue.

I would watch the plugs real close though after putting on the stainless manifolds. Only because Hotboat did an article about a year ago, where they were testing exhaust manifolds and when they put on CMI's the stock 502MPI, it ran to lean, so they reprogrammed the ecu to richen it up. I don't have any personal experience with changing exhaust, just passing info that I read.

Oh I just thought about another possibe issue. If you have the VST tank, which is located next to the ECU, and the float in it sticks open, the excess/RAW fuel will dump into the intake and create a very rich problem. If you have the VST, all you need to do is unscrew the inspection plug on the top of it to see what is going on.

I have the adjustable regulator and it works great. You have to be carefull, because you have to cut the end off of your stock regulator and then bolt on the adjustable regulator. There is a rubber diaphram in it, that could be punctured when you cut the end off. Just follow the instructions and you will be fine.
Your on the right track,

Sorry for the dalay in a show of appreciation... I'm only at this spiffy computer on weekdays !The fuel rating is certainly an interesting issue.The specs on our fuel is ....
Anti Knock index ..93
Motor Octane number..87
Research Octane number..98
We only use Shell, Esso is avbl.All same ratings and high price.. only one test rating for either island wide wether on land or by sea . I wonder if I should approach our 104+OCTANE BOOST supplier and see if he can get me a load of 'MUNDANE OCTANE' instead ! !

If any of you have had any experience dealing with Merc's tech people (at the upper end of the company) you might be interested to know that we've been persuing this with them for about two years.One guy,when cornered at the MIBS simply barked "What do you expect ?Its a big F'ing engine,it burns a lot of gas and makes a lot of noise !"(Reassuring huh ?)The rest of the tech team seems to think we have a problem but don't know what to do .Again .. reassuring !That's why this board is GREAT !!
Rockie.. thanks , ours has the fuel cooler setup but I will still get a baseline reading before any adjustment.And look for that float incase we have one too.According to TCM the standard fuel pressure on ours should be around 35psi.. newsetting will be @ 43psi.
OBNOXUS.. is yours a 502MPI ? Did you regulate your fuel pressure too ? DO you think you have compromised your engine's reliability ?

If its of any interest the exhaust system we are going to be using is made by an Australian company called "HiTek Marine products" AN all stainless fully water jacketed system which exactly replaces OEM equipment,comes with your choice of extension heights can be raw or freshwater cooled very easilly even has removable zinc anodes inside.Supposed to be tuned FAR better than OEM cast iron lumps but I would'nt think they could be referred to as racing equipment.

We will keep you posted of improvements (if any) when the stuff is all installed in a few weeks (I hope).Regards/Steve

Yes,,, 502 MPI Gil headers, and switchable exhaust, and Kand N flame arrestor,,,,,the fuel pressure regulator is factory,,,,,,,,,,,have I comprimised reliability,,,,,,,well, you tell me,,,, I have 600+ hours on the boat and have run only 87 octane from day #1 !!!!!!!!!!!!

Thats what I want to hear ... !

Figured you'd like that !!!!!!!!