I wish I had found this site 2 years ago,it would have saved a lot of agro trying to get answers/info out of Merc.Thanks to the large amounts of info here I am in the process of cleaning up my 502 MPI ('99 FWC 154 hrs).Already have the TCM billet breather/flame arrestor and the Maxflow adjustable fuel flow regulator,am waiting for a set of stainless manifolds to arrive in the next few days.
When my mechanic comes to install this stuff I'm sure he's going to have some questions so I hope you guys can help me get a jump on this.
What exactly is it about the design/const. of the 502 that creates this extreme rich burn situation ? I thought we had a lemon but have since found out that black transoms and unburnt fuel spitting out of exhausts is normal.With gas currently @ $4.98 a gal (US$) I need this 'foolishness' to stop !
What is the 'standard' fuel pressure setting on the 502MPI and how does this Maxflow regulator work ?
I have read a few similar threads but can't recall any questions exactly like these and apologise if I'm duplicating a bunch of answers here.

THANKS/Steve

I'm not expert enough to answer all your questions but i can say I've always "heard" the factory leans out the motors intentially to help save it from it's owners abuse and creating warranty issues/costs to them. I've got the new 496HO and it too on occasion will soot up the transom and I've "heard" it too is leaned out intentionally...Good luck with your project.

If the motor were to run "lean" it would be damaged. What you mean to say is that the motor is running "rich". When looking at a dyno curve, there will be an ideal point in the air fuel ratio. There will also be a rich max power and a lean max power. If the air fuel ratio lands between those two points then it is acceptable. If you run in Nascar and have to save on fuel and know that you have good fuel, then run it twords the lean side. If you have a marine application and varying fuel quality, then run it twords the rich side. Too far either way and things will get expensive. Merc sets their engines up to be safe. If you want to shave a little closer, then you have to run the risk of getting nicked on occasion. This is expecially true if you happen to get a bad tank of gas. An exhaust temp gauge will help prevent a melt down. However, you really want one on every cylinder, which isn't real feasible.

Guess what I'm saying is that for the amount that you've invested, you won't be saving a whole lot of money-and you're exposing your self to alot of risk. It's hard to pinch pennys when 5k motors keep on burning down.

BTW-check your injectors regularly. If an injector gets dirty or starts to fail, it doesn't really matter in a car. It will just run a bit rough. In a marine application, you'll be replacing internal parts.

Remember the saying, "A fat motor is a happy motor."

Rick.... thanks, I have heard that the 496's do the same thing !
CORD.. thanks too.I'm aware that this is done intentionally .. to a degree.I'm just hoping that we can get it in a happy medium,as you can imagine with gas @ 5.00 bucks a gal. we don't visit the upper RPM spectrum too often.

What's the deal with the original fuel pressure and the new 'reset one' ?

Because we thought that this could have been an injector problem they were checked recently for dirt and leaking and are O.K.

You can relax a little more regarding fuel quality if you install a 100 micron filter in the fuel line after the fuel pump & as close to the fuel rail as possible. Then no trash can make it into the injectors. I have heard of baseline fuel pressures from 27-45# so I personally would not commit to any of them. I run my '97 454 EFI Mags-Procharged at 45# base and 60# at 5# boost. I definetly would NOT like $5.00 gas......that's 4 cylinder prices!! Now we know why there are "blow boaters" out there!

You may have solved your problem already. My boat also has the 502 MPI and I had the same problem with soot on the transom. This summer I replaced the factory flame arrester with the K&N setup from Rex Marine and the soot went away.

I cleared up alot of the soot from my transom by installing an MSD-6M. As far as performance gains, I saw no measurable improvement. But that is what I expected from a basically stock motor.

Thanks for the continued input guys.
I kind of felt that there was a hint of 'at your own peril' in the response from cord.It seems though that everyone on this board agrees that you have to get rid of the stock Merc cast iron junk exhaust manifolds sooner or later and this is the point that we are at now.But does this start a chain reaction of events that compromise the reliability of the engine ?It seems that when you upgrade the exhaust you are also advised to improve the in air flow and then mandatory to regulate the fuel press.
Due to the outrageous costs of everything boat related here this is the absolute extent of what we will ever do to this engine,not to mention the difficulty in finding someone up to date with the technology.. I believe ours is the ONLY 502MPI (of any make) here !
I'm still curious about this fuel pressure issue though and how the new part works (DUH..).If its of any interest to know ,our fuel is supposed to be a consistent 93octane (lead free)..I guess that you should get something for $5 a gall. huh ?

I have the 496 HO, The k&N helped but Ive read a couple times that 93 octane is to high and excess octane doesnt get burnt, thus it ends up on the transom. I also always used 93 but before summer Im checking more into 87 or 89.

madbouys,,,, Now I am going to really hurt your feeling,,,,, part of your problem is that you ARE running 93 octane,,,, Merc stock motors do not have the compression ratios to support that high of an octane,,,, 93 actually burns slower then 87 octane and can attribute to some of the problems you are seeing, soot on the transom etc,,,, also, probably not noticable, but you motor will perform better and get better fuel economy on a good quality 87 octane.

That said, I dont know the fuel quality in your area, but if you can get a good grade 87 octane at an automotive gas station, by all means us it !!!!!!!!!! Like you said, you are not running yuor motor hard in the first place due to pricing, so if you happened to get a bad tank, it would not effect you like someone who runs full throttle most of the time.