I hate to see some one get no response at all, I just hope I don't get jumped on again!!
I read in another post that arizona speed does/did a lot of developement work with mercruiser & efi, so you might check it out with them.

W.W.> Not sure if your area runs winter time additives but they can cause some extra buildup. The HP motor also seems to run cleaner on 87 octane which you may be using I just know if you run 89 or 92 it will have the black transom you mentioned. You probably know this already but thought I would mention it. Tyler has a good rep but I am not sure how much EFI work they do.

Bill, you might want to try and get a hold of Lingamfelter performance. I know he helped Merc alot with the development of the 502MPI Mag. He might also have helped with the 500EFI. There is an article in one of last years PowerBoat mags about his company.
Eric

Wild Willy,

My 500efi sooted until I hit about 50 hrs. Then it dropped by about 80%. and runs as clean as expected now.

Has anyone looked at their sparkplugs and tried one step hotter sparkplugs. This sounds like a rich condition. The MSD will also help[ in this matter.

thanks guys for your input!!i will try next heat range in spark plugs and stay will 87 octane fuel!! BILL :p :p

I would think that hotter plugs would keep the plugs clean but wouldn't change the fuel/air mixture so you would still have the sooting problem. In order to get rid of the sooting you need to lean out the mixture.

I don't agree with leaning out the mixture. As Cobra Marty has mentioned, increasing the heatrange of the plug or adding an M.S.D. will lessen the transom soot to some degree. The reason being that the hotter plug or increased spark with an M.S.D. will more thoroughly burn the air fuel mixture. Personally, I have already installed an M.S.D. this winter. But, I can't tell you any results yet. It's still winter in the North East.

wild willy,
I am sending my ECM's to Tyler to be reprogrammed, I have talked to a couple people who had him do their ECM's and they are quite happy with the results, he charges $250 a piece to do 'em. I am also going to try #10 rapidfire plugs this year(they are a direct interchange from the MR43TS that are standard equipment).
Paul