I started this in the General area and should have came here first...

What could the cause be for a black transom if everything seems fine. Good fuel, runs fine etc...

Boat in question is a 2002 Scarab w/ 500 efi's (100 hours) and while running hard we come back with a black transom. Could the ECM be out of calibration causing the timing to retard at high RPM creating a run rich situation ?

let me guess he has an intergrated swim platform... merc had/ has the same problem with the 496 they call it trailering effect what happens is at idle/low speed the exaust gases get traped under the platform and cause this, merc did come up with a fix involing reprograming the pcm

have heard of it with the 500 but not often

What fuel are you running? Higher octanes have been known to cause this problem. Stick with 89.

He has tried all of them but usually runs 87 as recommended.

Any advice ?

Merc sets the ecu's to run rich. That way they are on the safe side. Better than lean and blowing holes in the pistons. I have had success with adding an MSD 6M-2 , it will clean up the soot a bit.

take it to a dealer, they have a service bulliten and a fix which involves sending the ecm`s out to get new maps. your dealer should know all about this.

Are both engines doing it. My 02 500 efi's were very sensitive to fuel brand and octane. Using torco 89 cleaned them right up and eliminated an occasional Spark plug fouling problem as well. Methyl tertiary butyl ethelene can be a culprit along with the aromatics used to increase octane in that sh*t cocktail they call gasoline these days.