i have 500 efi and have ran the boat at lake tahoe all of its life 6800 foot elevatiin. I brought the boat down to lake mead at 1100 foot elevation and put new battries in it. I went out and ran it. The boat ran perfect 5100 spinning 30 pitch props. When I pulled the boat out on the trailer I noticed a very black transom. The fuel in the boat was 4 months old. I added 15 gallons fresh 91 fuel to the boat. could the black on the back be from marginal fuel? Or the elevation change? the boat did not run as if it had any issues.

Personally I would never run an engine at 5100 rpm with 4 month old gas. Todays fuel degrades so quickly and its just not worth the risk. I always add new fuel and still keep the rpm's around 4k the first time out. Detonation is not a pretty sound!
This doesnt answer your question but I thought I would mention it.

Run 87 octane fuel in 500efi's. It will make the most power and burn the most completely. Higher octanes will not give any better performance and will leave a sooty transom. 4 month old fuel is fine as long as its not ethanol/E10. If its E10, it may have some water in it, but that won't cause the sooty transom.

factory EFI's are pretty rich to start with....

WHAT HE SAID

I can't comment specifically on the 500EFI, but we tested a 500HP and it was 11.6:1 A/F at idle. VERY rich. Idles just as nice at 13.0:1 and I thought that was still conservative and could have gone leaner.

As these guys have pointed out... I too was always of the mindset that I'd run 91+ octane "just to be on the safe side". I had the same very black transom problem. Switched to straight 87 on the advice of a friend and it literally cut in half the amount of soot on the back of the boat. Big difference for me.

Thanks fo all the replies, I feel the same on the fuel grade.the boat runs perfect. I hooked a DDT to it today and no codes.I also did a leak down over the winter and all cylenders were holding in the high 90sout of 100 psi. We all know that MAP will be adversly affected by poor engine condition.

Since you changed the battery it may have run extra rich for a short period of time until the computer "relearned" the parameters for your engine and elevation as well

I have twin 500 EFI's and have always had some soot on the transom. Spoke to Merc help line years back and he told me that is was normal, they set them to run a little rich. A little rich is better than lean for sure.