2003 500 efi black transom
 

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:drink:

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.

dirty transom
 

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.

There is nothing to relearn. These engines do not have O2 sensors for feedback.

Is it worth having them re-programed to get burn less fuel and have less soot on the boat? With gas prices getting higher and higher all the time, I would not mind saving a little gas. Could that help performance?

I think that leaning them out at idle would be a good idea. Not only does it save on cleaning the transom, those guys with alot of idling will be happy to not load plugs up all the time as well. 13:1 would be a nice consevative start.

That is where I set up my carb. 500. never had an issue in 150 hours.

I have not yet seen what the stock air/fuel curve looks like on the 500EFI's, but the 500HP before it was 11.6:1 A/F at idle and never crested 12.0 until after 3000rpm.

What you really need is a wideband O2 monitor to see where your engine is at now. Once you know how lean or rich it is, then you'll know what you have to modify and gain.

ECM's?
 

I have a pair of '05 525 EFI's in my boat and I also called Mercury Racing. They told me to have the ECM's removed by a certified Mercruiser dealer and send them back to Mercury and they would reprogram them, (under warranty), to make the engine run leaner at idle speeds. They told me their original set up was a little too rich at idle RPM's causing sooting on the transom. They are currently on their way to Wisconsin as I type this message.