SOOT Issue!!
 

06' 35 Outlaw with twin 496 HO's, with 320 hours. Recently had rear main seals and new coupler's installed. Only owned boat for 1 year, so just learning power boating with twin engines. I can not keep the rear of the boat clean. After about 20 mins of cruising the swim platform has alto of black soot. Not caked on, but if you walk across the platform your feet will get black. I have complained about this to my local Mercruiser dealer, and he says that with big power boats this is normal. I have posted this question before and have been told to have the ECM re flashed that there was a updated or something for 496 motors. Can someone please explain this? Why would the local dealer not suggest this? He did run boat and plug laptop up and check codes and performance, but did not make any changes.

My 496 sooted up the transom as well but only after I removed the corsa muffler tips. I was also running premium in it to avoid ethonal issues, some told me that was part or all of the reason for the soot. Whipple can futz with the computer and they say it will clean up the transom.

We have a 7.4 MPI—. 2000 all stock. When we first got the boat I likewise put premium fuel and soot on the transom. Cleaned it off and put 87 octane and no problems. First tank of the season 2 cans of SeaFoam and haven’t had any fuel issues.

3pointstar

Throw a couple bottles of Merc Quick Clean into the fuel tanks. That will help things drastically. There are three different bottles. Use the yellow one.

Only use 87 octane that is what the engines are programmed to use. Find a dealer that will check and see if the computers have been updated.

My Fountains have done that too. Impossible to keep the soot off. Always had a sponge handy to do a quick wipe down of the swim platform and hatch pad every time I stopped as to keep from getting dirty foot tracks on the boat. 1st Fountain had a 600SC, 2nd had twin 502's EFI. Everything I tried, just cost money with no improvement. I just chocked it up to the air turbulence created at the back allowing the exhaust to swirl around and end up back on the transom.

Are you saying a higher octane is bad for the engine? I assumed it was good but unnecessary and expensive. At my marina, they only have 95 so that's what I run. (Stock 496 ho) Haven't used it much but I've never seen any soot on the transom. I have my boat cleaned by the marina staff immediately after it comes out of the water, maybe they do an exceptional job or maybe it's too hard to see on a dark blue transom under an enclosed boarding platform?

RR

Pretty sure your 95 is not the same as rating of 95 in the US. https://www.etuners.gr/fuel/

Higher octane fuel has a higher burn point. The higher octane prevents pre-ignition and pinging because residual vapors won't ignite due to higher cylinder temps.
Higher octanes also will not burn as completely on an engine that is not designed to run them and thus soot up the transom more.

;)Now I'm totally confused....#*+% This is what we have: (lifted from a local fuel supplier's website)

"E20 is 20% ethanol and 80% gasoline
E85 is 85% ethanol and 15% gasoline
Gasoline 95 is 100% pure petrol in Octane 95.
Gasohol 91 is 10% ethanol and 90% Gasoline in octane 91
Gasohol 95 is 10% ethanol and 90% Gasoline in octane 95.........."

Only one petrol that is ethanol free so if you drive a Lamborghini or a Toyota and don't want ethanol, there's no choice.
Always referred to as 95 octane but Im not sure how it's rated. We used to have gasoline 91 (91 octane) but it's been dumped to get everyone on ethanol, except those who can afford the 95. :(

Got no choice, fuel is dear but the boat runs well so I'm ok. Ignorance is bliss.:)

RR