Black transom
 

So we are out yesterday for a little 100 mile cruise.
As we get back i notice my friends transom is really black !- The boat runs perfect , fuel was good as we all filled up at the same place etc... and he states it's been doing that for several months now- no matter what grade or brand of fuel he runs.
The boat is a 2002 38' Scarab w/ twin 500 efi's (100hours)

Could the ECM's be retarding the timing so there is too much un-spent fuel when he is on the throttles making it too rich ?
or ?

Re: Black transom
 

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

Re: Black transom
 

Nope- Flat transom....

Re: Black transom
 

Is he burning premium or regular?

Re: Black transom
 

You need to burn regular 87 octance with the 500's to prevent the sooty transome.

Re: Black transom
 

is water temp up to correct operating temp? if not trash in thermostat? if engine is temp not runing at correct temp it will flood with fuel.

Re: Black transom
 

Its those wonderful "additives" that the EPA and the oil companies are blending the fuel with. We see this on our dyno pulls on pump gas here in California no matter what octane or what tune we put on the motor. We switch to non-pump gas and the exhaust cleans right up to a nice light gray even with some fairly rich A/F settings. I am not a chemist or fuel expert, but I believe this is what they call "extra oxygenation" with additives. If you run pump gas in most states now get used to it, its not going away anytime soon! "sleep tight, your elected officials and oil companies are awake tonight!"

Ray @ Raylar

Re: Black transom
 

would running 93 in 525's cause the same thing?

Re: Black transom
 

Yes, you should run 87 you will also make more power on 87.

If everything in the boats that are turning black check out ok you can put an MSD box on and it helps clean them up a lot.

Re: Black transom
 

87 always - he tried a few other octanes even up to 101 for the fun of it but no better.

Generically speaking a few months...years....not sure it's just that i noticed how bad it was. Apparently it get's worse if he drives hard and is 3/4 - full throttle for long periods at a time.

Water temp was normal and never an issue in the past.

Any more advice would be great.