I have a 496 Mag (non H.O.) in a 2003 Velocity 260. At idle upon start up and many minutes after, I notice fuel in the water behind my boat. I realize it doesn't taek much gas in the water to look like a lot, but how much if any should I really see? I have also seen a decent amount of black soot on the transom after about 15-20 minutes of running. Some days it is worse than others. I had a mechanic plug into the computer to read any codes a month or so ago. At the time he also tested the IAC Solenoid and found it to be faulty. So I have since put a new one on. I have been told the 496 mag does run a little "fat" by design, but it seems to me mine is too rich.... I pulled and replace the spark plugs and they looked fine when I pulled them but figured I'd replace anyway. Also I have a clean Filter/arrestor and have also removed the throttle body and have cleaned that as well. I realize this is a broad question and is subjet to opinion..... But what is normal? Does it sound like my condition is rich or is this soot normal? I expect a bit of soot if out running hard all day, but I am seeing a generous amount after only running it for 20-30 min at 3500-4000rpm...

Some of the black soot that is appearing on the back of boats is now a condition that is caused by the oxygenated ethanol added fuels we are receiving now as part of the new emission standards .We see it on the dyno versus test fuel even when we are reprogramming the ecms on 496 engines. We also see a lot of negative spark timing (periodically up to -12 degrees) Mercury uses for getting the engine to idle down and slow when putting the engine back and forth in gear and out of gear.

These two condtions coupled with some boaters running premium fuel 91-93 octane in 496's makes the situation even worse.
Higher octane pump gas fuels burn slower and the burn tends to be incomplete when used in stock 496's with stock timing curves.
The 496 does not need anything but 87-89 octane fuel in stock form and even with our Raylar kits and Raylar prepared engines.

DON'T RUN PREMIUM FUEL IN STOCK 496'S!

On the 496's that are supercharged and with special supercharging reprogramming done by Whipple you must run 91 minimum octane.

Also excessive idling with your stock 496 will increase this sooting due to the late timing and poor burn of the stock 496 iron heads.
Just some info for consideration.

Best Regards,
Ray @ Raylar

I have been running premium..... When I bought the boat it had almost a full tank of gas (80 gal) that had been sitting for an unknow period. I have continued to run premium...... I just filled it up again with Premium...... I guess I will run it down and refill with 89. Thanx for the info!

Hey Raylar,

Will running Premium effect performance? Not that I plan to continue to run premium, becasue I hate the soot. But will it have any effect on the actual power/torque this motor is supposed to make?

Higher octane fuel has a higher flash point and does not burn as completely. The 496 engine is designed to run on 87 octane and 87octane is the best fuel for it. It will run the cleanest and make the most power with 87 octane.

Well this may explain the lack of power over 4000 RPM..... Maybe? I am just 200-300 RPM shy of where it should be at WOT. Bummer I just filled it up.....

After reading this I may have to reconsider my next fill up. I have a 2001 496HO all stock including ECM. I started using 93 octane shortly after buying in 2002 because I found it helped with the soot problem. Haven't had any problems but maybe I need to switch back to 87 due to the ethonal and see what it does. Thanks for the info Ray

put a fuel gage on it,,,there not much money,,,also make sure your fuel pressure regulator is working,,,

can you guys even get 87 at the docks? around here i only see high test or diesel

and look at the water temp. if it is cold at idle... 140 ish, it will sit at the rich end of the fuel curve thinking it needs the extra fuel until it warms up.