I need someone'e help here. I have twin 600 sci's Mercury, completely stock. My starboard motor on the starboard side of the motor turns my transom black...not soot...BLACK! but only on the starboard side of the starboard motor.

Its only on the starboard side of motor though. I've done compression and leak down test, all fine. I checked the plugs and 8 and 6 were a lil black but not fouled. I havent loss power, there is no blue smoke trailing behind me when im running along. I'm not hitting rev limiters. Ive checked the sea strainers and intercoolers...no blockage or trash in there.

My gut tells me its a computer issue and is making the starboard side run rich but thats just my thought.

What do I need to do next to track down and fix this problem?

motors have 130 hours on them.

thanks in advance guys!

swap the ecu's between the two engines and see if the problem follows or stays the same.

Great idea...not sure why i didnt think of that! thanks.

Also check your oil to make sure theirs no gas in thecrank case, would register over filled,smell the dipdtick for a hint of gas fumes...check crank case for blow by..could have some worn oil rings..

If the plugs are not showing an overly rich condition, my guess would be that it is burning some oil. If the engine was rich enough to blacken the transom that bad, it would show on the plugs. Oil burning will show real black on the transom.
If it is oil burning, the first place to look would be the valve stem seals if the engine leaked down good on that bank.
Does that engine have the injectors in the intake runners or is it in the rear of the blower?
Eddie

Eddie isent most new style merc's port injected??

Like on my 3.3 the brass screen/filter on the throtal thingy gets cloged up and runs ruff at idle,rich.Does the 600 have the same thing? Guessing here.

sometimes a very small oil leak from a hydraulic system on your transome will make black from your exhaust stick to the transome on and around the side its leaking from , i,e. trim hoses steering hoses fittings...it can be misleading

i have chased, false engine issues to this simple problem many a time in the passed

but if you still believe you are rich on one engine. wide band o2 sensor would be the way to find out. some performance shops have a portable version to check the air/fuel ratios when tuning

Eddie,
On a merc 600sci.. the injectors are in the intake ports.

Swapping computers side to side seems like a good idea.

I would check the cleanness of your air cleaner on that side also... this has caused my 600 to blacken the transom.

If all that fails.. do a leak down test as Eddie suggested... and he is correct in that it is most possibly a valve seal.

If you have done all that and it is still a problem... an O2 sensor will tell you if it is really running rich.. if that is the problem... you probably have a sticky injector... have them checked and serviced on that motor.

Hope this helps,

Chris

Since it is a multiport injection, it is real easy to determine if it is injector related. Any injector shop can test flow the injectors on that side of the engine. They would be able to tell real quick if that is the problem.
I don't thinkk swapping the ecm will do much unfortunately. It is EXTREMELY rare that anything goes wrong with the ecm's.
Westspeed made an excellent post. Check all of the ancillary things on the transom to see if anything else could be causing the transom to blacken.
Rob, the brass piece you are talking about is a filter for the IAC (idle air controller) Yes, it will cause a rough idle if it gets plugged up since air can not flow through it. Pretty much all of the air that keeps the engine idleing goes through that filter. I use a small K&N filter on mine to stop that from happening. They work much better than the brass ones.
Basically, if the injectors check out OK and the engine leaks good, then I would remove a header and look in the exhaust ports for oil. If a valve stem seal is bad, then you should see some oil on the stem and in the port.
Good luck.
Eddie

Eddie,

thanks allot for your input. I'll see if i can find a place that can check my injectors here in Chattanooga (too bad im not in nashville near you)
I will check everything on the back of the transom as stated.
Will also do a leak down test.
do i need to to get a wide band o2 sensor and check that?

thanks to you ALL for your help...hope to straighten this out soon!!