Topic: Black soot spits out of my exhaust????

[L.A. Nomad........]

So my motor is very warmed up, I get home from my 22 mile drive from work. I park my 1976 CB550(25,789 miles), the motor is still running and I rev it out a few times. My lower left exhaust blows out black Soot/smoke. Should I be concerned? My bike has 4 to 4 exhausts/ foam airbox/ stock motor.! Oh, I now run 20-40 motor oil. I just drained my oil and it was all black, I've put about 900miles since my last oil/filter change. Any thoughts anyone??

Nomad

[Gordon]

Pull the spark plug on that cylinder and see what it looks like.  Sounds like you might be running rich on that one, could be a stuck carb float valve.  Do you have any gas coming out of the overflow tube? 

[TwoTired]

Oil will turn black just from being heated while the engine is running.  Color doesn't mean much.  It can be black and transluscent and still be clean.  On the other hand when it gets murky black grey then it's dirty.
You did change the oil filter didn't you?

If I were you, I'd pull all the spark plugs, inspect the tips and compare them to the pics at:
http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html
This will tell you if the cylinders are running evenly and rich, lean, or just right.
Could be you need the carbs balanced with a (the cylinder that's spiting) slide is open more than the others.  Perhaps a carb float is punctured.  But, the evidence and pointer to where the problem lies is in the combustion chamber.  And, the plugs are your window into there.
When's the last time a complete tune-up was performed on the bike?

Soot does build up in the mufflers and they shed the exess at random intervals.  If the plugs show up even and the proper color, maybe it's just that one's turn to shed the build up?

Cheers,