[jclark003]

Some soot is normal. Is it happening more on one side? How many hours on boat? Salt or fresh?

After coolers if plugged on the air side can cause your engines to soot. You should check your engines maintenance schedule and past history to determine if a cleaning is in order. before heading down that route I would first inspect the air cleaner as if that is restricting airflow you will most certainly soot up the transom. Other culprits include fuel pressure (sucking air through a fitting, Racor or secondary filter), bad injector or warn turbine/hot housing off the turbo which may be accompanied by somewhat poor mid range acceleration.