I started to see a lot of black/brown soot when revving by motor so it determined it could be 1 or 2 things:
- Injectors were becoming clogged and therefore needed to be pulled and cleaned or
- Exhaust elbow was clogged with soot.
At first I was not looking forward to pull the whole exhaust portion of the motor off, but it is super simple and can be done in about 15 min if easy access. Disconnect the cooling water hose from the mixing elbow and then disconnect the whole piece of the exhaust manifold (mixing and exhaust elbows) from the motor by removing the 4 bolts.
I took mine home, separated the exhaust elbow from the mixing elbow, removed as much scale as I could with flat head screw driver, then soaked the both pieces overnight in vinegar.
Thoroughly rinsed and check flow through each with a stream of water. Reassembled, put on a new exhaust gasket and then put everything back together.
Fired up motor and black/brown smoke was gone.
My elbow is original to the motor, 1992, but my motor only has 700 h.