Black smoke can mean several things. Basically its unburned fuel
. You can get it temporarily if you overload the engine, which you might have done trying to get off the shoal. It can also mean that you have a problem with compression
in a cylinder or perhaps an injector.
One thing that is fairly common is to have the exhaust elbow
plugged up with carbon. Usually when this happens you will notice a decrease in max rpm
as well as copious quantities of black water coming out the exhaust
I'm afraid that if this is a common occurrence you'll need to do some diagnostic checking. The simplest to fix will be the exhaust elbow
. Remove, inspect. If filled with carbon gunk, soak in a dilute muriatic acid solution until clean. Inspect for other damage. Mine had corroded a hole the size of a quarter which made for a seriously dangerous source of monoxide in the cabin
If you can deactivate one cylinder at a time, (my old Yanmar
had decompression levers for each cylinder) you can sequentially decompress each cylinder. If the engine sound changes, chances are the cylinder is firing correctly. If decompressing a cylinder makes no difference, then that cylinder is not firing properly and will need attention.
This is not complete by any means but hope it helps.