Thanks! It seems to happen even when the motors are warm. I have a lot of no wake areas by me so I could be almost idling for 30 minutes to 1 hour. If I go from 1500 to 2200 i get a puff of black smoke. Thanks
A small amount of Black (fuel smoke) is common on mechanical diesels. What you are seeing is the less than completely burned fuel coming out the exhaust. This is caused by delivering more fuel to to the engine than the turbo can provide air for (Turbo lag). Once the turbo spools up the exhaust should be clear. Yanmar recommends a "Turbo Wash" process that will reduce this by eliminating soot on the turbine. Depending on your accumulated hours it might be a consideration.
Regarding the black smoke, I get it too if I accelerate onto plain quickly, one big puff between 1500 and 2200 rpms.
If I slowly accelerate up to speed, there is hardly any noticeable black smoke.