There will always be combustion build up on the holes in the injector nozzle.
This can cause smoking and rough running.
On a frequently used engine that gets good hours a year there are chemicals that will help .Stanadine is my first choice.
For 35 year old injectors with internal springs that age differently , along with the low load cruising common on rec boats that carbons the engine , a rebuild/renew is in order.
The fuel burn may only get a bit better , but the smoothness and noise reduction is worth the bucks .
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