260 hp nautical engine

Hello everyone, I go to you because my mechanics level is basic.
I have acquired a boat with a Mercruiser gasoline engine of 260 HP of the year 89 and I find myself that in the dead point (without raging the hell) the accelerations tolerate well, but when navigating (with the clutch gundy He told me that he also happened to him and that his mechanic told him that it was a problem of a sleeve that was chafado, but he repaired it and told him that he would not happen again.
In principle, this engine was full of the boat before the summer and it was made an exaustive review of an official mechanic, so I want to think that it cannot have a serious problem. I would appreciate that you give me ideas of possible causes and solutions starting with the easiest to repair since if you think it is very complex, I will not have a choice but to take it to a nautical mechanic and that they save me.
Thank you very much to everyone.

Is it gasoline injection or is carburation?

From what you say they can be two things:

draft problems in the ignition, the ignition sparks are not very good or jump at the wrong time, the gasoline comes out without burn and when when they reach the escape topa with the incandescent walls the flash sticks.

Lack of food, if the engine goes hungry, when subject to work is wrinkled, it heats up more than normal and yields little, sometimes they also petarde, but they do not usually give strong flashes as in the case of the ignition failure.

Another thing that can be is valve adjustment, but if the mechanics have reviewed it, it seems to me that the shots are not going there.

All the best.