Re: 1989 70 hp Merc
2 stroke idle circuits have to be overly rich, moreso then 'best idle' to provide the additional fuel necessary for the motor to accelerate.
As carbs dirty, the small idle passages show the first symptoms, poor acceleration, as the condition worsens then idle is affected.
Clean carbs, pay very careful attention to idle screw positions, G E N T L Y screw each one in until l i g h t l y seated and RECORD the exact number of turns so that once finished you can reset each idle screw to the SAME position it was before you started. This will save you LOTS of time adjusting idle afterwards.