1988 9.9 merc carb problem ?

[illinoisduck]

I have a 1988 9.9 merc that starts well and idles good . Upon acelleration it wants to be sluggish until it gets to about 1/2 to 3/4 throttle then runs fine all the way to top end . I have tried choking it which does not help . Any help would be appreciated . There is a brass screw near the top on the front of the carb , is this an adjustment screw for idle or fast ?? thanks , DUCK