Also, sync your carbs first(this will screw you up if way off), set the idle to spec. or lower for adjusting purposes.If you still get no responce, I'd remove the needles for inspection. Check the ends for crud. Then try the A/F screws again. +1 to seafoam or your preferred fuel treatment. It can never hurt.
You can replace all 4 needle sets, with springs o-rings and washers for $25 plus shipping, if you need them.