Clean the pistons and get back to the floats. You'll find out later if the staining was caused by poor tuning.
So he's telling you that unless he unscrews the mix. screws to somewhere down near 4 turns (on one carb only) it runs too rich? Someone has something backward. The farther the mix. screws are unscrewed (dropped), the richer it will run. Based on the info from your mechanic, you already know more about your carbs than your mechanic and have the desire to tune the floats yourself. The only thing that's stopping you, is you. Go for it. WE CAN DO IT.