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First thing... Use a float gauge. Honda makes a good one.
You might have noticed already, but there are little spring-loaded pins in the tops of the float valves. You'll want to position the carbs so the weight of the floats closes the float valves without compressing the little pins in the float valves.
Set your float gaugeto exactly 21mm.
There's a ridge of metal around the perimeter of the bottom of the carb. Place the arms of the float gauge on this ridge (two places), ensure that the float gauge is perpendicular to the plane of this metal ridge and measure the height of the float.
The floats are measured at the ends, opposite the float valve and hinge. Both halves of the float need to be the same height, and the height needs to be EXACTLY 21mm (+-) 1/2mm.
To adjust height, bend the little tab that sits on the little spring-loaded pin on the float valve. Do it a little at a time.
It's important to get the float heights as close as you can get to 21mm. Otherwise these carbs will give you trouble.