Paul - on a 161, the (3) YH's may possibly be large enough with a plenumless manifold. And the 3 would probably be too large if you used a plenum.
As Joe stated earlier, all three YH's should be identical as to tag number.
Start with the stock calibration for a baseline, including float levels. Fuel valve orifice values must also be identical. Make absolutely certain that all carburetors are synchronized before making any calibration changes. "Test" calibration changes (NOT PERMANENT) may be made my raising/lowering the fuel level in the bowl from specifications plus/minus 1/16 inch (yes, fuel level IS that critical). These changes would be roughly equivilent to one calibration step. These test changes can help you see what a change might do, without purchasing costly metering rods. With the YH's, recalibration is done by changing rods, not jets; as changing jets will not provide a uniform percentage change (calibration is jet area minus dynamic rod area). YH metering rods are not interchangable with other Carter series, so calibration parts will require custom machining.
With an engine this small, leave the choke plates in place; whether or not you use them for a choke. If you opt not to use the chokes, affix the choke plates in the open position. The presence of the choke plates will act as a "straightening vane" to help eliminate eddy currents in the air entering the air intake; thus improving flow.
Sounds like an interesting project. Keep us posted.
Jon.
