Grump,
You should adjust the carbs with both cylinders firing. It's my opinion (and just that kids, an opinion... not gospel) that the difference in taper(s) is relatively insignificant. The idle mixture screw works like a water faucet. Unscrewing it creates more of a gap between the tapered needle and the o-ring allowing more fuel to pass into the engine. Just like with the main jet needle, a longer narrow taper will give you more range of adjustability, whereas a short/blunt point would work more like an on/off switch.
But unlike the main jet needles, there is no need to re-jet when installing the thumb screws since this fuel circuit works primarily at idle. So if (for example) 2-1/4 turns out is ideal with the stock idle mixture screws, then with the thumb screws you -might- have to adjust then to 2 or 2-1/2 turns out to get the same results. The justification for installing the thumb screws is ease of adjustment.
Here in upper Mid-West, I find myself turning-in the thumb screws 1/2 turn during the hot Summer months.
Mike
