The idle needs to be at about 900rpms. If it's not there, turn it up. These bikes will idle as low as 600rpms when they're hot. Resist the temptation to think its okay. There's a 45 pound crank buried inside the motor. When the idle is set correctly, it is always making forward progress. You turn the idle to low, and the crank starts more of a jerky motion. Maybe you want that, but I wouldn't. It causes premature wear of lots of important stuff...of which egos are not one of.
My thought is they used the 42, because a 40 is plenty big for the motor. A 45 is actually on the big side for these bikes. There's only so much air you can suck into a cylinder(s). You want the correct air fuel mix. If you go to big on the carb
...ie a 45, you're moving more air, and would need to compensate with extra fuel to avoid being lean, especially at the top end as far as speeds and WOT
when your moving the most air. These carbs a re a simple afair, and they are constantly struggling for a compromise.