The Idle mixture adjustment does not make a noticeable difference in accelleration or top speed. It mostly just smooths out the idle. I think they leave it out for simplicity and cost, even the idle jet is usually just a drilled hole in the carb body and not replaceable.
The main jet and needle, if you have one, control everything else.
Every vehicle I ever owned ran better in damp conditions provided the ignition did not leak and cause mis-fires. Check your mixture, I am not sure if humidity makes your mixture richer or leaner (Fred, where are you?), nut mayby you are jetted on the edge.
Also, check that your air cleaner does not get wet, causing your mixture to get too rich.
You can fix this, we have the technology.