Actually, a hot, humid day CAN cause your engine to run richer. Hot air is less dense than cold air (resulting in a higher fuel to air mixture), so if the engine was tuned on a day that was say 15 to 20 degrees cooler, then you might have to lean it a little to get equal power. When cold winter days come around, I quite often have had to richen the engine by nearly a full turn to get "normal" power.

If you're not confident setting the engine though, I'd leave it for a more experienced guy.
Dennis-