Want to be confused?

When air cools and drys, you get more air in through the carb. More air compared to the same amount of fuel means a leaner run. So if the air temp drops, you often have to richen the mixture. Which also means you often have to lean out an engine when it gets warmer and more humid.

But that's the opposite of what you just experienced.

When it is really humid and hot, engines also run hotter, which can cause an engine to over heat. To reduce the running temp of an engine, you run it a bit richer. So on a hot humid day, you might richen to reduce running temp.

So, when it gets hot and humid, you're engine performance will probably change. You may need to richen, you may need to lean out a little, depending on the running temp of the engine and what the weather has been doing.

Also, running temp is affected by such things as prop size and fuel, so two of the the same engines in the same airframe may need slightly differnet adjustments in response to a weather change if they have different fuel and/or props.

I've been flying on days where we had a 20+ degree temp drop as a cold front came through. After the t-storms had passed, all the engines needed re-setting.

Oh, and the OS engines often have a rather broad needle valve (a good thing), so it often takes more "clicks" to make the same mixture change on them than it does on some other brands of engines. Some engines are much more "peaky" and go from too rich to too lean in very few clicks. These engines are also more sensitive to weather changes.