holley 1945 losing its prime

In-line filter with a check valve before your carb won’t let fuel drain back

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I've suggested those before......and always falls on deaf ears.

Regarding the evaporation. Usually, in cold weather, the humidity is low and evaporation rate is pretty high. When the weather is hot, humidity is usually a bit higher and that tends to lower evaporation rate. I'm no expert, though, since I've never experienced it.