fuel evaporating in carb?

This has been going on for some time, I have a Mighty Demon 750 Mechanical secondary, the fuel level visable through the sight glass drops over time, in a couple days it drops from the top notch to the middle, 2 more days and its at the bottom. Ive pulled the carb, replaced all gaskets, power valve and more. after much frustration I pulled it and put another carb on that i had laying around, that one does the same thing. Next thing I did was set up the 750 on stands on the workbench and fill the bowls with fuel, and monitored for leakage etc. It did the same thing, but no visable leaks,So Im wondering do todays gas evaporate that much, locally we have the winter blend with methanol? I think in it.
this may have been going on for quite a while and just never noticed it before. Any thoughts?