[Leonard]

If you blow into the melodica without any keys pressed you should feel no air movement, If you do it could be the spit valve or a few of the keys have missing pads.
Its not good, the joy of a melodica for me is the easy way they play, it should never be so much air.
Also the question about the mouth piece, that shouldn’t make any difference, from memory I think that the bottom of that melodica has alot of tiny screws they make a seal inside that is important, don’t overtighten them.