You need the thick (over .250") insulating gasket. I was running a 4175 until recently. I had the same trouble. Without that insulator gasket, no way was the carburetor tightening down. It was still very close with the right gasket.
I didn't like the 4175. I never could get it right. The idle mixture screws are opposite the "normal" Holleys, so keep that in mind when trying to tune it. Good luck.
