Before you give up on the Zenith, you might want to check the list in the link and make sure that it is the right one for the '48 WD. I have an original Zenith on a '51 WD and it works fine when clean & adjusted properly.
Sounds like you might be running a little rich. Recommended initial setting for the upper idle adjusting screw is 1 1/2 turns out. Recommended starting point for the lower main jet adjusting screw is 2 to 3 turns out, depending on fuel & atmospheric conditions.