I have 2ea. 228 Zeniths (well, one says Bendix on the float-bowl casting) on an Offy manifold, mounted on a 261 with Fentons and an "RV" cam. The engine will pull zero vacuum, as near as the gauge will tell, at 2900 RPM. Above that and the vacuum starts to climb slightly. I have no idea what venturi size is. The present truck came to me with a 235 and YF Carter. I put that on my girlfriend's pickup when she was in a jam and never got it back. So one or the other and now both of the pair of 228s I bought for $25 50 years ago have been on various rigs since then. I did put kits sourced from Zenith in them one time. Now they could use throttle shafts. As they say, "reliable as a stone axe."
Just as a point of reference on carb sizes, I have a 500CC Matchless-single scrambler that came with the equivalent of the 32mm Amal carb it now has. The optional "GP" carburetor was 1-3/8 venturi but had no idle circuit for the street. All this on 30 cubic inches per cylinder (times one). That would be like a 180 cu in six but turning 6,000 RPM.
