Not enough information- - - -like when does the backfire happen? Idle, acceleration, steady cruise, etc. Is there any fluctuation in the vacuum gauge? Is the .218 a lobe lift specification? What about lift at the valve, and duration in degrees? With a 1.5:1 rocker arm ratio, a 218 lift at the cam only shows .327 lift at the valves. That's a very mild cam grind, so your vacuum at idle is WAY low for some reason. I'd expect to see 19" or more at a 750 RPM idle. What's your initial ignition timing setting? Have you checked to see if the cam is installed correctly with the crankshaft and cam timing marks aligned?
A Holley 1850 is a 600 CFM, so you're not over-carbureted, but it's a huge step backwards from the Rochester Quadrajet that's supposed to be on there.
Jerry
