Help! Slant 6 keeps dying

My 67 225 was plagued by this, also. Never got to the bottom of it, but I suspected the Holley 1bbl carb body -- the idle mixture screw had been tightened so hard that there was a crack in the carb wall. The hole was also ovaled out so that the mixture could not be precisely controlled. Since new bodies for these are not available I decided to go 2bbl.

People always say "vacuum leak" but I never found one.