As far as I know, the "insulator block" is to keep the intake and exhaust manifolds from transferring a lot of heat to the carb base. This would cause the gas in the float bowl to percolate under some conditions, and that ain't a good thing.
These insulator blocks are available mail order, seems that I've seen them in the Chevy duty and/or Bowtie bits catalogs.
Some one mentioned that they may be to compensate for the space required by the governor assembly as used in commercial vehicles. Maybe - does' anyone have a truck with a governor AND a insulator block?
By the way, after really close examination of my carb base, I do have an insulator block. It was just disguised by grunge and dirt. It looked exactly like the manifold casting.
