Have you checked for a restricted exhaust system? Low manifold vacuum and backfiring on acceleration are both symptoms of exhaust gas that can't find its way out of the system easily. One of my customers was turning around on a narrow road and backed into a muddy shoulder berm- - - - -shoved about a foot of tightly packed dirt up the tailpipe. The car barely made it to my shop, and it took us a while to figure out why it was sluggish and wanted to backfire.

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!