Dave

idle 14 in.hg
Sharp rev 20 in.hg


14 idle is far to low, should DROP on rev & increase on overrun

Are you sure that cat or exhaust are not blocked ? does the cat rattle ?

Are the vac readings the same with maf disconnected. ?

need to check compressions AND valve timing I think.

does O2 switch at idle ?

Maf voltage is low, but that may be due to low air flow, O2 voltage should be high on acceleration, but could be effected by misfire.

Has the Fuel, pressure and flow rate been checked ?

possible that the maf has been damaged by the popping back. could be more than one issue here.

cheers Aidan