Okay, I'll take a stab at it.
Popping on acceleration would make me look at two things:
1. Lean fuel mixture,
2. Ignition ( plugs, wires, cap, rotor ).
I doubt that you would have a restricted exhaust problem, since you don't have catalytic converters.
Just to satisfy your couriosity, you could have somebody stab the gaspedal, while you place your hand near the tailpipe & observe the volume of exhaust coming out.
P.S. You say " timing seems okay ".
Check it, maybe somebody retarded it too much.