Back fire is just that, firing of the air/fuel mixture back up the intake tract, past the crab butterflys. Nothing to do with detonation.
Most common causes of back-fire are retarded timing and too lean carb jetting. What intial timing are you usiing, and are you sure all the valves are adjusted correctly?? If only one intake valve is not adjusted so it will completely close, back-fire can and will happen on that cylinder, like having a burned intake valve.
A vacuum guage would show a wildly fluctuating needle if one of the intakes were adjusted too tight, not letting the valve completely close.
Might confirm the initial timing is 12 degrees or so, with the vacuum advance dis-connected.
Back-fire can also be caused by an air leak in the carb, intake or any gasket for same.
Check this stuff out, and if you don't find a cause, let us know and we'll go on from there.