Stumped by a timing problem

[MAC71]

I have a 69 C20 that has a 396 in it. When I got it in December it was all stock including the distributor and carburetor. The PO had the vacuum advance disconnected and I discovered that with it connected the engine back fires threw the carb. That was with base timing at 8 degrees, but did not check anything else as I pulled the engine.
So now its a rebuilt 396 .030 over, 9:1 compression edelbrock manifold, Holley carb, HEI distributor with a Summit racing CD ignition box, stock cam installed at 0 degrease, and it still does it.
I also checked to make sure the timing marks are correct and they are.
There is no pinging when this is taking place that I can hear and its not a loud truck.
I have tried various timing settings and it keeps doing it so I have to run with out the vacuum advance connected, but I want to sort this out for fuel mileage as I drive it every day.
Currently base timing 12 deg, full advance is 34 deg and the vacuum advance adds in 8 degrees to those numbers if connected. I also have tried both ported and manifold vacuum.
Just as a comparison I did a test drive with a different carb and no change.
the backfire is only cursing over around 2200 rpm or faster.

Any Ideas?

Post are no good with out photos right?