Stock 1950 8BA

[Aviatur]

Hey fellas, this one has me stumped.
Stock flathead with dual exhaust, holley 94, load-o-matic dist.
New fuel tank, flex line, rebuilt fuel pump and carb with 7.5pv and #51 jets, rebuilt dist with new vacuum diaphragm, points, condenser, cap, rotor.
Engine idles and starts perfect, great throttle response but with no load it has a miss. It comes in abouround 1500 and gets progressively worse and will backfire through exhaust. If you stomp on the gas it runs great.

Any ideas?? I am stumped.

Thanks.