If I can be blunt, it's not a carb/fueling/timing issue. It's a package issue. The carb, cam, timing, manifold, compression ratio, displacement, trans type, differential ratio have to complement each other. However, it's pretty hard to make any comment without knowing the rest of the package. What trans and rear axle ratio do you have? The cam specs are also an absolute must.

I will venture this, though. The carb size and type are probably wrong for the engine, and the manifold is made for high rpm. Unless you have a manual trans, and a low rear gear, the double pumper is going to dump too much air/fuel mixture for the engine to run until it winds up. It appears you have a "package" that's always going to be soft on the low end unless you change some major components.