I just rebuilt a 350 for a guy and the thing runs pretty good cold but runs worse when warmed up. The plugs show to be black from fuel(no oil) and they smell like gas, but no black smoke from pipes. It has a stumble when taking off from a stop(normal driving), which after driving a few miles turns into a miss throughout the rpm range,and sometimes backfires.(I think its fouling the plugs) I've tried timing all over the place, 36 total, 40+ total, 30 total and it still continues to blacken the plugs and stumble.Runs ok at wot but should make more power. Its a brand new holley street avenger 670 and I've adjusted the idle mixture screws with a vacuum gauge multiple times, changed power valves(went from a 65 to a 45) reduced the primary jets from a 65 to a 64(thats the smallest I had) changed the secondary spring to a lighter one, and checked for vacuum leaks everywhere with carb cleaner. Wouldn't a vacuum leak show lean plugs? I wonder if the marks on the balancer are wrong, its a new professional products stock replacement balancer? Any help would be appreciated. Here are some specs:

Procomp angle plug heads 190cc runners(never buy again)
Comp Cams 12-234-2 adv duration 256/268 dur@50 212/218 1000-5200 rpm
I degree'd this cam in at 1 degree advance
Edelbrock eps dual plane intake
Engine Works ready to run dist. with petronix coil
Engine has exhaust manifolds, we could'nt get headers to fit his car