just rebuilt runs bad

just do process of elimination. 3 things to make an engine run, fuel compression spark. ok so you have fuel, check. you have spark? obviously if its backfires through carb. so that leaves compression. now youve checked ignition timing over and over again, good great, but have you check your cam timing? it could be a degree or two advanced. now, if that is good, then the problem lies within the valvetrain area, ie stuck valve or something... just tossing ideas out there. and i DO agree, the machine shop where you had this built should figure it out, not you.