black smoke and heavy fuel consumption

my toyota engine is model 5A and i did engine overhaul due a slow knok. i replaced all the requredparts.after the overhaul the engines rpm on idle went down very much to a point that the engone sometimes stops running on its own. again the idle is not smoth running and it has alot of miss when engine is running. when the vehicle is moving ,the engine usually stops when am changing from high gear to low one especially while enganging gear 1from 2.it produces black smoke and consumes alot of fuel.the check engine indicator does not show but oil pressure indicator shows when idle goes too down.engine shakes on idle. please help.