I own a 1990 civic dx... I've recently replaced the fuel filter , cleaned injectors , replaced oil pan gasket and change , replaced o2 sensor , replaced exaust downpipe gasket and replaced distributer cap and roter... but its still running rough... what can I do to help smothen my engine

. Clean the Idle Air Control Valve (IACV), throttle plate, and throttle body:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-7.html#post1054149

. Replace the Ignition Control Module (ICM) and coil:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-4.html#post265896

Um... where is the IACV located ? Picture would help

Backside of plenum--you can't miss it.