D16z6 Serious Problem With A/c

I Have A Honda Civic 3door 1993.with D16z6.i Installed A Used Air Condition,the Air Condition Is Working Fine But The Engine Is Not Able To Maintain Idle Prm (750),when The A/c Is Working And Sometimes The Engine Fails To Keep The Load.and Stops.i Need Help, Knowone Can Tell Me What Is Wrong.becides That I Dont Have Any Other Problem With D16z6.