Look at this thread.
http://www.peachparts.com/shopforum/tech-help/228018-420sel-rough-idle.html I know this is about a 420SEL but it applies to all 116 and 117 engines. You will find your answer. Read my post and the ones after it. BP5ES plugs are avaliable at Advance Auto if you have one in your area. DON'T ask for plugs for your car, ask for NGK BP5ES Part Number 7832.
Try the plugs only at first. If it still misfires at idle then check for vacuum leaks.
PaulM