I have had a lot of trouble getting petrol fueled engines to pass. Here in AZ, all 1967-1996 cars have to pass emmissions. What I normally do is to change the plugs, put in a brand new air filter, and step up the idle speed considerably from factory specs (usually 1200-1400 RPM). Sometimes the DEQ can grant you a waiver, since this is car was not made for the USA, but sometimes not.
Hope this helps...the alternative is a diesel benz...they never fail smog tests.