Usually that is caused by a bad O2 sensor.

 

My '90 NA had the same problem. Here's what I did:

(1) New platinum plugs
(2) New air filters
(3) Changed the oil (Yep... that finally got it through... it gets contaminated and causes the hydrocarbon emissions to be high)

I also discovered later I had a defective throttle position sensor... which I'm sure affected the emissions too.

Keep your idle up to around 2 or 3 thousand while you're waiting to get your test... that helps keep the cylinders cleared out.

Hope this helps,
Johann