Inspection trouble in Texas - emissions

[Stringer]

90 Suburban, 350 TBI, can't pass low speed emissions. Tried 3 times at the same place - same result. It's the NOx category that's failing.

This thing sat for a while before I bought it. I've burned a tank of premium and a couple cans of Seafoam through it. It runs fine - does not have any codes stored.

Anybody know what to look into next? Any sure-fire fuel treatments? Possibly timing? (I could go to another station but I'm afraid another one wouldn't pass the windshield!)

Trying to upload the results sheet. Not sure if it'll post or not.