Thanks, Roadtrip, for reminding me of Rule #1; When all else fails, read the instructions. I guess I saw 88 Spider instead of 80 Spider which means that not only does the car have Spica, but maybe I need TRI-focals!!
If the engine doesn't drop down to idle after a few minutes from a cold start, I'd suspect the following;
1) if the car has the four butterfly intake manifold, the rubber idle speed o-ring may have disintegrated (forget what year the single butterfly started)
2) vacuum leak(s)
3) Spica system needs adjustment or repair
4) distributor advance mechanism sticking
I'm not exactly sure what you mean by "throttle linkage will respond manually only"