My '85 12A developed an idle problem: fully warmed up, for instance at a long
stoplight, the idle would be normal and smooth, for twenty or thirty seconds, then drop to 600 rpm or so and falter. I replaced the fuel filter and fuel lines to the carb, and found a vacuum small leak in the rats nest. No improvement. This went on for a month or so, but the other day I was out driving and the idle all by itself jumped to 1500 rpm and stayed there. When I got home and opened the hood, smoke was coming out from underneath the firewall. This is a stumper. Any thoughts?

 

Welcome to the forum. What was the smoke like? What did it look/smell like? As far as the idling problem is concerned, I was sure it was just an idle mixture problem until you mentioned the jump to high idle w/smoke.

 

The smoke smelled like, something burning I guess. It was suggested to me that it might be the vac hose from the ACV to the intake manifold starting to burn, or clogged cat material. the mystery is in the sudden jump.

 

With the car off, look down into the carb. Are the two flaps (primary and secondary) that are way down in there fully closed? If not, start checking the linkage for something sticking.