[dimefury]

could be a bad intake gasket (manifold or carb) check in the normal way, with carbcleaner, brakeen, etc.

but it could also be a bad wax cylinder.

some of those motors' (z22) carbs had a wax cylinder that would hold the throttle slightly open to warm up the engine on a cold start. the wax would then warm up and pull off the fast idle.

look for a round cylinder on the side of the carb with a little rod sticking out of it (that may or may not be leaking blue stuff). it was not uncommon for these to fail and hold open the throttle a little and cause an un-tune-outable(?) fast idle.

this might be your problem, the kit to fix it is available through nissan.