I had exactly this problem when I attacked the TB of my '86 back in '96. I had just bought the car from a friend -- he had taken the car to an incredible hack, who took friend to the cleaners.

Friend was so disgusted he sold me the car and bought a new Suburu. We're both delighted by the outcome.

The hack mechanic was unable to diagnose a misadjusted throttle switch and dead CIS motor, so kept adjusting the idle by tweaking the mechanical setscrew (a no-no). This opened the throttle plate a hair each time the car was serviced.

But over the months of using the car, varnish built up in the TB, blocking the slightly open butterfly (did I mentioned that moron mechanic also didn't service the flame trap, so everything was PLUGGED SOLID?). The new varnish buildup lowered the idle over time, but moron mechanic would tweak the setscrew again, and so on.

When I washed out the TB, the butterfly was open enough to drive a P1800 through. Hence, high idle, like around 3000 rpm.

I readjusted the mechanical setscrew, butterfly, throttle linkage, and throttle switch. Installed new CIS motor. Presto, back to 850-900 rpm, and has been for 7 years.
--
Don Foster (near Cape Cod, MA)