It usually does not correlate to a speed, but a temperature. Sometimes
known as "closed loop".

The forums at www.toyotanation.com should have all the answers.

Toyota manual says the feedback system is active between 1,600 - 4,100
RPM, when the coolant temperature is above 17C / 63F, throttle
position switch is on, and the vacuum switch is off (i.e., vacuum is
below 3.35" / 85mm).  But even if the feedback system doesn't work
right it shouldn't cause much trouble because all it does is lean out
the mixture slightly at times, not enrichen it the way many feedback
systems do.

The factory manual says to hold the engine at 2,500 RPM for 90
seconds, and while maintaining this speed, measure the voltage at the
E (brown) and Ox (black-white) terminals of the socket.  It should
fluctuate between 0 - 7V at least 8 times within 10 seconds.

The feedback system won't work if the throttle position switch never
opens and closes.  It's supposed to close when the speed in neutral or
park reaches 1,800 RPM, and this is adjusted by disconnecting the
switch from the computer and measuring the switch's resistance while
slowly raising the RPM in neutral or park until there's continuity.
The adjustment screw is at the end of the throttle positioner
linkage.  There are two screws there, and the one farthest from the
throttle positioner is the one you want to adjust.  The throttle has
to be opened only a tiny amount to trigger the switch, and with the
throttle closed there should be less than 1/16" clearance between the
adjustment and switch.

Is it possible that SeaFoam caused varnish in the fuel tank to
dissolve and clog the fuel filter or carb?

The power valve opens an extra metering jet to let in more fuel when
when the the gas pedal is floored and the manifold vacuum is low.  I
don't know if a leaking secondary fuel cut solenoid valve can also
cause a rich idle (wouldn't the closed secondary throttle plate
prevent much fuel from being sucked into the engine?).

The only brands of carburetor rebuild kits I've used are Tomco and
CarQuest/Standard, and some of their parts seemed inferior to the
factory ones:

1. coil springs had far fewer turns.

2. Tomco's fuel cut solenoid o-rings turned rock hard in a few years,
and one broke off and lodged in the brass body of the float valve,
which had large slots instead of small holes that would have prevented
this.

3. Standard's accelerator pump shaft rubber bellows became brittle and
cracked in about ten months, allowing gas to evaporate out the carb.
CarQuest gave me a whole new rebuild kit, and I apply grease to this
bellows to prevent rot (can't use silicone spray).

4. Neither aftermarket float valve had a filter screen to block
debris, and this allowed a piece of old fuel hose that had shed off to
lodge and hold the valve open.

OTOH the directions and gaskets were fine (actually corrected a
mistake in the factory manual, where the two fuel cut solenoids were
mixed up), the o-rings for the venturi body didn't deteriorate
noticeably, and the auxillary accelerator pump diaphram was as good as
the factory original's .

If the heater and choke wires aren't installed exactly as original,
including their clip holders and the choke linkage cover, the linkage
can bind.

My lengthy reply is no indication that I know what I'm talking
about.  ;)