[T77911S]

you have other issues besides the decel valve(DV). even with the DV removed, the RPM's should drop and the car idle fine.

you could have air leaks. those need to be fixed first. there are lots of places for leaks. the hoses that run from the oil tank to the engine. the oil cap. the brake booster line. tested mine with a hand vacuum pump. pulled a vacuum and it held. that checks the line and the booster.
the intake runner sleeves. Orings, injector sleeves. even the aluminum pipe behind the CIS can rub and get a leak.
a smoke test is a good way to find the leaks.

next i would do a tune up. plugs, cap, rotor and wires if needed.
then i would check fuel pressures. system, hot control (HCP) and cold control(CCP).
check timing, make sure the advance works
then check the mixture

you can also check and adjust the DV with the hand vacuum pump. i would also check the vacuum lines from the WUR, to th thermo time valve (TTV) (if you still have one) then to the DV and then to the throttle body(TB).

you can also check the TTV with the hand vac pump, or just check for vac to the WUR when the engine is running. if you remove the vac line to the WUR with it running, then engine should run worse.

make sure you have power to the WUR and TTV and aux air regulator(AAR).

you have gone the the most of the CIS, you might as well check the AAR. make sure the idle increases when cold, then drops down after a few minutes.
you could pull the AAR and make sure it closes when power is applied.