Decal on Support says "DO NOT TOUCH..."

[Suburbanite88]

A little help?

1988 V20 TBI 350/400 California Truck, 164,000 miles/40,000 on rebuilt TH400... Normally very strong, absolutely no hesitation, stall, or lag whatsoever, loves to run. Smog Check, tech said engine idle speed too high, showed me on computer screen 1200+ rpm at idle... Wanted to bring it down a little, I said "sure... Why not?" down to 900+ still steady, smooth and quiet idle...

After test, died at red light, then again, after that 50-60% of the time... Next day, filled up regular gas, then immediately rough idle, stalling more frequently, increasing hesitation, general loss of power, hard starting and backfire...

Condition worsened to UNDRIVEABLE... Lots of advice from ASE Cetrified Techs, to Backyard Mechs; everything from Fuel Pump, Tranny or Computer, to Vacuum leaks, Throttle Body Rebuild, to Bottom-end Rebuild...

Started with the cheapest thing first... After inspection, found: TBI Gasket Vacuum Leak; (5) Intake Manifold bolts took 10-15-degree-or-less turn; Distributor Cap, Rotor, and Plug Wire boots some corrosion...

With all issues mitigated, starts right up, but still won't idle...

Next, all new: IAC Valve, fuel filter (priming at 13psi); AC-Delco plugs, wires, coil, cap and rotor, buddy with a Timing Light tried to dial it in just right, but could not keep engine operating, between about 1500-2500rpm sounds like it's missing...

Replaced with new: MAP, TPS, ATS, knock sensor, fuel pressure regulator, ignition module, EGR Valve and Solenoid...

Starts easily, fires up great, prime burns off, rpms dive right past idle and...

Dead.

It all seems to go back to Smog Tech guy adjusting the idle from the idle set screw, now I just wanna get my Girl back on the road, any thoughts?

Thanks...