Howdy,,,, 

   Now, I know there is a classified section here,,,being that I been a member for several yearsbut I noticed the heading  to include  "Trucks" so,,I thought,,,maybe I can slip this one in without getting blackballed,,,is that even a word,,,,??

 Anyway,,I am need of a  TBI type carb for my 1988 GMC 3500 with the 454 engine and automatic. The unit it came with was so erratic, so I got a gasket set but did not help at all. It tends to surge and die out with a rough idle.

 Anyone got any fresh ideas or maybe a magic wand in rear pocket,,,,???

The item you're looking for is the Rochester TBI-220.  It appeared on everything from the 4.3L V6 up to the 454, with the difference between all, being the injector size.  Off the top of my head, I believe the injectors fitted were something like 28lbs/hr in the 4.3, the 305 were 32, the 350 was 40, and the 454 were 45lbs/hr.  fuel pressure was either 12 or 15psi on these units.

But wait, Dave... how could they use the same throttle body- that'd be way too big for the little six, or way too small for the big V8...

Yeah, but no.   It's possible for the throttle bore to be too small for the 454, but in a throttle body or injection system, there is no circumstance where having too LARGE a bore will cause the problems commonly associated with 'overcarbeuration', because the throttle body's function is simply to limit the amount of airflow INTO the manifold.  Fuel metering is not done by a venturi, it's simply a throttle plate... the fuel volume is managed by the ECM (in yours, it's referred to as '7747')  based on manifold absolute pressure (MAP), throttle position sensor position, crankshaft RPM, coolant temperature, and (when in closed-loop mode) the 02 sensor.

Now, there's a device on it called an IAC... an Idle Air Control valve.  It is a stepper motor with a leadscrew and a plunger on it.  The plunger moves in and out... opening or closing off a passage that bypasses the throttle plates.  When running at idle the throttle plates will be fully closed off (there's no 'idle stop'), the idle bypass circuit, the IAC plunger, and motor bypass whatever air is necessary to maintain proper idle.

Now, the IAC motor is controlled by the 7747 ECM by looking at all the sensors, and a few other things (the park/neutral switch, the AC compressor clutch status, etc), and deciding wether the idle speed is 'below' or 'above' the target idle RPM.

So the IAC motor/plunger MAY have something to do with your idle instability, BUT...

It may also be a case that your ECM is getting incorrect readings from any of the sensors... OR...

The ignition system (which was called the Delco EST-HEI) used a really slick ignition module in the distributor base that, instead of a mechanical advance, the BASE advance curve is built into the electronic module, so if the ECM is NOT connected, it will run beautifully under the base curve.  IF the ECM is connected, the 7747 will send 'alterations' to the distributor module, and change the timing THAT way.

The reason why I'm pointing this out, is because that module, or something ABOUT it, can cause your idle to be bad.

There's also a sensor board that connects to the ECM's distributor signals, and allows the KNOCK SENSOR to arbitrarily retard ignition when it 'hears' knocks.  A failure of this module, or a lack of a good sensor will cause the module to crank back the ignition timing for no apparent reason.

 My Friend, Dave Kamp,,,,,,,,It good to hear from youse!!  Many THANKS for responding!!

  Altho,,,,Ahem,,,altho,,,,I say this with a generous amount of depredations at the apparent amount of possibilities of even one little thingie being the culprit amongst all the possibilities this system has that can be the bad boy ,,,,,? My memoriy has been joggled to bring back many tales of other TBI owners being exasperated with the system,,,,,

 My question is this,,,,With the correct distributor,,,UM,,,"pre-computor",,,an adapter plate for the manifold, that will accept a simple Holly of 750 CFM or even a (gasp) AFB,,I believe Edelbrock markets them old AFB's now a days,,,would this "modification" give a fellar a lot less headache with it's simplicity,,,???

Holley makes TBI replacement units, if look into these or the reman units currently out there they are either Painted or otherwise Coated to prevent Vacuum leaks.