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.

I know the TBI-220 and 7747 ECM controls fairly well.  Back in 1996, I acquired a 250ci inline six Chevy from a houseboat (identified by MerCruiser Marine as the MCM165), and disassembled, blueprinted, and 'worked it over', then fitted the marine intake/exhaust manifold with TBI.  Initially I used the Holley Marine Pro-Jection, which was a big, very expensive mistake.  I ran it for two summers before yanking it and correcting my errors.

I switched to the TBI-220, and burned a custom PROM chip, and that resolved all the problems.

The 7747 and TBI-220 combination is actually a good, solid, simple setup.  IF you have porosity leaks, they're not difficult to fix... but I haven't found any porosity issues with the TBI-220's die castings.  Realize realize, by the time the TBI was developed, Rochester had been die-casting things MUCH MORE intricate and complicated than the TBI unit for upwards of 6 decades... the marine varients of this engine, as fitted by MerCruiser, came with either a Rochester single (1MV, B, etc) or a Dual-Jet... and the 292 version had a small Quadrajet.

The fascinating thing about manifold leaks on the injection system, is that if the ECM is running in closed loop, it notices that the MAP sensor's indication is a bit low, and the O2 sensor's reading is a little off the center of the AF map, so it increases the injector's pulse width to correct for the leak.  In the worst case, if it sees that all the parameters are skewed out a bit, and the throttle position sensor is not IN an appropriate spot for that combination, it MIGHT give you a Check Engine Light.

A vacuum leak with TBI is just as simple to find as with carbeuration-  use either some spray, or an unlit propane torch waved around suspect areas and listen for change.  If no response, it's not a vacuum leak. 

Is is NOT-at-all difficult to resolve a locateable porosity issue.  Start the engine, and spray the area down with carbeurator cleaner.  Leave it running, at ldle, then whip out a bottle of your wife's fingernail polish (or yours, if you're not willing to risk her wrath), and paint the area liberally WITH ENGINE RUNNING.  Any point of porosity will draw the polish into the casting... and it will rapidly cure and seal it up.  After that, as long as you don't hoze it down with acetone, it'll stay sealed.

The TBI-200 has throttle shaft bushings and seals, and they DO wear out.  No different from a carb in theory or practice, but they're usually bigger in diameter than most carbs, and easier to pop out and replace.

The HEI-EST distributor would be the next place I'd look.  The shaft bushings wear just like the AC-Delco distributor used on farm tractors and industrial engines, they need to be knocked out and replaced every-so-often, but they're sneaky...  see, it's an electronically-triggered ignition module, and it will continue to fire just great, even if the diz shaft is 'gyrating' around the inside of the housing.  Problem is, that gyration makes for one extremely unstable spark timing at certain speeds.  The oil pump is driven off the tang at the bottom, so when you're up off idle a bit, there's a fair amount of oil pump load, which is to say, lots of damping keeping that shaft fairly stable... but get down to low speed, the shaft rattles around a bit... especially in say... a boat... 

And don't ask me how I know...

Another silly thing that DOES happen to early TBI motors, is an apparent loss of vacuum, with a Check Engine Light, and after you've replaced every sensor, every module, every gadget, it STILL indicates CEL...  until you cut the lead pipe loose from the front of the catalytic converter.  Once the plugged-up converter is disengaged, vacuum comes right up, it smooths out, and runs great...

Again... don't ask how I know...