Dave; You are going to need to be a bit more specific than 'sputters'. ('81 GM products don't use points). Could be a secondary misfire, lack of centrifugal advance, ect.
Loren
SW WA

once car shifts from 2nd to 3rd gear [automatic] it feels like its not getting a steady flow of fuel or something causing back pressure because it starts stalling and backfireing [out of carb] if i flutter petal sometimes it goes away

Dave:

At the age with the so called "tune-up" done and backfiring up thru carb it suggests a timing issue that could be a jumping timing chain especially at the age. Age alone counts as most used a plastic gear that almost no extreme care could prevent a problem over such a long time!

Other issues could do this too - plugged converter with something loose moving around in front of it out of you sight of course, over fueling, up-shift too quick for load on engine - the list goes on.

Please tell us more about the history on this - what exact engine is in it and how many miles do you believe it has,

T
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Tom Greenleaf - MetroWest, Boston

i just cut the catalitic conv out and installed test tube also pulled check valve off of fuel filter. still no good! its a3.8 with 160.000 well maintained miles its also bone stock

dave; If the distributor has a vacuum advance, disconnect the vacuum line, plug it, and test drive. (I'm thinking pickup coil).
Loren
SW WA

already tried how about accelerator valve/pump in carb?

The accelerator pump diaghragm can cause a hesitation at tip in, but doesn't sound like that is the problem you are having. If you are under power when this happens, sounds more like a secondary misfire or starving for fuel. Have you checked fuel pressure and volume?
Loren
SW WA

     

Best I could find is this DOES use a vacuum advance dist. This may be the issue as Loren suggests at the speed mentioned which seems to trigger the sputter/backfire is about a shift to highest gear also and a load so the vacuum advance may move right then to cover any knock. When this type moves - look carefully at the pic (please show up) at the wires to the ign module with two at each end. The end towards the shaft of the vacuum advance wires are close even in the pic and are normally but should never touch the wall inside. They can and it will run like crap when they do or not at all. They move some and can fail inside the wire's insulation too.

More: If you have a hand vacuum pump test the advance to see if it holds tight. If not, move the rod that it moves when it gets vacuum and with hose disconnected put your finger over the end and the thing should stay put then move when you remove your finger.

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Timing chain on orig 1981 is highly suspect at both age and miles. To test - look at this dist with rotor still on and timing mark on crankshaft on TDC. Turn the engine with the bolt in the crankshaft damper (large size) one way till rotor turns then count the marks turning the crank bolt the other way how many degrees crank turns before the rotor in distributor turns. If more than about 5 degrees the chain is about at it's limit of stretch or wear for proper performance. At 6-10 degrees my guess is they can jump while running and for some unknown reason are capable of jumping back on the same correct tooth again which is all unseen.

I'm near certain the original cam gear to the timing chain would have nylon teeth on the metal sprocket. No amount of oil changing or preventative care will stop that from wearing OR throwing chunks out! IMO that was "Planned Obsolescence" by all the major US auto makers for ages!

Carbs if OE "Rochesters" are fussy to adjust but good carbs when right. This may have two pulloffs which should be checked as well but leave that for another thread and I can explain.

Good luck,

T
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Tom Greenleaf - MetroWest, Boston

fuel pump tests at 4.5 psi

The fuel pressure spec is 4.5-6.0 psi; So, you are at the low end of the spec. That is questionable. Under heavy demand? But, I'm still suspecting ignition system due to the symptoms. hmmmmm. You did try driving it with the vacuum advance disconnected? And no change? I'm kind of wanting to start from scratch...
1. Remove cap and rotor; Inspect coil connections, carbon tower in the center of the cap, ground strap from coil connected (some use a bent metal piece rather than a 'wire', some have both).
2. Remove and check spark plugs for proper gap and check the porcelain part of the plugs for cracks. (very easy to crack one during installation).
3. Check base timing. Check centrifugal advance, report. Check total advance, report.
4. Adjust air/fuel mix. At warm idle, turn one A/F mix screw inwards until engine speed starts to lower, then back off just until it recovers. Do the same with the other screw. Redo the first one, again. That should be your best lean idle.
5. Set proper idle speed.
Loren
SW WA

i'm going to give all that a try firstthing in the mornimg i'm done for today thank you guys for all the help! i'll let you know how it goes dave

Ahhhh..Must be beer:30. Especially when you're tired, sometimes just best to walk away and deal with it later. Have a good rest.
Loren
SW WA

i just got done with checking everything [cap,rotor'vac adv.,timing,i changed fuel filter and plugs. still have same problem! i just ordered fuel pump it won't be in till wed. pray for me fellas i'm going nuts! thanks again for the help