1983 LeSabre 5.0 engine sputtering under load

HI all, I'm trying to help my dad figure out what is wrong with his 83 lesabre with 5.0L (307) engine and quadrajet carb.
It starts and runs fine in neutral and park. It'll idle fine in gear but
when you try to accelerate it starts sputtering. When the engine is cold, it's hard to get 'er above 35 MPH. We checked the timing because it felt like a timing issue, and the ignition timing is fine. It got a new timing chain and gears last fall and had been running fine since then until now.
The exhaust does not smell of fuel so I assume it's not an excess of fuel. The fuel filter was replaced already today.
Any ideas?
We did notice during out 'array' of tests that the choke plate on the two back carb barrels never open (well, unless you open them by hand).
Thanks for any help you can offer.
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You will never see the secondary plates open until open full throttle and the choke would have to be off, these are not choke plates. Did you replace the small fuel filter in the carb inlet? Many things can cause this, if you are not fully familiar with checking the coolant temp sensor and the operation of the carb mixture solenoid and tps you may need professional help with this.

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Thank you for the help.
I think I'll suggest he buy a repair manual for his car (I always buy one for my vehicles). I can't believe the carb has a tps! Would it be used for timing advance only? Hopefully the test procedure for the tps would be in there.
I'm starting to wonder if the fuel pump is going out.... and the carb looks like it could use a good cleaning.
If the tps only controls timing advance then I'm inclined to rule that out. I have driven carbeurated vehicles without operational timing advance and this doesn't feel like that sort of issue. I also wonder about the carb mixture solenoid. If the solenoid were stuck closed I'd have the opposite problem, wouldn't I? If it were stuck too far open then I'd probably smell gas.
I think I'll suggest he picks up a loaner fuel pressure gauge too.... I'm starting to wonder about the fuel pump. He told me he changed the fuel filter; I'm not sure which filter!
Thanks for getting me closer to a solution. 8^D

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looks
I don't know if I'd jump to this one yet (fuel pump). If I understand what you say correctly, the car won't even gently rev up enough to drive it above 35mph or so - correct? In other words, if you are sitting in park and are just trying to rev it to 3K or so a few times in a row, what happens? Does it rev right up or does it hesitate?
Some things to look for when you're doing this - take the air cleaner off and watch and listen - do you get a good spray of fuel when you pump the carb? Can't really describe well what you want to see/hear but the best I can do is to say you should hear the spray when you pump the carb. Do it a bunch of times in rapid (sort) succession. Does it rev up nicely every time?
There's a filter screen in the fuel line input to the carb that gets clogged. It's worth looking at. When it gets clogged you can experience starvation problems that will sometimes make themselves apparent by the above procedure. Sometimes not. It's so easy to check though that it's worth simply pulling the line.

out.
Man... this is taxing. I'm trying to remember back to my Delta 88 with the same motor. It almost hurts to try to remember back that far! An old memory is a terrible thing. I think I'd still go back to the question of how the carb performs in the garage with gentle input - does it take the engine up to 3K?
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when
sputtering
when
It'll
going
Hey - a thought just occurred to me. Have you covered all of the basic tune up checks yet?
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