choking out and sputtering

[grandpa's75]

Ok. Just acquired a '75 K20 Custom deluxe 2 wheel drive. She's got the original 292 i6 with 4 speed manual. Fired her up to bring her home. Starts good. Idles good. 1st and 2nd can run through just fine. In 3rd gear and more so in 4th gear when I get her up to speed, around 40 to 45, it's as if she starts choking out and sputtering. Can't get her to go up an incline with out it happening either. I was thinking carb issue? Rochester mono jet.

 

[grandpa's75]

This is what it looks like under the hood.

 

[Quadrajet Power]

Since it will start and run some, I would suggest looking for a restriction in fuel or air delivery.
Check fuel filter and air filter.
Check all rubber fuel lines including any return lines for cracks and deteriation. Today's fuel will rot the old lines and let air in the system.

 

[MadOgre]

In 1st and 2nd how high do you bring the RPMs ?

 

[grandpa's75]

No tach but can wind her up further than I dare keep going with the old girl.