Agree with the post above, a lot of parts have been replaced, but what diagnostics have been done?
What I'd do:
warm engine up, remove spark plugs, have throttle set wide open, disable ignition and fuel pump. Perform a compression test that is a baseline that you work from. If you have poor compression on one or more cyls it will never run right no matter how many parts you throw at it...also note condition of spark plugs, are they burning clean with light tan or grey deposits on the center electrodes, or black and carboned up (fuel mix too rich or high oil consumption) or blistered white (fuel mix too lean) spark plugs are a good view inside if you know how to read them.
If compression is good then move forward if not then you have to find out what is wrong. A cylinder leak down test is the way to find where the compression loss is.
Going forward I'd first test:
fuel pressure and volume, make sure it matches Volvo's spec
if good then move on if not find out why, ie restrictions in fuel delivery system, fuel vent clogged? anti siphon valve on the fuel tank rusted and seized up? crud in the bottom of the fuel tank? Dump out the contents of the fuel filter and let it sit in a mason jar. Do you see crud on the bottom? Water?
The carb, well yes I tried a new Holley 4160 once thinking my old Quadrajet was too old to rebuild and I was wrong. I had no end of problems with that Holley, it would start and run but ran very rich, fouled plugs and just did not run right. So in frustration I pulled the old Quadrajet off a shelf in the garage and rebuilt it in 2 hrs and it ran like it always did, just fine, not smooth like a modern EFI vehicle but just like my '72 Chevy Impala and my '75 Olds Delta 88, ie good enough!
So my opinion is Holley's quality control is not the best, but you have to make sure that carb is set up right. When you say only one idle screw, you are probably referring to the idle speed screw. They probably have those asinine caps over the mixture screws which I hate with a passion. My Quadrajet didn't have them and neither did the new 4160 I bought have them. I'd see if they can be removed so you set the idle mix the way it needs to be for it to run right. Had the same problem on our Suzuki DF 2.5 had to remove the crap epoxy they covered the idle mix screw with and give it one half turn rich, that was all it took for it to finally run right....
Last thing, make sure that there are no intake leaks between the carb and the intake manifold.
Carb troubleshooting is old school auto shop 101. Learned it many many years ago.
PS finally found out what was wrong that fool Holley 4160, the rear metering plate was loose!
I'll take a good Quadrajet anytime over a Holley!
A good second choice is an Edelbrock 1409
you have a 2bbl I assume so I guess you have to make this Holley work
What was wrong with the original?
go on to Holley's website they have a lot of videos and service information.
https://www.dropbox.com/s/ncpyim8e196lc ... k.bmp?dl=0read this for starters, its out of my 1970-1977 Chilton repair manual
I did this in about 2 hrs......
https://www.dropbox.com/s/zu1hut15udcnx ... d.JPG?dl=0https://www.dropbox.com/s/jwp8tkdvyu2wn ... t.JPG?dl=0PS if you are running bad at idle and low rpms your idle system is suspect, air bleeds idle jets. Carb has a float system, idle system, transfer slots, main system and accelerator pump.
If you let it idle a while on the water hose then pull out the spark plugs and see how they look...
White = too lean, brown/tan good, black too rich.