Carb is off and will be disassembled and cleaned today.

Number on the carb is 80443 so it's a holley 4150 850 cfm marine carb

Get rid of those (ie: replace) plugs too. Recheck firing order when putting wires back on.

I'm sorry I didn't come in on this sooner but after going through the entire thread here's my Ripley's believe it or not story .
I had the EXACT same problem some years back . Same carb but on a 351W . Would start as soon as you hit the key and blipped the throttle but once warm it would die below 1700rpm. It sounded as though the carb had gone completely dry and there was nothing but air going into the engine.
Hit the key and throttle at the same time and it would fire right back up , but as soon as the engine speed went below 1700 rpm .. kaput .
The guy who did my trailering at that time said 'it's the carb' (with no details) so I got a carb re-build kit , cleaned and put it all back together and NO change . So , carb ruled out ? ?
Got a decent mechanic involved and it drove us mad. Chasing non-existant vacuum leaks I paid a ton for a new inlet manifold .. NO change . We removed the engine from the boat and literally re-built it. NO change !
My trailering guy again said 'It's the carb' .. still no details but I had a factory rebuilt one (trying to economize) sent in. I was suspicious from day one as it was coated with a black sort of paint , not all nice and shiny . NO change and many more hours of mechanic's troubleshooting .
Finally the trailering guy said 'It's the carb , get a NEW one , not a re-built , I've seen this before' (at last , some detail)
My neighbors had recently bought a new supra ski boat that had the same base engine and carb so they let me 'borrow' their Holley for a few hours.
As soon as I cranked it I knew I had found the problem . Engine ran flawlessly , hot or cold , no more acting like it was out of gas below 1700 rpm. I promptly ordered a new carb .. bolted it on and the problem never came back .
Don't ask me why a complete re-build or a factory 2nd hand re-build didn't work but lesson learned .. Listen to you trailering guy once in a while , even if he's not a certified mechanic.
My advice ? Try to borrow someone's perfectly functioning Holley and see what happens !

Thank you for the comments. I do have another holley double pumper on a sbc I can take off and try.

Hoping I can rebuild this carb though and find out what the issue is.

Couple things of concern came up when I was disassembling the carb. The first thing was that there is zero transfer slot showing on the secondary side.

And then the accelerator check valves were different. The shorter one is on the primary side, they should match right?
https://cimg2.ibsrv.net/gimg/www.off...c1935690b0.jpg
https://cimg7.ibsrv.net/gimg/www.off...ae2ff7ef75.jpg
https://cimg8.ibsrv.net/gimg/www.off...e5edfd8b1b.jpg
https://cimg4.ibsrv.net/gimg/www.off...845d0a5eba.jpg

The end on the short one broke/fell off

Thats what I figured, thanks!

Rebuild kit will be here today, hoping between tonight and tomorrow night I can get that wrapped up and get new spark plugs put in.

After installed, I cut off the nipples (right over the bulbous part holding it in ) so not to interfere with anything.

Not sure if this has been mentioned or maybe its obvious, but carb needs to soak in chem dip for a while to loosen up crap in the tiny idle circuit passages. Then follow up with B12 being shot throughout the idle circuit using the red nozzle/tube that comes with the B12 to confirm passages are clear. Dont forget eye protection. B12 stings like crazy in the eyes.

Also, a blown power valve will make it run pig rich. Some of the old Holleys dont have a check valve to prevent damage to power valve during backfire events. Not sure abt the 4150's. Hopefully rebuild kit comes with new power valves. If it does, need to confirm old power valves have the same vacuum rating as the new ones.

Thank you. I did already spray out all passages of the metering block and none of them were blocked. This carb is very clean and has for sure been rebuilt. (Was told it happened this summer)
My new rebuild kit comes with 2 new 6.5 power valves which are the same as what's currently in there. Im wondering if when this carb was rebuilt new power valves were not put in.