Sounds to me like your pilot circuit is lean not rich.
I believe the stock pilot is 22.5, for US models which might be different to your Oceania model.
Reading through your other posts you say "With the standard pilot jet, the best the bike runs is with the air screw wound all the way in", which, again makes me think your pilot jet is too small and you're making it richer by restricting the air flow with the air screw to compensate.
I think MarkT already said he runs a 25 pilot jet.
If it were me I'd go back to stock jetting and work back from there.
Good luck with it.

I agree with Pete - backing the screw out adds more air, in less.
My early 70s bikes all seem to run better (eg. reduced surging) and not as hot (darker vs. lighter spark plug) with a slightly larger main and pilot jet.  Not sure if my theory is correct, but the EPA was formed in 1971 I think and I would recon that by 1980 these 2 strokes where probably jetted way lean to meet standards...but just a hunch
 

I did think that it was too rich at one point, but I could never get past the starting setup. With the 20 and 22.5 pilots, I could never get the bike to start with the choke on, and I had to apply throttle. By reducing the the pilot jet, it is getting closer to starting with the choke on without throttle. It has also always had a surge, or inconsistency at idle where it doesnt respond well to changes in screw settings, or idle adjust settings. And at every set of lights, the idle would sit at a different rpm

I realise that I say some conflicting things, but its because I am still learning, and generally incorrectly changing multiple things at once which realistically causes no end of issues. But as someone eager to solve the issue as fast as possible, I'm kicking myself in the nuts as I go

Still playing around. Something I have found has made the bike more reliable is to religiously turn off the fuel tap after turning off the bike. Most fuel components are new, but I seem to get an issue where the fuel somehow syphons into the crank case, and that seems to contribute to needing to start with the throttle wide open

Did you put the double-lipped seals in?
Have you renewed ALL the brass parts (float valves, jets, needles etc) in the carburettor with genuine Mikuni/Yamaha parts?
 

Yes I have put double lipped seals in both sides of the crank.

And the entire carburetor is a brand new item, Mikuni VM24-512. I changed it for a bunch of reasons, but mostly because all the jets in the original carb had different threads and sizes compared to the new carbs (for what reason I dont know) so now I can buy 'standard' VM24 parts and be comfortable knowing they will fit. Also, the original float needle had a brass cone, and while I couldnt confirm it didnt work, I wanted a new one with a rubber cone for better sealing

A VM24-512 is not at all the same carb as "original".  Both are made by Mikuni, but similarity ends there.

It's bit like saying you've replaced the processor in your old Intel 486 processor computer with a Intel Pentium processor and now wonder why it won't boot up...  both processors are "Intel" but not the same.

So the question I have is what "standard jetting" are you using?  I'm certain the original DT175 jets and slide needle will not work (or in some cases even fit!) in the -512 version of the Mikuni carb.  For example, the pilot jet is a completely different series as are the slide needle and needle jets. 

It's not that you can't get your new carb to work...  but you'll need to start with the original -512 jets and tune from there without ANY reliance on what Yamaha used on their original bespoke version of the VM24.  Correct pilot jet should be up around 35 or 40 for example. 

P.S. the reason the "jets threads are different" on those carbs is you are buying cheap Chinese knock offs (copies) of the original carb.  They can often be made to work as well.  If you don't want to fiddle with the jetting and tuning, then the only choice is to get a TRUE original Yamaha Mikuni for your exact model and use the original jetting in it.  Used is likely the only thing available but there's nothing wrong with a used carb that has been cleaned properly.