HI Barry, A few thoughts. First is do what you can to verify you have no air intake leaks. Valve adjustment & timing should be verified correct as well. Compression test is a good plan just to be sure it's ok.
Assuming the above is good. Meaning zero intake air leaks, valve adjustment & compression are "good enough" the bike should start & run mostly decently.
As Stuart said there is no provision for cold fast idle like on a car. You must keep rpm up & blip throttle as needed after start & during warm up. Reliable idle may not happen until 5 miles or so depending on ambient temperature & fuel you are using. It's fashionable to remove choke, but some motors really like the choke & it can greatly improve running after start up.
Your bike should start first kick, & stay running with holding rpm about 2-3000 ish & blipping as needed. Using choke will allow for slower cold running, but you'll still need to blip throttle at stop signs until motor is well warmed.
My personal experience on premiere with its stay up float, if float level is low it takes longer to tickle carb. I've worked on a few premiers now. Until I learned better I used to reset premier float level lower to the .070-.080" below float bowl surface. This is way too low for premier! It won't work right. Longer time to get the tickle volume was the results. Harder starting after tickle & poorer running cold was also the results as well as tool lean across the board. Chased my tail for some weeks until I learned the proper Amal specs for premier carb float level.
At the point you are now assuming the motor is sound condition, I'd recommend removing bowl, pilot jet, mixture screw, carb top.
We'll start with carb top. Look down at needle clip & verify it's in the desired notch & the clip is fully seated into slide with spring on top. If clip is not seated, it will modify needle position & can even wind up the spring. Carb will do the oddest things. Just double check.
If you're using choke, pull air filter & verify the choke slide goes all the way down & all the way up when you move lever. Again, I always leave the choke on the bike. We ride in the upper 20s & lower 30s in the winter & choke makes life easier.
With the pilot jet out, blow carb cleaner spray in the jet hole in carb. Spray cleaner it the mixture screw hole. Place finger over opposite sides of carb. Fluid should exit into float bowl area & out the smaller hole at carb intake horn for air filter. Some may come out the large hole here also. Use compressed air also if you have it.
Blow cleaner spray into the tiny hole at jet tip. It should freely exit the 4 side holes. Again use compressed air if possible as well.
Take the float & needle out of bowl. You see the 2 holes coming up from bottom of bowl. Check the lower edges for paper thin casting flash. If you have any flash, break it off with a screwdriver or needle nose pliers. The flash can fracture from vibration while riding & will get sucked up into the carb body. I've seen this a few times already.
Finally check float level. A rough check is place tip of thumb on pivot pin. But don't allow thumb to touch anything else but pivot. Hold bowl upside down. The proper level on premier will be very different from older concentric. It will be just above & level with gasket surface.
Don't check by pushing pin or float tabs down to seat needle. Hold it upside down as this seats needle & compensates for play in pivot etc.
The true most accurate way to set float level is with a clear tube & measure the actual liquid level. Do not use skinnery tube than 1/4" or results may/will be skewed. If you have an old drain plug (or drill new one), drill & epoxy a 1/4" tube into plug. Get some 1/4 clear hose & put a spring in it so it won't kink. Mark outside of bowl with sharpie to the center of the high/low range. You'll find the upside down float level will get you fairly close to middle spec. But it's the liquid level that is true level.
Do not move the needle seat to set float level. Simply bend tangs. Watch the pivot loops. They may open up when bending. Bend them back as needed. Check the pivot play first, so you know what normal play looks like.
Back to pilot jet you should have a .017 or .019. number of rings on snout will tell size. Depending on your fuel one will work better than the other. But the float level is the very first thing you must check. The part # is Amal 622/502-17 (2rings). 622/502-19 (3rings).
Use a drop of oil or some grease on the orings for pilot jet & mixture screw. Dry ring likes to tear on the sharp edge of carb bore. lubed they go right in. Doesn't cause them to loose adjustment later.
For cold starting don't short tickle. Choke on or not, your choice & experiment what works best. I always use full on choke for cold start so I don't have to rev so high. In summer I turn choke to 75% right after start up. In winter, I turn choke to 90% right after start up, depending on how cold. You'll know how my by if motor want's to die or you must rev too high.
Push the tickler down. Let fuel stream right down side of carb freely & it will fall onto top of trans. Free clutch. I do one slow easy primer kick. It just helps. I clock kicker to horizontal as it gives more leverage for old guy like me resulting in faster cranking speed.
Turn on key & give a swift kick. Once hot idle speed & idle mixture is set I give zero throttle during the kick. The motor fires instantly. I then give some throttle to keep it revving. This is where I find choke really helps on my bike in winter.
Seriously 99 out of 100 or more times, bike starts first kick & keeps running. I give it about 20 seconds warm up in driveway as push garage door remote & wait for door to lower & verify it's staying down.
Getting under way if I don't use choke it will buck & spit back. So I use choke. 4 houses down, is corner. I clutch it & keep it revving while I slow & check for traffic. It would die if not blipping throttle. Depending on temperature, I'll start backing choke off as I ride. I've done this so many times I've developed a feel for how much to back off by way bike runs. Sometimes I forget to turn off choke.... At larger throttle the bike runs bad & will 8 stroke or like it might die. I then remember & turn it off. I usually remember, but traffic can be bad & my pea brain is occupied with avoiding cars.
Back to mixture screw. Base setting is 1.5 turns out. Mark line on screw with sharpie for witness mark. After 10 miles or so set mixture screw to "best running idle". Set slide stop ( idle rpm screw) until motor idles at desired speed. I like about 1000 ish. I wouldn't go lower than 950. You probably won't have tach, so just make your best guess. Then trim mixture, then idle. Ride another 10 miles. Now motor is almost fully warmed at 20 miles. Go over the mixture screw & idle speed again. Idling too slowly leads to low oil pressure & poor cam lubing at idle. Not to mention unexpected dying at stop lights. You'll figure that out.
When you get your mixture & idle rpm where you want it. Use your witness mark & see where mixture screw ends up. If you're more than 3/8 turn or so from 1.5 base setting, you may want to go to different size pilot jet. If you're in too far use larger pilot jet. Out too far, use smaller pilot jet. Yet.... it's how the motor runs & responds. If it runs good & you're happy with it. Just leave current jet.
I can't explain why but I've seen a few cases now where if you lean the mixture hot, it really wants to die & is definitely harder to start cold. Meaning it fires, but then will die before you can pick up the rpm with throttle. So you may have to purchase an .019 for a test if you experience this. This is not rocket science, but sometimes getting close to it!
The big thing is getting the float level right or you'll chase your tail forever.
We haven't spoken of air filters & exhaust. If you change these from stock type, you'll often have to compensate with carb jets & slide. I'm not a fan of the paper filter elements. I find they are very hard to tune around. I like the original wire/gauze like what factory used.
I have enough experience on premier now to promise you, when you get the carb right, the bike will start first kick, run very, very well. As good as they did in '66. Tip: if motor won't start heat soaked. Tickle it like it was cold, but hold full throttle. Be ready when it fires or it will go to redline instantly.
Here's a few photos of what I'm talking about.