Gremlins

Been done. All clear. Had extra new set. Now I'm gonna sleep on it and consider my next move. I may end up putting a pair of those wassells on it when I get tired of piddling with it, which is clearly on the horizon and has been under consideration for a while prior to this little nuisance. Oh, about 18000mi., but the needles and slides were replaced before I put it away in '80.

If your jets are 18k mi old, I would look at new ones maybe.

Raising and lowering the needle is going to effect the transition from the idle circuit, ie as you pull away from the lights. It should not effect the idle circuit itself, which is like a carb within a carb.

At 18,000 miles the carbs as a whole are basically worn out.

It should still run better than this tho...

Well...To settle this strange bird let me say...…..Top ring on needles tamed the beastie. I know that it shouldn't, but there it is. I recall when first going into them years ago the needles were in this position, which I promptly placed where the book stated. The bike ran okay, though felt a bit in decline as I recollect. One never knows what the PO did, so you put it back. Right?
Once my age fog cleared being backed into a corner I figured WTF. Immediate start, smooth idle & warm up, no stutters or pops, & all of this off the bat at 1 1/4 out on air screw.
Anyway I'm going to put it out on the road for a short run and see if it sticks & check plugs as soon as things are snugged up.

Since it hasn't been brought up yet - check/replace your needle jet, this one of the most commonly overlooked wear items in the carb that causes problems at the rpm's we spend most of our time at:

"If your bike is running rich at lower or mid-range throttle openings do not assume, even if you see Amal stamped on the needle jet, that it is the size it claims to be! This applies EVEN IF THE JET IS NEW. Measure ALL needle jets (use CS-0398 measuring standards) before installing them. Very little wear, or being out of tolerance as little as .0005" over the specified size, will cause the motorcycle to run very rich or lean."

https://www.princeton.edu/ssp/65-cub-data/library/amalbritbike.pdf

"
It’s the needle jet stoopid:
If there is one thing you come away with, from the time you spend reading this, is how import the needle jet is to every day performance of your motorcycle. To put this in perspective, a change in size of the needle jet’s metering orifice equal to 1/3rd the diameter of the average human hair, (one thousandths of an inch .001″) , will make the needle jet too rich or too lean. In practice the typical 4 stroke needle jet’s orifice is .106″. A needle jet that has an orifice of .105″ will be too lean to a point where it could introduce unwanted detonation. A needle jet that has an orifice of .107″ will be too rich where it will significantly reduce fuel mileage and could foul the spark plug. The needle jet is in play in the throttle positions where we ride these bikes the most: closed to half throttle.

Caution: There are about ten people manufacturing needle jets around the world. When you see reference to using plug gauges to verify the needle jets size we are referencing checking new needle jets. Not all needle jets on the market have the jet orifice that is marked on the jet. The new .106 could have an orifice anything but .106″. While you can get an idea if a needle jet is worn with an smaller than standard orifice plug gauge, it is better to routinely change them from a know supplier like Burlen."

https://vintagebikemagazine.com/technical-articles/amal/

Great info and very much appreciated. I now have it on file for easy access in the future.
Just got back from road test and am very pleased with the results. 850 now has all of the characteristics of a cat. from laid back napping to screaming and hauling ass when doused with scalding water. Quick crossover transitions & smooth even cruising. Redline in a heartbeat if so desired also.
The guy I bought it from made no alterations, but someone owned it prior to him so I have no knowledge of the phase of it's history. I do know it has always had a strange not so even idle with a lope to it...When flogging it on it goes and then suddenly blasts off like a gunshot & you'd best shift fast or cut throttle fast, or hit 7K. I learned quickly to not push on it too hard and be prepared for the burst coming.
Whatever else it doesn't abide by the center ring on the needle which is where it is supposed to be. So I tinker no more, and subdue my curiosity until the day I have to open this thing up. I only hope it's not in small pieces, or more than NV put in.