Having run and tuned GP carbs for many years, perhaps I can add a bit of info regards the "enrichener", air circuit and 'this 'n that'.
The GP carb was designed as a racing carb and has a neat feature to make it easier to be adjusted for max engine performance.
The "enrichener control", enriches the mixture throughout the carb operating range by restricting the "air correction jet", air flow.
It is not a choke as is typically thought of, but was designed for tuning purposes. However it can and is often used by some riders for a slightly richer starting mixture. I've also read that some IOM racers used it to compensate for the altitude changes on the course. I dunno ...
With the spring loaded enrichener plunger all the way down, with a 'slack cable', the mixture is enrichened approximately the equivalent of 3 to 4 main jet sizes.
The way it was intended to be used was with a 'guesstimated' slightly lean main jet installed, the bike is run at top speed in top gear and the tach reading is noted. Then the enrichener is used to slightly enrichen the mixure. If higher revs are noted, the main jet is obviously a bit too lean and is replaced and the bike is again run for max revs.
The enrichener could also be used for testing for optimum needle and slide settings at lower speeds.
That all being said, I and many others have removed the complete plunger and cable assembly and plugged the hole with a bolt. This leaves the air correction slot feeding air to the correction jet wide open at all times.
Having tuned and run Goldies at various altitudes from sea level to 5-6000 above sea level, I and most others have found the OEM fixed air correction jet size to be OK and it's not overly critical.
But of course the main jet, needle setting and slide cut away need to be changed as required for changes in altitude. The GP is quite sensitive in that regard.
I live at ~ 5000 ft and it's a ritual to change tuning whenever I ride in CA or other areas at lower altitudes. I typically richen the main jet 2 sizes and raise the needle a notch when riding at sea level and adjust the idle as required. I sure enjoy the extra power riding at low altitude!
Here's something many guys have overlooked when first fitting a GP carb on their road bike. If the GP was set up for use on a megaphone equiped racer in it's previous life, as many of these carbs were, the low and midrange setting will be quite lean. There will oftentimes be a 3GP6 'lean' needle vs a 3GP and a #6 or 7 slide vs a #3 or 4 installed for road riding with a "twitter".
The reason for this is due to the pesky exhaust reversion pulses present with an engine having long overlap cams. The low and mid range carb settings are usually set quite lean to compensate for double/triple loading of the intake air when the revs are low and the engine is "off the pipe".
It's interesting to be able to see the little cloud of fuel enriched air hovering at the mouth of the carb at low revs when weather conditions allow it. Increase the revs and the cloud disapears.
However if you have properly modified your exhaust with 'modern tech', you have nearly eliminated exhaust reversion problems and the dreaded stumbling "megaphoneitus".
BTW, using the ignition retard control on the mag can also be used for setting optimum timing in much the same manner as using the enrichener.
With the timing set a few degrees too far advanced, the bike is ridden and the spark advance control is then retarded a bit for max revs at top speed.
With the bar control left in that position the timing is measured and the the mag is then reset to the measured timing advance value with the bar control at the max advance stop.
And again with all that being said, I and most racers eliminated the bar spark advance control and run the mag at full advance at all times.
But it sure is nice these days to have a Boyer
MP sparkler system with the auto advance! This makes a typically fussy highly tuned Goldie a ****y cat to tune, start and ride slow.
Accurate big sparks are a wonderful thing!