Has a little off-idle stumble when the throttle is opened quickly
Well done! way to be persistant!
The stumble you speak of can be the fuel mixture varying as the carb's idle screw settings interact with the needle jet influence as you raise the slides.... I use a subtle method to tune the idle screw setting to cure that stumble. I find that the idle screw has almost a half turn where the bike idles well, but turning the screw has seemingly no effect. I think that's the idle screw's setting showing the range of fuel proportions that it will tolerate at idle speed. (Once you go out of that range the bike will begin to stumble) Within that range of idle screw positions where the bike idles well, there's the narrower range where it will more smoothly transition from "running on the idle circuit only" to running on both the idle circuit AND the early influence of the needle jet/needle.
I get the bike warmed up, then I find the idle screw range where the bike idles without stumbling on both carbs. Then I gently raise and lower the slides to run the engine up to 2000 rpms, while I micro-adjust the idle screw setting on each carb, going back over it numerous times looking for the smoothest transition to 2000 rpms. You'll find that you can smooth out that stumble this way. (It makes pulling away from a traffic light with some impatient car on your ass less of a "pray it doesn't stall" event)
My explanation of this issue and method to fix it is simple. The idle screw setting has a range where the mixture is changing the fuel ratio, but that ratio still works for an engine that is only idling. Once you lift the slides at all, you add the influence of the needle jet, so the fuel mixture ratio is altered by this secondary influence. By micro-adjusting the idle screw as you lift and lower the slide, you are trying to adjust the idle screw influenced fuel ratio to eliminate the momentary stumble as the secondary influence is added.
It took a lot of reading about amal carbs before I realized why the idle screw had a wide range of adjustment where the bike idled well but turning the screw while the bike idled seemed like it did nothing. My theory is that turning the screw slightly is actually changing the fuel ratio as you turn it, but so long as that ratio is in a certain range, the bike will happily idle, if everything else is right. That idle screw influenced ratio becomes more critical when you lift the slides and add the needle jet as a secondary influence to it. If that combined ratio goes outside the workable fuel ratios, you get a stumble upon accelleration.