Here's what I think....
When you set up your amals, the air idle screw adjustment is comfortable in usually a 3/4 turn range.... That means you get the screw in that range and your actual Air/Fuel ratio is about 12/1 at the low range screw adjustment and 13/1 at the high range. YOUR ENGINE will tolerate this range without giving you any feedback to help you find out actually where you are in the range... read this again if you are not following before you read further...
So, you have your idle air screw set and your bike idles away happily,.... But now you go to accelerate by lifting the slide and needle.... You've now introduced a new variable to your carburation! So, how did that effect your actual A/F ratio??? Was your idling A/F so lean that the momentary addition of your slide and needle lifting caused your bike to run below a working A/F ratio?? (This is why some bikes "cough" when you open the throttle quickly) The solution is to use that 3/4 turn range of your idle air screw to richen up that setting, so when the effect of lifting the throttle causes a momentary lean condition, your new richer setting of the idle air screw adjustment has kept your carburation with a working A/F ratio... and you no longer get a stumble.
I know this is some heady shit to absorb,... but stay with me, I'm getting to your situation...
So we have this carburetor that has overlapping circuits so there is interaction between them, but when you are in each circuits range of carburation, that circuit dominates the A/F result of that RPM range, but the other settings are still contributing to the overall A/F.
When you set the idle air screw, you can NOT just set it without lifting and lowering the throttle as you set the adjustment because that's not how your carburetor is going to be used and you have to factor in how well is my idle transitioning to early acceleration. (meaning: Is it stumbling? or smoothly accelerating?) Often you can cure a carburetor "hiccup" by setting your idle screw with this technique (lifting and lowering as you adjust the screw looking for smoothest transition to midrange RPM's)
NOW, in your case, you have what sounds like the exact opposite condition, but based on the same effect. When you quickly shut the throttle, it's inducing a lean condition momentarily beyond the workable A/F ratio, so your bike stalls. The thing to do is the set the idle air screws slightly richer and see if you can make it go away....
Also,... this same lean condition induction by opening or closing the throttle quickly can also happen on the rich side of the equation. IF your A/F ratio is too close to rich and something you do momentarily enriches the carburetor you can foul out the engine that way too...
I know this is some heady stuff to wrap your head around,..... but it's a thing related to amals and the reason people who don't learn this give up on amals. I have straightened out a few people's amal carbs that had an acceleration stumbles by using the "lift and lower adjustment technique". Most of the time they say, "I've been trying to cure that for years".
I think it's pretty likely that shutting your throttle quickly is inducing a lean A/F condition and making your engine stall... So, now after you've read my lecture,... What is the solution??
