Starting at the beginning. Bing carburetors suck for performance other very basic.
The big thing I see so far is the slide travels to far at the top of opening an by doing so it pulls the tapered needle out of the atomizer way to far the making it run too rich and possibly causing four stroking, that’s why you need to back off on the throttle to stop it from happening. Now for the fix of just this one issue (more will come) the carb slide travel must be limited by installing a hard plastic tube on the cable between the top of the slide and the bottom carb top or installing a shim in the throttle housing and the throttle tube to limit the rotation throttle tube thus preventing the slide from traveling too far up in to the carb.
If anyone reads this and has more to add then please do so tell me I have it right or wrong or I’m full of it.