I just browsed through the posts and without really trying to get into your problem I noticed one thing. If you think it’s have something to do with the carburettor, don’t forget the throttle cutout. It’s often overlooked and a too big cutout can lead to lean conditions and a too small can lead to rich. It mostly the region between idle and small throttle lift that’s affected. Every “stage” in a carburettor must perform faultless. Unfortunately I have no idea of what the right settings for your carburettor are.
Best regards, Leif