Hi
Just a few points observations. Get the choke off as soon as possible every time. The problem is likely to be the carburettor throtle pump diaphragm. The throttle pump (At leas I think that's what we used to call them!) injects a glob fuel when big demands are made such as throttle down/blipping etc to avoid the inevitible flat spot you are experienceing due to an over-lean mixture.

Might be worth a carb rebuild kit or maybe pick up a good known carb and just swap them over.