The fact that the engine rpms drop when you apply pressure to the butterfly linkage indicates that there is no binding in the idle step linkages.
So as SFRY said you are probably going to need to replace your bi-metal choke. (relatively simple if it has screws holding it on).
Before removing it, mark what position it is set to this will be a good starting point for installation and adjustment. There should be a small notch or groove on top of the choke and reference marks on the body of the housing.
If you need any other assistance let us know.