Theres a very small clip that holds the accelerator pump linkage arm that extends out of the housing and secures it to the throttle linkage. I had that clip come off which caused the pump linkage to disengage the throttle linkage, caused the exact same problem as you describe. This is on the outside and is very easy to see and check. Likes been said don't tear into the carb until you've done all the easy checks.
Dave
