The problem sounds like it may be associated with the main jet or high speed jet. which is behind the fuel solenoid and held in place by that spring you talked about. Most of the time with that carb I just replace the entire fuel bowl assembly
When you speed up the engine quickly or put it under load the butterfly opens and tries to get fuel through the main jet up the emulsion tube if there is a problem with that jet, or damaged O rings on the emulsion tube could cause the symptoms you are having. When not under load the engine actually runs on the idle circuit of the carb.