Your carburetor throttle shaft could be binding at the 1800 rpm range or maybe the cable is sticking on the outer sleeve, when you Blip the pedal the throttle returns to idle faster and gets bast the bind.
Check the fast idle cam to make sure that is not hanging up. Make sure the secondaries are closing all the way, make sure the secondary spring on the shaft is connected correctly. Check the choke linkage isn't interfering. Try loosening the carburetor mounting nuts, give the carb a little shake and retighten, sometimes if they are not tightened evenly or are over torqued they can cause a bind. Disconnect the linkages from the throttle and rotate it to feel for a bind in the shaft. Maybe disconnect the accelerator pump while you are checking everything out so you don't flood the thing. If the throttle snaps back with the cable disconnected but doesn't with it connected there may be a binding in the cable.
The two spring idea is to keep the throttle shaft bore from wearing, don't just add another spring to solve you sticking throttle problem, fix that then add the second spring.
I've seen the throttle shafts bent, but yours is most likely something simple like the cable or interference from some of the other linkage.
