with the can capped making it better you might check Rotor Phasing (my baby) and also total timing at the RPM where the problem is. yes the center heat riser openings ain't an intake port as you know but if the intake is warped enough or the gasket is bad it could be pulling a vac leak in at that point which it definitely sounds like it is doing, just make sure that the spray did not get pulled into the nearby carb airhorn or carb base & give a false symptom. not likely but pulling the intake is work & it never hurts to keep Mr Murphy at bay. I use a piece of cardboard to keep things isolated when testing. yes pull the intake & go from there