I'd be concerned road grit is causing vacuum piston to bind..
Hard to tell how long unfiltered air has been entering carb..
Remove air filter, press on outside of rubber duct so it folds into itself, deformed into somewhat of a C and shove duct lip into air box.. little lubricant WD40 or silicone helps.
At that point you should be able to lift vacuum piston with finger and feel binding is there is any..
You could also check piston operation while engine running.. may need to use cell phone camera to video rise and fall as throttle operated..
if piston raises fully w/engine running at higher RPMs torn diaphragm would seem unlikely to me..
With duct pushed inside air box, 1/4 of the work to remove carb has been accomplished should you decide to have it gone through...
96 VS800 Intruder (sold 2018 ), 87 CMX250C Rebel,
79 CB400 Hawk (sold 93), 75 CB350 (sold 83), 71 CB350 (stolen 74)
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