Carb air piston hangup

[KKSMC@aol.com]

After replacing the leaking dash pot 'o' ring on my '74-6 I find that the air 
piston is hanging up the inside of the ZS carb cover.  The cover is aligned 
properly with the lower carb body.
 
Nothing I have done so far will enable the air piston to travel up and down 
freely inside of the cover whether at idle or 'goosing' the throttle assy.  
The air piston remains in the carb cover when removed from lower carb body.
Any suggestion on a remedy other than sanding the o.d. of the center tube of 
the piston?

                                                                              
                                       KEN C.