I have two of these things, one 1969 and one earlier. The motors are not that strong. The '69 has that 'smell' (like it is working hard) and from the original owner I understand it has always had that smell. I have never seen one that didn't struggle. The spring is balanced, i.e. it is the same up as down (pretty much).
Any tips? Is there some place that needs oil or grease or are they all underpowered?