I'm with Bill on this one, it sounds like the shop is looking to make some extra revenue off you. Are you running an oilbath air element; if so how is the oil level? I rebuilt my Zenith myself and since then I've only had one complaint: It's sensitive to tank sediment and the float sticks when the fuel filters need to be replaced.
Have you checked the fuel pump and filters? I found so much sand in my fuel system I was expecting a beach ball and surfboard to pop out of the lines.
