Nate,
  Flipped through a few of the old 850 books and did not see a
regulator.  Do not remember the cars having one (from the
factory anyway).  Perhaps the owner is refering to the fuel pump?
  We had a customer locally who had a similiar problem after
driving for a while.  He, and many others, have had to pull the
gas tank and have it cleaned.  Trash, rust, and even pieces of
the pump diaphragm would clog the carb.  
  If it has an aftermarket regulator on the car - take it off
and throw it.

Good Luck
_____________________________________
VICK AUTOSPORTS - Largest Fiat parts 
Distributor West of the Mississippi
WEB:      http://www.vickauto.com
E-MAIL:   vickauto@aol.com
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