I concur about the O2 sensor...if it's not doing its job, then it can't tell the ECU to adjust the mixture to correct the situation.
I don't know if a check engine light comes on for the O2 sensor on the '85. Obviously, it's not sending any codes...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car