ON NEWER BIKES:
you can find a 4 digit serial number lightly stamped onto the headbadge.
The first three digits are the numerical day of the year,
and the last digit is the last digit of the year.


You've got a Schwinn from July of 1988. Or possibly 1978.

http://www.angelfire.com/rant/allday...nnCodes1.html/

http://re-cycle.com/History/Schwinn/SwnB_Serial.aspx

does your headbadge look like this: