This is what I have found in past:
1 Leak air into primer pump
2 Low fuel pressure,( Pressure regulator).
3 Weak lift pump, damaged or leaking ck valves.
4 Vacuum on fuel tank.
Number 4 easy to ck ,remove fuel cap, listen for the whoosh. Fuel cap or tank vent.
Number 1 get newer style pump.
Number 2 If two piece regulator, stretch spring to 19mm or renew spring., (member Greazzer) might still have some, Or renew. complete regulator.
Number 3 After doing others If still not fixed, renew pump ck valves and seal.
Yes there could be other sources of air leaking in But after this many years old, IT"S TIME to renew these parts.