Fast blinking is indicative of a burnt out bulb, or replacement with LED bulbs. Basically it isn't drawing enough current.
Post a picture of your blinker switch.
If it's like the one in a 1971 114, fast or slow blinking can also be indicative of the switch/relay being bad, in which case the solution is to replace the switch/relay. Not sure if a 71 113 has the same switch/relay, as my car is a 1967, but I can tell you from a picture of the switch.