My '73 (just got the newly rebuilt L28 running in her) has a weird signal quirk. Right works fine, but left stays solid when at idle. When I give it a little gas, it blinks normally, just like the right one. Strange.

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"Go into emptiness, strike voids, bypass his defences; hit him where he does not expect you" -Lao Tzu

Sounds like you have a problem with voltage to the left,try checking the indicator output voltage from the switch.and comparing it with the right hand side,probably a poor contact/connection.My 73 240 has been suffering from corroded connections from the age of the wiring.

Hope this helps.