Left turn signal better than right

I've seen ammeter needle twitch with signal operation too which shouldn't occur, even with a much lower output alternator. The negative ammeter reading at idle suggests the entire system is suffering from low voltage at idle.
A different signal flasher might cure the 1 slower signal issue. There is still a fault somewhere. System voltage (+) and grounds (-) are not 2 different things either. Think circle. A fault in any part of that circle yields about the same result.
So the common culprits, bulkhead connections, ammeter connections, are always suspect.
In a 73 model , 2 white plastic harness connectors are also suspect, 1 in the engine bay, 1 under the steering column, all on the (+) side of the circle. Chassis ground path is what it is. Clean tight connections throughout and sometimes supplemented with a actual wire where needed.