I just went through something similar with my 142 -- check the 140-160 index -- I received some good advice there. In my case, the turn signals worked in both directions, but the indicator light on the dash only worked on a right turn. It turned out that the PO had installed too small a bulb in the left front turn signal. If that's not it in your case, I would check the wires that go from the left-side contacts on the flasher unit socket (in the 142, marked "L" ) to the left turn signal lights and the wire from your turnsignal stalk to the flasher unit socket.
Bob S.