G'day ValvO,
If the distributor does have a circular thing on the side that this little tube from the inlet manifold connects too it may be a vacuum advance diaphram. If it is partially blocking it may be sticking on and keeping the distributor advancing which will effect the idle. The early 122's only had the centrifugal weights that did the advance. so it may be a later model distributor (like the 140's). You could try clamping the tube completely shut (or disconnecting it and plugging it) to see if it stops the problem. This will stop the vacuume advance working tho, so will effect your performance, but will at least hint if this is what is causing your problem.

Then at least you know if the kinking tube is the problem, rather than having to chase carbie problems.

If it started about the same time as you changed your cover I would start with that (co-incidences usually mean something).

If it is the tube, time for a longer tube!!

Rgds Gerard.