Cold exhaust, hot valve cover.

It sounds to me as if you are trying to bandaid the problem with excessive idle-timing; possibly indicating a rough normal idle.
So then, I would look in the carb.
I would check the transfer slot exposures under the throttle plate, looking for one long and one short, due to a misaligned throttle valve, causing the hot side to be doing all the work.
If not there, I would look in the idle wells.