With insulated pliers pull one spark plug wire at a time with the engine idleing. Compare the amount of change in each cyl. That could point you to cyls that may be effecting the rough idle. You can tell if the vacuum advance works by watching the dist as you accel. the engine. Ifv it is working the dist will turn. This does not effect thje idle though as the 235 uses ported vacuum for the advance & doesn't advance at idle. Take a look at a couple of spark plugs to see if they have carboned up from all this idleing. You probably are going to have to run a compression test.
