My orange '76 242 is running poorly. Could someone give me some advice?

Problem with 1976 242DL w/CIS (K-Jetronic):
Loping, inconsistent idle. Sometimes it runs high RPMs, sometimes low, sometimes stalls, all depending on weather, humidity, rain, sun. Because of this, I am constantly adjusting the idle speed knob to find the proper idle speed. Often I must re-adjust when engine warms.

I know that the cold start injector, thermo-time switch, and auxiliary air valve are functional. The aux air valve is sort of broken in such a way that I can rotate it to control the amount of air entering the engine when cold.

I have replaced the fuel injector seals. When warm (and cold) the engine exhaust smells of gasoline, and often is slightly white even on a warm day (too rich?). The timing is 15 deg BTDC with the vacuum retarder on the distributor unplugged and capped. I have removed the air pump, and disconnected the EGR and any peripheral vacuum lines for the emission system in order to make troubleshooting easier.

The compression is 150 across the board. The vacuum is a consistently meandering 13-15 inches, and the needle on the gauge states the ignition timing is too late, but like I said, it's a confirmed 15 deg BTDC.

The ground strap on the valve cover is filthy and covered in grime but intact, and I recently took the rubber boot off of the air mass meter venturi and cleaned the boot and the little metal disc.

I haven't troubleshooted the fuel pressure regulator, and don't quite know how, I'm a bit of a newbie with this. I thought it might be the regulator, but I'd like to know other people's opinions.

Three things:

It does sound like an intermittent miss fire caused by a bad ignition component, which is aggravated by weather.

The miss fire could also be due to a lean mixture caused by a vacuum leak hence the low vacuum gauge reading.

I cannot remember if the EGR recirculates exhaust at idle or above idle. It may be necessary for proper idle. The EGR valve may not operate correctly with low vacuum.

Have you ruled out simple things like the distributor cap, rotor, wires and spark plugs? -- the fact that humidity makes a difference to how it idles makes me suspect an electrical issue. But this would not account for the evidence of fuel-rich exhaust -- are you getting unusually poor gas mileage too?

An easy thing to do that can cure rough idling is to remove and clean the throttle body with fuel injector cleaner.

I have a similar idle speed/roughness issue with a recently purchased k-jet equipped 240. Haven't sorted it out yet -- in my case the throttle body was dirty but not causing the problem. I thinks it's best to replace the wires etc. to rule these out before proceeding further.