I drove the car fairly regularly and it gradually started running worse and worse. Then hardly at all. It only has 53,000 actual miles on it, but it hasn't sat for any great length of time. All of the hoses etc seem to be in good original condition.

 

PROBLEM SOLVED !



So here is how I discovered it:


There was mention in this thread of the vacuum line to the 16CU (thanks for that).
I had already checked it and verified proper operation a week ago.
But that got me started in thinking along the lines of too much vacuum.
And that led me to try, of all things, disconnecting the vacuum line to the distributor vacuum advance pod. The car immediately started idling more smoothly. I thought I had solved it (too much advance). But not really. Soon after that the car started idling badly like before, and I couldn't get it to run well again no matter how much I plugged and unplugged that vacuum connection.


But something definitely did happen briefly when I disconnected the vacuum advance, and that something was happening inside the distributor. I had already gone through the entire ignition system over the past week. I verified that there was a good spark coming out of the coil, and a good spark to the plugs (I had previously tested the first couple plugs in the firing order).


So then all I could think to do was to test the spark reaching ALL the spark plugs. I pulled the wire off the third plug in the order and tested it for spark. The results ... bad. The spark was weak and intermittent. What the ?!!??!


So for the umpteenth time I removed the distributor cap and inspected it. It is a Lucas unit, almost brand new. I cleaned the inside and outside of it (again), although it didn't really need it. The arc contact marks on the terminals inside the cap looked perfectly normal.


I inspected the rotor and it was of some unknown brand, almost new as well. I cleaned it also, just in case.


Then I remembered I had an older used Lucas rotor. I decided to compare the geometry of the newer brand-X rotor with the older Lucas rotor. The arm length and height measured the same.


But I did notice one slight difference, and that one little thing had been the source of my misery over the past couple weeks.


Here is my vanquished enemy, the brand-X rotor:





I cleaned up the old Lucas rotor, installed it in the car, and all is well, finally !


The Lucas rotor is basically straight and level across the top of the arm (the long horizontal red line illustrates the upper plane of the Lucas rotor arm).
Here is what made the brand-X rotor no good (see arrows, the arc point should be on the same plane as the long upper red line, not deflected this much):

I think the brand-X rotor had been warping a little bit over time, during the past few months that I was running it in the car (occasionally).

Thanks again for all the suggestions and brainstorming. All the effort certainly wasn't for naught as I have learned a great deal more about the operation of this vehicle.
.

 

PS:
And here she is, all better now




.

 

Congratulations!

Finding something like that is very difficult. Just goes to show how a basic part can cause a major headache.

Mark

 

Good for you. There is a lesson here for all of us, too. The basics HAVE to be right, and OEM grade parts HAVE to be used. I am really delighted for you.
Greg

 

Well done.

 

Nice job Holmes.

 

Good sleuthing !

Cheers
DD

 

Went for a drive around town today.
The car ran smoother than at any other time during the several years that I've owned it.
I think overhauling the ignition system (beyond just the rotor) made an improvement.


But even when I first got the car a few years ago it had a slightly uneven idle. One of the other things I did yesterday was to retighten all of the intake manifold bolts. That made a difference I think. And it isn't really very hard to do if you have universal-joint for your socket wrench. That job only took me 15 minutes, unlike changing the spark plugs which took me most of the day (I did that job without removing the A/C compressor or throttle capstan bracket).