The distributor and the points see thousands of volts. The capacitor is on the primary of the coil and sees several hundred volts.
(confusion between distributor CAP and CAPacitor)
Interesting. I thought the condenser, being in the primary circuit, only experienced voltages in the 12 volt-15 volt range. You are correct, my capacitance measurement test was only done at low voltage, so the condenser remains suspect. As a counterpoint, the engine performs normally at rpms off idle, which seems inconsistent with a condenser failure.
BTW, I did buy a replacement already, which cost – sort of hard to believe for a capacitor – $14.
The capacitor, with the primary coil inductance, form a resonant circuit, which, when the switch (points) open, causes an inductive kick to hundreds of volts. The transformer ups that to thousands of volts for the spark.
What you say makes complete sense @BillRussell . Just curious, what’s the expected symptom if the condenser isn’t working? I’ve always heard it shows up as burned and pitted points, the condensor’s job being to limit the voltage at the points to prevent sparks jumping there between the two point contacts.
On the few condenser failures I’ve seen the symptom was similar to a carbed engine that is flooding. Bucking and jerking, some black smoke, etc.
The engine would die now and then, sit for a while until things cooled off a bit, and then may run again for an unspecified amount of time until the symptom resurfaces.
Aside from something simple like a Spark Plug trying to foul out… I am in line with the vacuum leak diagnosis. If you know how to use the Good Ole Vacuum gauge…its amazing what those can “tell you” if you know how to read one properly… Its almost a lost art today. I would invest some time to check into vacuum leak. You can use the gauge…which I highly reccommend…or the old spray lube trick… Spray lube around areas of possible vac leak and listen for idle disruption or actual improvement.
Blackbird
This AM I finally had some dedicated time to work on this, and can report the problem is now resolved. 70’s Ford truck 302 V8 engine idling and running smoothly again.
The major problems turned out to be:
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The gasket between the EGR spacer plate had a hole burned clean through it, all the way out to the edge, creating a vacuum leak directly into the intake manifold. I replaced that gasket, cleaned the mating surfaces (which was a pretty big job), cleaned out the EGR holes in the spacer from carbon deposits, and replaced the EGR valve w/a new one.
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Some of the carb passages were clogged. I think the main problem was a clog somewhere between the jets and where the idle ports emerge higher up, near the venturi booster ass’y. So I bought a carb rebuild kit, which included a new float valve ass’y, power valve, accel pump, and various gaskets and clips, all of which I installed. The technique I’ve used in the past for rebuilding the carb by removing the carb from the vehicle, then removing the air horn, the jets, the float ass’y, the accel pump, the venture booster, and the power valve and spraying all the passages with carb cleaner spray wasn’t effective. I tried that 2 times, worked for a while, but didn’t take. On the third time I decided to bite the bullet & remove the entire choke ass’y and various other gadgets, then thoroughly clean the exterior of the carb of all the gunk. That part is time consuming for a diy’er without a parts washer, that’s the reason I wanted to avoid that task, but it was necessary this time. Next I soaked the entire carb body in a bucket of Berryman’s carb soaking solution for 20 minutes. Then I soaked the carb in a water bath to clean off the Berryman’s, then sprayed the carb & passages with carb cleaner, then used some 30 AWG wire to explore and verify the passages remained unblocked. One thing good about this wire task, I think I better understand how this carb works & how it was manufactured. Finally I sprayed all the passages with compressed air in the reverse flow direction using an air gun and compressor set at 35 psi.
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Contributing factors were fuel and engine air filters that were past their prime, which I replaced. I added a second fuel filter in line with the oem filter, with a clear housing, so can visually check for any debris in the fuel lines. So far I don’t see anything accumulating there.
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While everything was removed, decided that was a good time to replace the spark plugs. Access to number 8 can be beastly, but with everything removed and the pathway clear, and a little tinker-toy engineering of the ratchet configuration, number 8 popped out in a snap. Visual inspection of the old plugs showed them to all be in good condition, the gap pretty much unchanged, but there is a slight indication of a problem on number 6, the insulating ceramic adjacent to the gap was gunked up, which I think might be caused by a dodgy spark plug wire causing an occasional misfire. I think that b/c that spark plug wire zapped me a couple of time when I touched it near the boot, and none of the others do.
Upon reassembly and reinstallation of the carb on the engine, it started right up and idled smoothly again.
Many thanks @“Rod Knox” who suggested the soak idea, and investigate the EGR spacer, and what that triangle gadget is (choke pull-down, which tested ok), and to finish off the job with a compressed air spray through all the passages; to @TwinTurbo for the suggestion to secure the correct tool to remove the jets; to @“Honda Blackbird” for suggesting heat to remove the jets (I used cold rather than heat, but same idea); to @wentwest for suggesting thin wire to check and clean the passages; to @Nevada_545 for suggesting to investigate the power valve (turned out to not be the problem); to @“the same mountainbike” for suggesting to make sure the float floats, the fuel pump pumps, and dizzy dizzies (none were the problem); to @Tester for suggesting it might be caused by ethanol in the fuel (probably not the problem, but keeping this in mind if future fuel system problems develop); to @insightful and @pleasedodgevan2 for suggesting to check the spark plugs and wires and ignition system; and @ok4450 for suggesting to check the intake manifold vacuum (seems ok, measures 18-20 at idle now).
For reference purposes for those interested, the following thread was a spin-off from this one.
http://community.cartalk.com/discussion/2307884/stuck-carburetor-jet/p1
George, thank you sincerely for the followup post. It’s good to know you got the problem solved, and I’ll bet we all learned a little bit from the thread.
Happy motoring. 
Congratulations @George. Funny how things fall right into place for those who are willing to go the extra mile and do things correctly. Lazy people call it luck.
There not much on this truck imho that will cause idle surgeing, and that is a fuel or vaccum issue. Bad float, bad pump, bad diaphram, bad vacuum hose or booster etc.
There not much on this truck imho that will cause idle surgeing,
I watched a u-tube vdo where a guy rebuilt an ancient autolite 2100 carb on his gravel driveway, and all he had besides a few hand tools was a 5 gallon bucket to sit it on. About every 45 seconds he’d say “remember, this is from a Ford, Ford makes such simple stuff, turn a couple screws, wipe it clean with some gasoline and an old t-shirt, it’s fixed.” Then he’d say " Anything made by Ford can be fixed practically just by looking at it" … lol …
After rebuilding a few they do seem simple. And being able to operate the engine with the top off the carburetor makes setting the float level and idle jets very easy. I guess I could rebuild one on a gravel driveway again but I’d prefer working in a somewhat clean shop. Dropping a part in gravel can be a real pain as I recall.
Back in the Good Ole Days, when you saw one of these
you knew you were in the presence of a real Ford Pro.
I finally learned to take carburetors apart very slowly, and only at a table or workbench that I cover with a towel first. Those little parts, springs and jets and such, just go flying off into the unknown otherwise. And I bought a magnifier that clips on the frame of my glasses (it’s a jeweler’s glass) so I can see even the tiny print on jets. But slow and steady, on a table, with clean hands.
Sweet…Good Job @GeorgeSanJose …well done. I wish I could find a way to sell the feeling of “Empowerment” it’d be a big hit…