Interesting.
A couple weeks ago my extended family was home for my grandmother's funeral and most were out at my parent's place for the after-party (remember my pickup stays there). I asked 2 of my gear head cousins to come listen to my pickup. They are both even younger than I am but have a good hobbyist understanding of mechanics.
One of them opined that since I tried about everything else we could think of, maybe it was the cam or the distributor since as far as I know they have never been replaced in 210k miles. We pretty much dismissed the cam thought because it idles so well. Maybe there is something to that theory.

Please let me know how yours turns out.

I have been way to busy with family/work/studies to do any more diagnostics. I will eventually get the compression test done and let you know what I find.

 

So in the process of tearing my engine apart and checking things, I noticed my distributor vacuum canister was leaking as if there was no diaphragm in it at all. I don't recall checking that, but like others have said, it sounds like a vacuum leak. I had one and didn't know it.

 

And in particular, a leaking distributor vacuum can will cause some really weird timing jumps that will make the engine run funny. I know this because my '79 400's vacuum advance was all messed up; it would run fine at idle or if my foot was in the gas, but cruising it would pop and spitback like crazy. I unplugged the vacuum advance, capped the hose, and it smoothed out like a dream.

 

I put my original re-built carburetor back on.

I did a compression test, the results are as follows.

1) 141 psi
2) 142
3) 143
4) 140
5) 146
6) 143
7) 142
8) 141

Looks pretty good to me! Here is what the plugs looked like:





My understanding is this dull, dry black color with light carbon deposits can be caused by a rich mixture, plug range too cold, weak ignition, dirty air cleaner, faulty choke, or excessive idling.

I don't know if I can determine anything from the plugs. It has been doing almost nothing but idling since I put the plugs in about 18 months ago when this problem manifested, and if you've been following this at all, you know I have done lots of experimenting with the choke.

I noticed a pin-hole leak in the fuel line on the front of the engine and will have to get that replaced. The leak was from the line rubbing on something.



fuel spray!


During the testing, it did not surge or misfire when cold or warm. When I revved the engine it seemed fine. I didn't get a chance to drive it because it's packed tight in the back of the machine shed and i didn't have time to dig it out. I hope to drive it again soon to see if it will act up. I really haven't changed anything since the last misfire (other than the carb re-swap).

If the problem persists, and I expect it will, I'll be suspicious of the distributor for a couple of reasons.
Firstly, about a month ago my brother and I finally fixed a persistent problem on one of his old tractors by swapping the distributor. The engine would run beautifully until it warmed up. Once warm it would misfire. Under load it would clear out and run fine. The distributor was causing the trouble.
Secondly, a friend of mine who is a self employed mechanic (his shop is called "Shade Tree" - hilarious!) suggested trying a new distributor, but he was working with only what I could tell him about the problem and what I have already tried.

 

Hard to tell much from plugs of an engine that has only idled... only idling and very short drives before the choke opens fully can make for some dirty looking plugs...

 

The compression is nice and uniform. Unfortunately compression tests don't tell a whole lot as they tend to mask valve leaks, head gasket leaks, etc. But a leak-down test will provide info on those areas, and would be my next step if you still have problems down the road.

As for the plugs, those show a wide range of "color". But, as you said, idling doesn't give good reads on the plugs, so I'd wait until I dug it out and got it on the road before worrying about the color.

Having said that, ignition problems can certainly cause color variations like that - but only if the problems are downstream from the dizzy. If the problem is the dizzy then all the cylinders will probably suffer and you won't get the contrast you are showing. So, if that contrast continues after you start driving it then I'd be looking for vacuum leaks where the intake hits the head, or where the carb goes to the intake.

Looking at the plugs, 1, 2, 4, 6, & 7 aren't too bad. Cylinders 3 & 5 are very rich or are burning oil. And #8 either isn't getting fuel or is running way lean. If #4 was as white as #8 I'd say your PCV valve is dumping lots of air into the base of the carb. But, with #4 being darker I don't think that's the case. So, I'm confused as to what could be causing the problem - assuming there is still a problem when you start driving it.

 

Actually, valve and head gasket leaks should stand out like a sore thumb on compression tests...

Was the really clean sparkplug recently changed?

 

In my experience they don't stand out. I've had engines with leaking head gaskets and bad valves that didn't show at all on compression tests, but glared on leak-down tests. So, I no long bother with compression testing.

 

All the plugs were changed at the same time.

Thanks for the input again. I will let you know what happens after I get it out. Hopefully soon, but who knows...

 

Coolant leaks into the combustion chamber can also cause ridiculously clean plugs.

 

Good point! As the coolant flashes to vapor it scrubs things such that plugs look new.

 

Are you suggesting that for #8?

 

Sorta. If, after you drive it, the plug continues to look that clean then you need to figure out why. On the other hand, maybe idling it is getting just exactly the right air/fuel ratio to totally and completely burn w/o leaving a deposit, while many of its brothers and sisters are getting too much fuel/not enough air.

In other words, don't worry, be happy - until you drive it.

 

I learned something yesterday.

I put the fuel line on and went for several short drives in the 95 degree heat. Everything seemed to be working alright.

Later at dusk, I took another short drive. I drove up the lane with no problem, turned on the lights and MISFIRE! Whenever I turned on the lights it would miss.

This explains why it always runs poorly at night! I thought it was temperature related.

I remember this happening in the middle of the afternoon last summer but I don't remember if the lights were on for some reason. But clearly there is an electrical issue.

So... what on earth would be causing that to happen?

 

Remind me - what ignition are you running? And, how is it wired?

I ask because something is pulling the voltage to the ignition system down when the lights come on. Maybe your alternator isn't charging and the battery voltage is quite low and the headlights pull it down too much to get a strong spark?