Zed Head

Man, I was going to suggest that after it cools it would probably get you home. Didn't know you had to wait. Good luck.

1, 2, and 6 are missing.

Still significant that the other three cylinders run clean. That's something that removes many other potential causes. Adds focus.

He has an old set of NGK plug wires. Swap those on and if the problem is exactly the same odds are it's not the wires. Put an old cap or a new cap on, see what happens. Odds are the problem won't stay with 3 and 4 through two different distributor caps. A good mechanic, even with no meter but with the thought that it might be plug wires, would just swap a "good" wire for a "bad" wire. Clean the plug, let it idle, recheck. The good news is that after all of these years (yes, years) "the problem" is narrowed down to 2-3 cylinders (still not clear if the half-dirty plug is #2 or #5), and the problem shows up in 5 minutes of idling. This is Saturday afternoon futzing around kind of work. Make guess, make a change, run a 5 minute test. Learn, rerun process. Our guesses almost don't matter any more. A good mechanic should be able to look at all six plugs and have a good diagnosis within an hour. Three really dirty plugs and three really clean plugs, and test results in 5 minutes. Can't get much easier I think. For all of the time involved, he'd probably just put all new parts on to discount any wire, cap or plug issues. A basic tuneup. The basic problem here, on the internet, is still the same. Incomplete information, incomplete followup (resistance readings on the bad three but not the good three), actions suggested but not taken.

I don't know who your mechanics are but a good one should be able to do something with this picture alone. Show them this picture and tell them that it happened immediately after installing new spark plugs. Don't say anything else.

Driving and looking at plugs has already been done. What will doing more of it tell? You need to find out where the "fuel" is coming from that's fouling those plugs. Why are cylinders 3 and 4 dirtier than the others? This assumes, of course, that spark is good and strong in 3 and 4. What are you using to hold your plug wires as they cross over the engine? And what brand are they? Maybe 3 and 4 are shorting to the engine somewhere and even though the resistance is good, the spark is still bad.

If it's not fouling from excess fuel and all of the spark plugs are getting good spark that leaves something specific to each cylinder. Oil fouling from leaking valve seals or bad oil rings seems reasonable. Oil fouling from bad valve seals would probably show some signs in the intake runners in the cylinder heads and the backs of the intake valves. 3 and 4 will be oily, the others not so much. A bore cam ran down through the carb throats might show something. Or remove the carbs and intake manifolds to get a really good look. There might also be signs from above, under the valve cover. Obviously misplaced valve seals can probably be seen through the valve springs.

Seems like that would show up on the cylinder pressure test. He says that they're all 180 psi. which is actually very high for an engine with a performance cam. It's an odd problem, but at least it's a new one.

That's a definite possibility. If the fouling is from excess fuel, then 3 and 4 could be getting a rich fuel mixture from the carbs and the outside cylinders have air leaks that lean it out. The difference is very distinct though. 3 and 4 are terrible and the others look almost lean.

1, 2 and 6 would be nice, to round out the picture. But your measurements so far are normal. So, to sum up, the question is "what would cause 3 of 6 plugs to foul very quickly?". So far, it seems that the problem is mostly a low RPM, or idling problem. The history has been about driving in traffic and idling and hot weather. A higher RPM shutdown would be interesting but might be painful, due to hot plugs. Ponder...thoughts...ideas.

Keep measuring plug wires! We're so close.

He's editing as he goes. Making progress. Not clear though, if the measurements are from the contact inside the cap to the center electrode. That's what is needed.

Edit - replay to site's #121 post (anybody notice that Patcon's post is 101, after site's 121? Base 120???) I took a look but didn't see anything. Searched Far311 and fouling and misfiring. But really, I'm just glad my car runs so well and that we seem to solve a lot of problems on CZCC. Lots of good advice here for people that want to use it. Horses and water and drinking and all that.

One... https://www.taylorvertex.com/faqs/#ohmsonwires http://www.ngk-sparkplugs.jp/english/techinfo/resistor-wire/01/index.html

Seriously? This advice is not good enough? I don't see how it could be better. Not clear that Jalex really wants to fix the problems. I have to admit that sometimes I miss my car's old problems. It was something to work on. Good luck. https://www.zcar.com/forum/10-70-83-tech-discussion-forum/418272-spark-plugs-3-4-getting-sooty-black-my-1972-240z.html

He's so close, right now, to figuring out the poor idle in the heat problem. Those two center plugs and the dirty one next to it are huge clues. Much more informative than many of the previous posts. The clean plugs, 1, 2 and 6, tend to remove any choke setting, or carb-tuning, or fuel percolation, or Far311 camshaft issues. Unless, the fuel overflow runs straight down the 3 and 4 intake runners. With that focus a bunch of new ideas come to mind. But we need to know if the plugs are getting their voltage or not. Gotta test those plug wires.

Depends on how much oil is entering the cylinder and whether or not it's burning with combustion. You need to work on the simple checks so that you can zero in on what it really is. Focus on those spark plug wires. Here's another possibility - the newish spark plug wires aren't fully seated in the cap. That would give a sporadic misfire if there's no contact between wire and cap contact. That will show up in the multimeter test, which I'm sure you're doing while also searching the internet.

"Powder coat" is not inherently excellent. Like all coatings, surface prep, application, and curing determine the quality of the final product.

Yes, the center electrode should have a good path back to the contact in the distributor cap. You might see high resistance since you probably have resistor wires, but all six plugs and wires should show the same number. Wiggle the wires around once you get your meter set up, for each wire but especially 3 and 4, to be sure that you don't have an intermittent break in the circuit.

Just a loose end. This is why I haven't bought a new cap. Trying to figure out how to fix that gasket instead. https://www.courtesyparts.com/oem-parts/nissan-cap-filler-17251n4501

That's what Mark was saying to do. Put the plug in the end of the wire then measure from contact inside the cap to the center electrode of the plug. He said body but I think he meant center electrode. Do it for all six so that you have a good comparison.

Cap and rotor. Circling back... Oil rings don't show up in a cylinder pressure test. This a California Datsun engine after all. Valve seals can leak a lot of oil also. Focus on the plug wires and spark for now though. If you can show that it's definitely not spark related then you can move on to other possibilities. Eliminate one possible cause at a time.

That's a good test. But, does Jalex have a meter?

Another quick test of the wires would be to swap 2, 3, or 4 for any of the clean cylinders. It might be a stretch because the wires are custom fit but should be doable. Or just buy a single new plug wire and replace 3 or 4. If it's the wire, the problem will follow the wire. You're in to the process of elimination now.

That is a great picture. It shows a lot. We might all ponder the various causes of fouling for a short while. As I recall from much past experience with poor running engines, a fuel fouled plug often looks like 3 and 4 here, but then when it fouls, it gets wet with fuel. And it fouls quickly and the carbon coat is not so smooth. These plugs look fairly dry, and the other plugs look almost lean, like they're not even close to fuel fouling. Weak spark fouled plugs typically look wet also, like they were wet from the beginning of missed sparks. These plugs look like they were firing well, with a good even coat of sooty carbon on everything exposed, until the spark started shorting to ground through the carbon. In short, they look like they had great spark until they didn't. #2 (or maybe #5) looks like it is fouling also. But the other three look very clean. Considering how the carbs and manifolds are set up it doesn't seem like it could be a fuel problem, unless fuel is running down the intake runners and in to those cylinders. Is that possible? Is there a path for raw fuel in to the center runners? If he parks the car on a slant so the fuel runs away from the cylinders would that be a clue? If the spark is good and the fouling is fuel or oil related, and fuel is discounted, that leaves oil. Which could be rings or valve seals. It could also be valve lash, possibly, causing poor combustion. Just some thoughts on other possible causes. On situations like this, I tend to go around in a big circle on all of the possible causes until one wins. There's definitely a cylinder specific problem though, which narrows the possibilities down.