Mark Maras

To Carl.

I think the third pic is correct.

How many extension cords do you have?

@jalexquijanoOn this site search "valve guide seals". DAVEWM (as well as others) have good descriptions of the procedure to change the valve seals. You only have to remove one valve spring, #4 intake. Stuffing the cylinder with rope, using the valve spring compressor shown earlier in this thread, remove the valve spring retainer, lower the valve slightly, rock the stem back and forth to try to gauge the guide wear. If there is little to no sideways movement between the valve stem and the guide, use a straw to install the new (you'll probably have to buy a whole set to get one) seal and reassemble the spring. If there's considerable sideways motion of the valve stem, the head will probably have to be removed to repair that guide. Once again, before you buy a spring compressor and a set of seals, do your homework. Search this site and probably YouTube too. They have lots of videos. Should you decide to go ahead, you'll have the cost of a valve spring compressor, a set of valve seals and maybe a valve cover gasket. You might also check to see what a local shop would charge to inspect and change that one valve seal.

YouTube. Search, 240z valve seals. Pics of what you find would be helpful.

Hopefully it's just a valve seal.

You're good to go, reinstall it.

Concentrate on the float levels (primary mixture adjustment) and balancing the carbs first.

Sounds like gas-lighting to me. I'd pull the valve cover and look carefully at the valve seals. A good light should help when looking between the spring coils. Compare #4 with the rest and see if you can see any difference between them. I'd be looking for a torn or distorted top of a valve seal or one that may not be pushed down all the way.

I doubt that the PCV is causing the fouled plug. The PCV is just a one way valve. Shake it, can you hear it rattle, good. Now blow in each end. Blowing in one end will be impossible but the other end should free flow. If it's blocked either soak it in lacquer thinner or buy a new one.

Although COVID19 spreads mostly through the nose and mouth, scientists now conclude the greatest risk comes from assholes.

I had an interesting conversation with an old friend who wears a bandanna but can't (his words) wear a mask. When questioned about no mask he said he has panic attacks when he dons a mask. Knowing that he came from a screwy abusive family I asked him what his reaction would be if I came up behind him and put my hands over his nose and mouth? His reaction was OMG, don't ever do that I don't want to hurt you. He'd had that happen many times as a child to the point of passing out. As it turns out, his mask phobia may be caused by childhood trauma.

Good advice above. You could also carry a can of starting fluid with you. When the engine dies, give it a shot and see if it starts.

New valve seals and checking guide wear would be my next step. It's possible the #4 seal was damaged when it was installed.

Back in the day, pulling a spring from the advance weights for quicker advance rate was a common tweek if you were going to spend the day at the track. Is it possible that the PO didn't reinstall it?

"Do it in a Datsun"

Before proceeding to valve guides, I need you to test the #4 plug (use a new plug) against a ground by turning the engine over with the starter. That will ELIMINATE a possible electrical cause of the fouled #4, then we can move on to oil fouling. But while I'm thinking about it, how much oil does the engine use in around 1000 miles or 1600 kilometers?

Yeah, Put a hotter plug in #4. It's a band-aid but it may buy you some time until we discover WTH is going on. Is the dark color on #4 plug oily, smell like fuel or is it dry carbon? I'm beginning to suspect a bad valve guide or valve guide seal on #4 if the colorization is oily. Looking at your latest pic of #4 it looks like the plug is trying to fire (clean near the center electrode) but is being overwhelmed by either fuel or oil. Fuel can be crossed off the list so it is likely oil. One more test please. Pull #4 plug, put the wire on it and ground the plug (With an insulated gloved hand) while turning the engine over with the starter. Watch the spark and note the color. It should be bluish, not yellow and you'll probably be able to hear it.

The early 240s valve covers I've cleaned (usually with lacquer thinner) had no coating. I believe that's true of the later ones too.

Ok, the closed throttle explains the low numbers on all of them, however it still doesn't explain the misfire on #4. Back to the simple things. Are you absolutely sure the number four wire has been securely attached to the plug. It should have the same noticeable click as the others when you snap the plug wire onto it.

@jalexquijanoWere those readings with the throttle wide open? Also before or after checking the valve lash?

Yes, and after they've been reset another compression test would be helpful to rule out low compression in #4.

That has been my understanding. I hope I haven't been wrong for 50 years. I once owned (before Zs) a dragster (BB/GD) with a blown (671) 327 small block chevy. The center electrode was recessed down in the porcelain on the plugs that came with that engine.

The switch in the upper rt. corner is the antenna switch from an early Z that had the AM radios like my early 71. You may be able to use that existing switch if you can source an early faceplate. I don'r recall when the change was made but it does seem a bit weird that you have an old switch in a 73.

Have any of the #4 plugs you've pulled in the past had no gap? Is it possible that you dropped the spark plug or bumped it against something that closed the gap previous to installation? If it were mine I would re-gap that plug, install it, disconnect the coil wire and turn the engine over with the starter for a couple of revolutions. Then pull the plug and see if the gap is closed again. If the gap is OK, replace that plug with a new one and take it out for a test run. If the gap closes again take a pic and report back. The resistance readings on the two wires look OK to this non-electrician.