26 minutes ago, emccallum said:

I would eliminate everything I could before pulling the head, especially since you have good compression, no smoking and no signs of a bad head gasket. You may want to remove the rotor in the dizzy and check the dizzy shaft for play. It should have no play. If it has play that can cause inconsistent firing. I dont know if it could cause a single cylinder to always missfire or how a pertronix would be affected. I do recall you getting better running when you were messing with the ignition wiring. Maybe swap a plug wire just to be sure it isnt one of the wires.

 

  I agree. In the beginning, this problem looked to me like it was a distributor cap, rotor, wire, or spark plug because it was only fouling one plug. It was either that or an internal problem. Countless plugs, new wires, cap, and rotor haven't made much difference but just to put my mind at ease, swap a plug wire with #4 and see what happens. Be sure all the wire connections are on all the way and secure. My son's Subaru had a plug wire that kept backing off the plug. He cured it by lubing the inside of the boot with dielectric grease. The boot slid all the way on the plug with grease. 

1 minute ago, Mark Maras said:

  I agree. In the beginning, this problem looked to me like it was a distributor cap, rotor, wire, or spark plug because it was only fouling one plug. It was either that or an internal problem. Countless plugs, new wires, cap, and rotor haven't made much difference but just to put my mind at ease, swap a plug wire with #4 and see what happens. Be sure all the wire connections are on all the way and secure. My son's Subaru had a plug wire that kept backing off the plug. He cured it by lubing the inside of the boot with dielectric grease. The boot slid all the way on the plug with grease. 

Okay but do you want me to disassemble the complete distributor to see what happens? it has a new cap, rotor and wires. 

No, I would just make sure that the shaft doesnt have any play (wobbling around). Remove the cap, remove the center rotor (part that spins), then wiggle the metal shaft left and right. If it does have play, then you have issues with the dizzy bushings and it needs work. It will have play up and down but should not have play laterally. 

Take the #4 plug wire and swap it with the #3 plug wire. Swap them at the plug AND the distributor cap. If your fouling plug moves to #3 then you have an ignition problem, if it doesnt you have a problem somewhere in the motor. 

3 hours ago, emccallum said:

I would eliminate everything I could before pulling the head, especially since you have good compression, no smoking and no signs of a bad head gasket. You may want to remove the rotor in the dizzy and check the dizzy shaft for play. It should have no play. If it has play that can cause inconsistent firing. I dont know if it could cause a single cylinder to always missfire or how a pertronix would be affected. I do recall you getting better running when you were messing with the ignition wiring. Maybe swap a plug wire just to be sure it isnt one of the wires.

 

I dont see any play whatsoever

I guess part of my suspicion is based on the fact that the #4 plug doesn't foul unless it's idled too long. If there was a ring problem then you would expect to see more of it off idle when the vacuum is higher in the motor. Or smoking on acceleration or deceleration. So if it's not a ring problem and the valve seals have been successfully replaced then I would look at what hasn't been replaced. I believe the only thing that hasn't been touched is the dizzy and the pertronix...

It could be cam related but that seems like a stretch. I guess all the lobes could be mic'd to verify but still seems like a long shot

Jalex,

Did you physically move the shaft around in the dizzy? It needs to be more than a visual inspection.

Edited April 3, 2024Apr 3 by Patcon

While you have it open, you can suck on the vac advance rubber hose and make sure the advance is working properly. Plate should rotate and return. I dont think that's your issue but gives you some piece of mind that advance is working. 

Replacing pertronix (as Patcon suggest) is easier than removing the head.

 

Edited April 3, 2024Apr 3 by emccallum

Seeing as how this is an earlier California Datsun head, its time to pull it.

Check the valve guides and any evidence of oil coming from the head gasket.

12 minutes ago, gundee said:

Seeing as how this is an earlier California Datsun head, its time to pull it.

Check the valve guides and any evidence of oil coming from the head gasket.

Valve seals on #4 have been replaced

My uncle used to work in TWA AIRLINES as  an airplane mechanic he told me to use a temporary spark plug extender if the plug does not foul its a damage valve guide.

Thats worth a try

1 hour ago, Patcon said:

Thats worth a try

Any chance damaging the engine if you use this anti fouling extensions permanently?

I would think permanent use is not a good idea