Hasn't #4 plug been an issue from the start?

If mine I would pull the head. Need to check the valve guides on the cylinder.

On occasion (although very few) a guide have been found to be cracked right at the head. 

I had a E31 head with a cracked guide and the plug looked just like that.

If they replaced the guides when the head was rebuilt, they can break one if not careful.

Was the valve seal replacement a head off replacement or head on replacement.

Were the guides and seats replaced during the build to handle unleaded fuel 

14 minutes ago, gundee said:

Hasn't #4 plug been an issue from the start?

If mine I would pull the head. Need to check the valve guides on the cylinder.

On occasion (although very few) a guide have been found to be cracked right at the head. 

I had a E31 head with a cracked guide and the plug looked just like that.

If they replaced the guides when the head was rebuilt, they can break one if not careful.

Was the valve seal replacement a head off replacement or head on replacement.

The thing here is that i do not want to open the head until i have all the seals, guides, valves, etc. I bought this car with the specifications i sent previously. I have never open the head. 

You are not going to uncover anything until the head is removed IMO. If you don’t have a competent machine shop in Panama I’m sure there’s a member on the forum that can recommend a shop here in the states closer to you where it can be disassembled and diagnosed.

9 hours ago, Patcon said:

It's cheaper to try a new ignition system!

How do you conclude its the dizzy?

You have messed with everything but the major components of the ignition system. I would swap it out before I tore the engine down.

Let me see where i can find a rebuilt single distributor.

53 minutes ago, Patcon said:

You have messed with everything but the major components of the ignition system. I would swap it out before I tore the engine down.

Will this one work? Remember im using the pertronix module and coil.

https://zcardepot.com/products/electronic-distributer-new-complete-l6-240z-260z-280z-280zx

Yes, that should work. I don't believe you will need the pertronix with that distributor. Which is a good thing IMO

That particular distributor is the same one they sell on EBay. Two lead hook up. Red positive side of the coil, black negative . No ballast resistor. Everything is internal and as you can imagine not very well made but will possibly provide hints.

1 hour ago, Yarb said:

That particular distributor is the same one they sell on EBay. Two lead hook up. Red positive side of the coil, black negative . No ballast resistor. Everything is internal and as you can imagine not very well made but will possibly provide hints.

Can you guys possibly point out which distributor is recommended? I bought a rebuilt one prior to pandemic but after installation it did not turn the engine.

1 minute ago, jalexquijano said:

Can you guys possibly point out which distributor is recommended? I bought a rebuilt one prior to pandemic but after installation it did not turn the engine.