20 minutes ago, Zed Head said:

How about the old standard of mixed up firing order? Just watched a pro do that. It happens.

Correct firing order?

1-5-3-6-2-4 CCW?

Edited 5 hours ago5 hr by Patcon

I've also seen old plug wires crossfire or short to ground. Waaayyy back in high school working with old worn out car parts. Used to have to arrange the wires to avoid problems.

Also seen that visible weak spark does not always spark under cylinder pressures. Had a weak ignition module once.

You're probably running points though?

Edited 4 hours ago4 hr by Zed Head

Plugs are new numbered Nissan. Don't suspect them...

Distributor is new electronic from Ebay. No resistor. The cap doesn't locate very positively. So being made in China makes it a suspect...

21 minutes ago, Patcon said:

Distributor is new electronic from Ebay

I ruined a GM HEI module by running the engine with two spark plugs disconnected. I assume that a stray high voltage discharge, or some sort of "flyback" (it's a thing), made its way through the module. After I turned the engine off and reconnected the wires the engine wouldn't start. I checked spark and got a weak orange spark. The engine would start with starting fluid but would not without. Put a spare new module in and it fired right up.

How about some details, just for fun, on the new electronic? Is it a ZX type or Pertronix or something else?

Edited 4 hours ago4 hr by Zed Head

Probably not your issue, but thought I would share. I had an issue with my white Z where it ran great driving to a show 20 miles away and then horribly on the return drive. Cylinders 2 and 3 were bad. I was convinced it was fuel and wasted a couple hours rebuilding and then replacing the front float bowl shut off valve. Eventually I took off the valve cover and saw that the #2 and #3 valve rocker arms had 'relocated'. The end that was supposed to depress the intake valves was no longer touching the valve. Long story short, took the head to the machine shop and was told the valve stems were all gummed up from running old/bad gas. Sadly, I do not drive the cars enough. Anyway, put it all back together and it runs great now. Machine shop cleaned all the valves and guides. Fortunately, no damage was caused to anything except for my wallet. You can see the two valve 'nicks' on cylinders 2 and 3 in the pic.

More info:

Pulled the float bowl lids, fuel was within 0.10" of the same level in both bowls.

Pulled the front carb. No obstructions as far as I could see with my ancient borescope. Clear into the head but couldn't see the valve stem.

Took the valve cover off and all valve actuate as they should appear to have the proper lift and return. No wiped out lobes

Reassembled it all and tried it again. #1 & #2 are both dead. #4-#6 are over 100dF within 15 seconds. The front two are 80dF. Rev it up and hold it for a short while and no change...

Next weekend the exhaust will come off and we will look for FOD...

Hopefully I can get it apart. The exhaust paste I used is pretty tenacious and I haven't found anything that will dissolve it yet

Distributor I'm running

Cody thinks the car starts on all six and then starts misfiring. An exhaust obstruction might do that...

eBay

New L6 Electronic Distributor Replace For 1970-1983 Datsu...

1970-1983 Nissan Datsun 240z 260z 280z. Product Type: Electronic Distributor. 1 x Electronic Distributor as Picture. (The Compatibility Is Just For Reference. -Will dramatically improve the sporty app

I have had bad spark plugs before...brand new ones. Ended up cleaning an old one and sticking that back in.

Your compression test would seem to rule out anything mechanical in cylinders one and two. And yes, front carb supplies all three front cylinders, so I would not suspect a "no fuel" issue. How confident are you that the carbs idle settings for the carbs are where they should be - so both carbs flow the same amount at idle? And, when revving?

Spark plugs can be dead right out of the box. I'd move plugs around and move plug leads around with them. I'd try swapping known good, perhaps from six to one and from four to two and vice versa.

I think a Z engine will have a very hard time running if it has two totally dead cylinders. They may be intermittently firing a bit.

So, new plug wires, new plugs, and new distributor cap?

Disconnect at the manifold/header exit and run your borescope up that way.

That distributor looks like just a magnetic pickup distributor, that uses an external ignition module.

But your exhaust blockage sounds reasonable. Good luck. Post pictures.

Maybe try one of these cheap plug testers. At least you could visibly see spark getting to the plug and rule out ignition. What do the plugs look like?

https://www.harborfreight.com/90-in-line-spark-checker-63593.html?hftm_sc=532&hftm_source=google&utm_source=google&utm_medium=cpc&utm_campaign=21901271210&campaignid=21901271210&utm_content=171677806542&adsetid=171677806542&product=63593&gad_source=1&gad_campaignid=21901271210&gbraid=0AAAAADAHb4cCWCcSJ4qCMytUVmlv7v7bX&gclid=CjwKCAiAlMHIBhAcEiwAZhZBUlzpLzH9TR7YSC9i3f7bzmYEm7gw-Ai3uRRfne-Qjhh-zYy0O4G-khoCpwwQAvD_BwE