Random Misfire while cruising

[Johnny wick]

I talk to the owner of the business and he said that engine was built perfectly.

[siteunseen]

Just now, jalexquijano said:

I talk to the owner of the business and he said that engine was built perfectly.

What else would he say? Those rings have to be staggerd. The oil control ring is a worm ring that has a top and bottom solid ring also.

Have you had this problem all along or just recently? 

[Mark Maras]

  I expected that answer from C.D. No crystal ball needed.

 C.D. is a likely person to point a finger at but the question is ,where do we go from here? He's not going to offer any help. If it were mine (I wish), I'd either pay someone with a good borescope to take a look in all the cylinders or buy a good borescope and take a look myself. The cost of either option may be similar, IMO. Looking down the road, I'd also be keeping my eyes open for a used engine.

[Patcon]

My only problem with the oil blow by theory is that it looks like carbon. I am not truly convinced that we have eliminated ignition as a culprit

Boroscope might help...

[Mark Maras]

 We narrowed an ignition problem down to cap, rotor, wire and their connections because it affected one cylinder. I thought we had ruled those out in the beginning. Thoughts?

[Zed Head]

You will get a low number if one of those tube adapters is used in the spark plug hole.  It increases the effective combustion chamber size.  Five 110's and a 100 looks pretty even overall.  

Four is the dirty one though and he shows three as the low pressure.  So, bad oil ring seems to fit best.  Bummer.  I'd run a hot plug in #4 to keep it clean.  I think it's been suggested before.

[siteunseen]

You're right Charles. It's not oil blow by. Once again I bow out with Mr Panama. 

[Patcon]

49 minutes ago, Mark Maras said:

 We narrowed an ignition problem down to cap, rotor, wire and their connections because it affected one cylinder. I thought we had ruled those out in the beginning. Thoughts?

I know it was discussed but I'm not sure we ever got good enough feedback to rule it all out.

If it were mine I would see if the former 240z racing mechanic had a good dizzy cap and wire setup and swap it all out as a test. Also check #4 plug for a helicoil. Maybe there is too much resistance right at the head

It is CaDatsun after all

Edited August 25, 2021 by Patcon

[Mark Maras]

 Both are excellent suggestions. As I recall he has a reasonably new set of NGK wires and we tried swapping two wires. He eventually bought a Bosch cap and rotor and verified a hot spark. I hadn't thought of a helicoil. 

[Johnny wick]

Plugs 4, 5 and 6 after 5 to  7 mins stop and go traffic today! What next? Ngk Bp5es spark plugs!!

[Patcon]

If all 3 on one carb are like that, I would start there.

Maybe stuck choke or choke cable binding up

[siteunseen]

Yes, pretty much narrows it down to the rear carburetor.

I'm not sure if the SUs have an idle circuit but that's what it sounds like.