Random Misfire while cruising

[Johnny wick]

Maybe i should try a hotter plug bp5es in that cylinder instead of bp6es

[AK260]

I pulled out plugs 4 5 and 6. 

These plugs in the above photos to my eyes look surprisingly OK / perfect. Were they pulled after idling or normal driving?

At the risk of adding any confusion - Referring to your photo which I edited below: if I knew nothing about your engine and saw this photo, I would have said you have an electrical issue on 4 (not firing strongly / consistently enough) and that your front carb is running leaner than your rear carb. But then also would be asking why 3 looks OK, given it’s on the same front carb.



Don’t go pulling the head yet - I strongly agree with what has already been said about doing a wet / dry comp test again and share with us please.

[Mark Maras]

@jalexquijano Do you have access to a borescope? It would be interesting to look at the cylinder walls of #4 as well as a wet, dry compression test before you consider pulling the head.

[siteunseen]

They're cheap now that they bluetooth to a smartphone.

I still say lack of spark to #4. Don't you have some kind of aftermarket ignition?

[siteunseen]

And you finally found a Datsun 240Z mechanic in Panama! Congratulations, don't argue with him and maybe he'll get you straightened out. Good luck Jorge.

[Johnny wick]

Yes. Guy used to race a datsun 240z in the 80s in Panama. I have a pertronix ignitor with module.

[Johnny wick]

On 8/14/2021 at 5:36 PM, Mark Maras said:

 A couple of questions? Did you use a straw over the valve stems to prevent damaging the new seals? Are those plugs a new set? Don't pull the head yet. You need a good reason to do that. If you're absolutely positive you didn't damage the new seals, I'd do another compression test, dry and wet.

Compression test results

[Patcon]

Is that dry???

[AK260]

Thanks for sharing that.

I was looking for problems on 4 but you compressions are very consistent across the cyls. What I find intriguing though is that they all seem rather low. I was expecting over 150, below 200!

It could just be the gauge that is at fault but what is important is that your cyl 4 isn’t low compared with the rest.

So, are you still having issues or has it improved since you did the seals?

[Mark Maras]

 @jalexquijano Give us the details of the compression test. Throttle open or closed? Oil or no oil in the cylinders? I'm pleased and bewildered that the compression is that even. I too expected something closer to 160 psi and #4 lower than the rest. If the throttle was closed during the test, I'd like to see the #s again with the throttle wide open.

[Mark Maras]

 I shudder at what I'm about to theorize and I hope I'm wrong. I'd like to hear everyone's opinion on this theory and if there's a method of confirming it or hopefully blowing it out of the water (borescope?). So far, we've crossed off ignition problems, valve seal has been renewed, several compression tests have always been within specs or explainable if they weren't (throttle closed), carbs are tuned, valves have been set, I'm now down to asking myself what could foul #4 plug and I'm left with a broken oil ring in #4, If the top ring compression ring is good, that could explain no compression loss. What do y'all think?

[siteunseen]

California Datsun motor, that's the problem and most likely Jorge has had this problem since day 1. My opinion from all I've read over 10 plus years about California Datsun. He sold a motor to a dude in Panama. No chance of any blowback.

No shudder here Mr Maras. 

Edited August 24, 2021 by siteunseen