The fact that 3 and 4 are different, makes me think you have an internal engine issue. If my plugs exhibited the same, I would check valve clearance and do a compression check to investigate further. Apologies if you have already done these checks and I am being repetitive.

I have already done the compression test and there is now issue whatsoever. When i bought this car the engine was assembled by california datsun. 

That is strange indeed, typically one side or the other will be darker.

 

Some other thoughts and forgive me if it’s already been said.

 

Let’s eliminate potential electrical issues:

 

1. Swap 3 and 4 with say 1 and 6 and see if you get the same effect. Could it be a faulty pair of plugs? And are the gaps the same (although that shouldn’t make a huge difference).

 

2. If 3&4 soot up again, try rotating HT leads from other plugs that aren’t sooted up.

 

3. Check the 3&4 electrodes on the inside of the distributor cap to ensure they are not corroded.

 

Looking at the mechanical side:

 

3. Have you checked manifold gasket for leaks?

 

4. Have you checked valve clearances lately?

 

5. On your comp test, what was the difference between 3&4 and the other cyls?

 

3&4 aside your plugs look a little too hot to me, how have you adjusted your mix? I have found that idling for a long time with a lean mix on a hot day, the car starts to misfire as if there is an electric problem, but it’s a heat problem.

 

 

 

 

what about getting a spark plug extender. I still think it could be a valve seal as this engine was remanufactured by California Datsun. I am about to open the engine and change all the valve seals. Any other option? 

On 6/23/2019 at 6:32 PM, Mark Maras said:

 They look pretty good. Odd that 3 & 4 are a bit darker. As you said, clean 'em up and try again.

Mark,

Sorry i couldnt keep up with your recommendations on solving this weird misfire issue on the car´s 4th cylinder but the starter engine went south and i had to purchase a new remanufactured one. I have read so much on this matter and even though the compression test results were good i still believe there must be a valve not working properly. I drive the car a couple of blocks or leave the car running for 25 to 30 mins and the 4th spark plug starts fouling. If you step on the gas pedal, the engine starts missing power until the spark plug is cleaned. Any other option other than getting inside the engine to change the seal or valve in that cylinder?  Should i purchase a 14 mm spark plug extender that avoids the spark plug getting fouled? 

https://www.amazon.com/Dorman-42000-Spark-Plug-Non-Fouler/dp/B000BYD9Z8/ref=sr_1_2?dchild=1&keywords=14mm+spark+plug+extender&qid=1602790752&sr=8-2

 

 

Edited October 15, 20204 yr by jalexquijano

  Glad you got your starter problems resolved. I wish I could give you an opinion on the spark plug non-foulers but I don't even have an educated guess. I'm not familiar with the product at all. It's cheap and I doubt it can do any harm. I'd be interested in hearing about any change if you try them. Anyone else want to chime in?

 Have you tried all the things that @AK260 suggested in post #15?

Those ngk wires are new, cap and rotor also.

I’ve had misfire issues where the tip of the spark plug was loose. The end where the plug wire goes. On some NGK plugs, the end cap threads on, and I have found many that were loose. Check yours and tighten thoroughly with pliers.

Here is a screen grab showing plugs with and without the end cap and how they thread on

Should i try a hotter spark in cylinder 4 which is fouling? BP5ES? I want to test everything before opening the engine! Please advise.

 I'd try a hotter plug. It won't tell you what's wrong but it might buy you some time.

Should i try a hotter spark in cylinder 4 which is fouling? BP5ES? I want to test everything before opening the engine! Please advise.

My friend, you have to find out why it’s fouling - what you are suggesting here is a band aid not really fixing the issue. Until you solve the underlying issues, they will keep finding you.

14 minutes ago, Mark Maras said:

 I'd try a hotter plug. It won't tell you what's wrong but it might buy you some time.

I bought this car already modified by california datsun so i really dont know if they rebuilt the engine correctly or not!