I tried searching, but only found a similar post on nissan forum. All of my spark plugs look exactly like the one in the picture (picture is from that nissan forum). Actually more defined HALF/HALF.
I had my injectors cleaned and flow checked at the shop.
I checked o2 sensor as per Helms manual - all readings are within specs.
Compression test is ~190+/-2. Leak down test is within 5%. Block leak test hasn't detected any hydrocarbons (exhaust gas) in the coolant - I don't think it's a head gasket.
I'm having overheating issue, but I pretty much narrowed it down to old radiator (may be partially clogged).
Car is idling fine with very (extremely) minor (can only feel it at the exhaust pipe) "burps" (approx. once every 2 seconds) or you may call it misfire, I guess, again - VERY minor.
While driving it sometimes has flat spot (a miss) in acceleration when I let go off the throttle and step back on it.

 

hmmm... your next step is to check timing.... you will still get good compression if your timing is off by a tooth but you can exibit hesitation / burping if its off my a tooth... also check the dizzy timing with a timing light make sure your at 17 degrees ( right? 17 degrees is stock?) lol

 

checked timing, I was a little off - put it right on 15 BTDC (red line). I was off by 2-4 degrees may be, burping/acceleration hesitation is still the same.

 

If you haven't fixed your overheating problem do that first. Spark plugs are basically heat exhangers. The temperature of the motor affects the operation of the spark plugs.

 

Engine temp affects it by making it half black?

 

white = lean
black = rich/oil

so the answer is somewhere in your combustion chamber something isnt happening as it should. i really would have guessed a bad valve, but you say your compression is good so assuming your correct on that i really cant give you any further useful advice lol.

 

I think leak down would have shown a problem with chamber: valves, etc.