I've been trying to chase down the cause of a miss/cough that seems to only happens at specific RPMs, there are 3 or 4 specific spots btwn 2k-3.1k. The car runs strong and smooth above 3.1K RPMs.

My spark plugs are a couple months old and 3 of them look as they should, but the #4 is different. It has a layer of carbon around the rim and down inside the inner dia. of the plug body, there are white speckes that appear powdery all over the carbon, and the insulator is darker brown than the others. The same cylinder has a different color injector than the other three and I am wondering if it matters. I can't seem to find the source now, but I remeber reading somewhere that there are different flow volumes for different injectors. Does this sound familiar to anyone?

I have recently changed all of the vacum tubing, new cap rotor and plugs, air filter, checked timing, did the flame trap, TB and TPS, and cleaned the IAC valve. The AMM checks out fine with a digi multi meter, as does the O2 sensor (though it may be a little slow).

Any ideas? I appreciate any input.
Thanks -JonB. 83 245DL