One is the primary plug were the flame front starts and is showing the correct A/F mix. This plug is fires BDTC and fires from the coil throught the center electrode to ground. The second is the waste spark. It fires ATDC (the A/F mix is now lean and the plug also shows the correct lean condition) and fires from ground to the centre electrode and finally back to the coil. This all happens in a split second. You need a lab scope to see it.

Posted from my Motorola Milestone

 

Just to throw off your theory, the two pictures are from different cylinders.

Each pair from a cylinder look identical (both brown in cylinder 1, both white in cylinder 6 etc).

If all the plugs were a in a range between the brown and white shown I wouldn`t worry but four cylinders are exactly as per one photo, and four cylinders are exactly as the other.

If we were still running carbs that would be a definite sign of mixture imbalance, added to which it just doesn`t run as smoothly as it did for the first 10K or so of it`s life, no changes or mods made at any point and no different since this plug change.

 

Well I guess it was fouled injectors, cured slight missfire after using two cans of Seafoam in a tank and a half of fuel. Now runs smooth again, I'll check plugs again soon but Seafoam looks to have cured problem, I had tried some other injector cleaner a while ago which made no difference.