I have a 1967 MK2 3.4l badged "340" which has sooty plugs in the following pattern. Plugs 1 and 6 fine tan condition. Plugs 2 and 5 somewhat sooty. Plugs 3 and 4 sooty. I've made all the adjustments on the carbs and was wondering what else could be the cause of uneven firing. Hard acceleration from a roll in 3rd or 4th causes engine to stutter which smooths out once RPMs increase. Runs great at speed and in 1st and 2nd gear. The car has aftermarket distributor and resister wires using standard (non-resistor) Champion plugs N12YC.

 

I would check the plug caps, measure the resistance across them and if any are open circuit try changing them. I had this recently with some Champion Caps where one failed causing a misfire.

 

The central cylinders tend to run richer due to the layout of twin carbs and a log manifold. The effect may be exacerbated by ignition problems failing to burn off the carbon or carburettor adjustment making the mixture sometimes too rich. The starting carb may be staying on too long or set too rich or the oil in the SU piston damper might be too heavy.

 

The plug colours have been changing on my old cars and bikes as our fuel has changed here in Old Blighty and I don't worry about it. I only look at them when they've cooled, and the old oil has drained out, after a decent hot run and the plugs' colour varies along the cylinders on my '66 S Type because, I think, of their distance from the carbs and the radiator. I just check the gap and always shove them back in where they came out.

I've been using Brisk plugs now for a year or two because they're. apparently, still glazed in the old way, before lead became such a scary thing. I'm very happy with them, particularly in my old bikes.

Ray..

 

If the spark plugs and the absence of glazing is the source of the problem, others should be experiencing it with older, carburetor fueled XK engines - I cannot judge as my car hasn't run for a few years. If it is the plugs, there are a few possible actions:
Buy old stock plugs. The Green Spark Plug Company has old stock. They may also be able to give some good advice. They even have Golden Lodge, which back in the day, gave longer life than Champion (and much longer than Bosch). I used Golden Lodge in my Mk2 in the 1970s and 80s. In those days they cost more, but were worth it. I had to buy them from Alfa Romeo garages.
Weaken the fuel-air mixture a little. But not too much!!!
Make sure the corners on the plug electrodes are sharp. Even cut a groove.

 

In my experience, Champion plugs lost their way back in the '80s when production moved out of the UK, initially to France, I think. I went onto Bosch for my bikes but had to run twenty miles to get them. I moved onto NGK in the late '80s because of the easy availability from my ;local motor factor.

NGKs have played up in the last decade or so after they changed their porcelain glazing process. If they get wetted up they'll rarely spark again. In old bikes that have to be tickled to start, this can easily happen, or if I don't get the choke off quick enough. I changed to Brisk on several bikes over the last couple of years and have had no problems since. I fitted Brisk in the 3.8 S Type when the time came for new ones although I'd had no problems with NGKs in this.

I've had no problems with modern bikes - a set of NGKs in my 2005 1050 Triumph did 28,000 miles a few years ago when I mislaid my service record. Just my take on plugs - we all have our favourites..

Ray