TR6 Carbon Fouled Plug

I have a 72 TR6. Last March I had the cylinder head rebuilt and the car ran great. Today I drove the car on a two hour round-trip. On the return home, the car began to miss ever so slightly at lower rpm. Tonight I pulled all six spark plugs. Most of them are in normal condition. Only #4 is carbon fouled and has bridged the gap almost completely. Everything I have researched has touched on the subject of an over-rich carburetor. The problem with this theory is the aft carb feeds plugs 4,5,and 6.
If this aft Stromberg carb is running too rich to foul #4 why did 5 and 6 survive? I am wondering if I should suspect a bad valve guide or valve itself? I will pull the valve cover off in the morning and check the adjustment and see if the valve is working. Beyond that, I am at a loss for explaining the carbon on this one plug. I am using the recommended Champion plugs and the timing is set properly. Has anyone else had this experience?

I am wondering if a compression check might be in order here.

All 3 indicate a rich mixture. #4 just caught the worst of it.

I'm with Poolboy - if the other 2 cylinders looked better I'd be more worried about #4. For now I'd look at tuning your carbs including the front bank if they look the same. There is a slightly higher temp champion plug out there that people use which might help too.

Looks over rich to me too. I'd clean them off, lean out the back carb and drive it a while and recheck after the same amount of miles since the head rebuild and reassess from there.

Thanks for the support guys. Tonight I finally got around to investigating the problem. I checked the valves and the lash. Everything was in limits and working normally. The compression check told a different story.
#1 90 wet: 110
#2 121 wet: 145
#3 135 wet: 160
#4 70 wet: 80
#5 135 wet: 140
#6 95 wet: 130

I had this cylinder head overhauled almost one year ago. Compression readings have dropped since then; especially on 1, 4, and 6.....

Does this appear to need a lower overhaul? My gut reaction is yes.. unless something in the valves has failed,, like a guide? Anyone have an opinion?

If you had the head rebuilt and left the lower end as it was you will need to have it rebuilt sooner rather than later. I would do a sniff test of the oil to see if it has any gas smell as well...if so a rebuild is in your very immediate future.
Rut

I thought rings on the first post. Does the car sit a lot?

Thanks to everyone who contributed advice; it is greatly appreciated. The wife hates the Triumph and it only get driven on nice days and when I'm going somewhere alone. So, it sits a lot. I never abuse the car but drive this car with spirit and sometimes autocross it. But I guess it is time to remove the engine and do a rebuild if it is to remain in the stable. I have done a lot of mechanical work myself such as clutch, drivetrain, brakes etc. but usually refer major work like transmission or cylinder head to professionals. How difficult is it for an average guy to do the lower unit himself? Should I have a reliable machine shop deck the block? Is a more intelligent decision to haul the engine to a shop and have it done professionally? The last thing I want to do is attempt this myself, do something incorrectly and have to pull the engine a second time. This is a time consuming job.

If the plug did not foul before head was rebuilt I would suspect a guide too big and oil fouling plug. Would be hard and dry after running. Check that plug after a run and it has sat for a while. Might just need that cylinder might use a valve oil seal.