I was at the Banbury Run yesterday, over 600 pre 1930 bikes being taken out on the various road courses and returning, also there is a very good autojumble.
One of the stalls there is a chap called Sparkplug Man, he's something of an expert on spark plugs, I was there looking at getting some caps and a chap asked him what plugs to use in an A7. He said iron headed beezers were best on B6hs, I told him I use B7hs and he asked me if I find that they suddenly fail without warning or reason [they do], he said that B7hs's will wet up easily, and when they do they wont work again. he recommended I try some B6hs so I did.
I put them in and went out for two hours in the sunshine, the bike never missed a beat. It started easily, ticked over from cold easily and ran great. I was doing town, fast roads and country lanes.
Anyway, the moral after this long story is that I think mine runs much better on B6hs or at least so far so good.