NGK 9's, gapped them at .6, then .7mm, always the same.
I have just been out after cleaning the plugs and gapping at .7 from .6, they were covered in black soot after a mile or so.
0.7 is worse than .6 cos now she misfires as soon as I hit 3k, even if I feather the throttle whereas .6 only misfired badly on wide throttle, I could rev her up with careful application of throttle.
I would try .65 but I can't find my feeler blades, my other gauge is just bits of wires of various thicknesses, can't add two together to make .65
I had a similar problem when I put the mongoose exhaust on, spark blowing out. Maybe the new valves are creating more compression and the spark plugs can't take it, or, it is time for a new set of HT leads, maybe they were fine before but not now that the compression has gone up ?
It's definitely a misfire, she makes a "barrp barrp barrp" kinda noise.
Out all day tomorrow in london won't be able to do anything until monday now.