My oily plugs (after 200 miles the front one had the electrodes hidden in a carbon slag heap) were completely cured by oversize OD liners in rebored muffs, and new pistons. I was told, by Trevor and others, that the muffs gradually lose their grip on the liners, and that the interference between the two is essential. I'm now completely certain of this. Wear is one issue: time is another.
I subsequently replaced the valve guides with new sealed ones, but it was the bores that made the big difference. None of this made much difference to oil consumption, because oil was being chucked out of the breather. After a great deal of faffing about with standard timed, standard with extended timing, standard series D, and series D with a non-return valve breathers, I realised that the problem was too much oil UNDER the pistons, in the crank chamber (put there by my two start pump). So I put the restrictor wires back IN to the rocker feeds, and the problem virtually went away. The scavenge now empties the crank chamber without the rocker feeds dumping it back in again before it ever reaches the oil tank. I detailed this in the last MPH.
I am now convinced that had I concentrated on reducing the clearance between piston and liner, and not dealt with the lack of interference between muff and liner, I'll still be faffing about with breathers.
Incidentally my final breather system is identical to the one Eddy arrived at, but with the timed breather plugged.