Of the two 600 singles, both Vincent's I have just done, one using twin plugs the other with a single plug, the one using twin plugs definitely had a stronger idle and very crisp response as you crack the throttle. By comparison the single is fine, and probably very normal as we know them. They both run on normal pump fuel, the twin plug set at 26 degrees full advance and the single on 34 full advance. Seat of the pants performance of both seamed very good, although the gearing for both bikes was quite different, the twin plug being a stock Comet and the single plug in a Slimline Featherbed geared much taller, similar to a twin. My take on twin plugs is that the larger the bore size the more effect it would have, plus having a piston with a very tall dome with the plug over to one side, running two plugs must help. I have recently also had some discussions with a very experienced engine tuner of classic japanese in line four race engines. I would say that high reving smaller bore engines like these suffer from a poor "Burn" in the combustion chamber possibly because of the high revs they run. By using a tapered squish band on the piston and combustion chamber and twin plugs, the burn improves very much, and he is getting very good HP figures from these engines now. This is mostly on Methanol fuels for both solo's and outfits.