Villiers Carb Help

I have been riding my Greeves Scottish TAS for a year now with the Villiers S22 carb and have changed over to a Mikuni VM28 as the Villiers was p____g fuel everywhere and it's never been that great due perhaps to wear or lack of use - I've only got to leave the bike a couple of weeks and something playes up like the clutch dragging or the carb running weak. Can someone please advise on the needle jet size for the Mikuni as this new carb is causing severe detonation in the midrange - this is nothing to do with the ignition setting as the bike is excellent through trials sections and off the bottom end, it's just that I ride a lot on the road and need to get this sorted before the engine seizes again. Allen's couldn't advise on supplying me with a needle jet with smaller holes (for a richer mixture) and while this will probably do the trick I'm asking if there is anyone out there who has already played with the jetting on this setup?? Probably unlikely but I'd thought I'd ask anyway.....