Norton carb needle fouling

[GJM_T120]

I've had problems with the Amals on my 71 Commando and have solved most but this one.

I'm getting this fouling/varnish on the needles after very little run time.  (Also getting clogged pilot jets but these are easily cleaned with a guitar string.)  If I pull the needles and clean them the bike runs like a top, but this is alot of work for every time I want to ride.  

Any ideas what's causing this?