Thanks Nelly,
The school of thought I have followed was to balance the throttle bodies with Vacuum gauges (This should in theory give equal CO in both cylinders) then set the overall CO on the trim pot. I seem to have acheived a good balance thru the revs on the butterfly linkage alone (i.e. both air bleeds shut).
I might check this again in case the Gunsons CO I bought is not working correctly