I have normally only found that to be with vacuum secondary carburettors.
Not with double pumpers/mechanical secondaries, as the DP carbys usually
can accomodate large camshafts. Whereas the vac. secondary carbys idle transfer
slots are normally highly exposed with large camshafts. Before drilling holes
in your primary throttle plates, you can try adjusting the secondary throttle
plates so they open slightly, this can reduce the transfer slot position on the primary