Like Bob, I think that feels a bit on the low side. I have most certainly been down to that sort of figure but with a sidecar fitted and someone in it. I do believe that the change to the SU carb was an attempt to improve mpg but the thing which seems to affect mpg most on a Square is compression ratio. Assuming that you have the 5.8 to 1 pistons which are standard for a 4G or MK I, I don't think you will ever achieve high mpg. But you will have a much smoother, more flexible engine which to me is worth the poorer mpg.
I suspect the only useful thing you can do is to check the usual suspects such as ignition and valve timing and distributor wear.
36 4G, 37 VH, 54 KH(A), 75 Healey 1000/4, 52/53 ex ISDT KHA (project).
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.