Graham,
15 - 18 mpg is not very impressive , most 6 cylinder Bristols should achieve 26 - 30 mpg on a steady trip at 70 mph and about 20 in city driving .
Attached is a simple float level sight tube similar to what I use .
It is made by drilling a hole in the centre of the hex of the main jet holder. Then solder a small piece of copper tube into it .
I suggest you check the diameters of the main & idle/pilot jet to see if an earlier owner has drilled them out . this was a common practice in the past.
I have found the best way to check the mixture is with the use of a mobile gas analyser, it can give the mixtures right through the rev range under various conditions .
Geoff
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