you should be getting much better than 12 from a 216 , these are easy to get upwards of 20mpg from with simple fine tuning.
Last trip I did to Blenheim (310kms)I sat on a steady 55mph with the deck full to level and averaged 22mpg. (Imperial)
Check the float level , and the timing can be enhanced a little with a couple of degrees more advance (depending on the grade fuel you choose to use).
Oh yeah ,, keep the revs down ,,
,, I do 55 'cause I have a 3.9:1 diff from a '55 and the wee 216 loves it.
