Have you got one of those old school distributors with a contact set (points) ?! If so, then I'd start there and check the gap is correct which ultimately effects how well the coil gives a spark and your performance. While you could use some feeler gauges, the more modern approach is a "dwell meter" which is normally part of any good auto multi-meter - you set the dwell to 55 degrees in the X1/9 case +- 3 degrees by varying the points gap in the normal way - but this approach takes into account wear/wobble in the distributor. Once this is done, only then check/set the ignition timing as it's dependent on the gap. It could be the dwell and/or ignition timing is out, so you're compensating with heavy(er) right foot, hence worse fuel economy.