Gas Mileage 1300 pre-1979

Here is the info from the service manual if you are going to check the jets:

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One other thought: If WD desmogged the car, depending on what they did could result in some unexpected ignition advance issues. That distributor has a very large centrifugal advance range as well as vacuum retard. The vacuum retard gets tempered by a delay valve in the line but if that was "desmogged" without any other changes you could get some strange effects. I would check the static timing as well as that at full advance (about 5000 rpm for that distributor) and see if they are in range.
Thanks, the fact they did that 'hero' work does have me concerned that maaaaaaaybe the master mechanic didn't check all the boxes. When she goes in to have the leaky boots replaced, then all of this will be scrutinized, hoses, routing, timing, etc.
 
When she goes in to have the leaky boots replaced, then all of this will be scrutinized, hoses, routing, timing, etc.
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.
 
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.
A pic of what she came with....thanks for this info too :)
 

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That looks like a stock 74 Ducelier distributor. Just for laughs, have a look at the vacuum line connected to it and see if it has a delay valve (black & white plastic thing with axial input/output ports) in line near the distributor. From there, the line should go to the thermal valve on the front of the head (near temp and oil pressure sensors) and then to the carb. Depending on how it is hooked up can affect how you want to set the timing. One desmog trick from 74 was to flip the delay valve which blocks the vacuum since it can only suck air in one direction. Of course you could also just disconnect the vacuum line but then it would look desmogged. If the vacuum line is blocked, it will eliminate the 10 degrees of vacuum retard. Obviously, you would set the timing different if this was done. You'd like to get it somewhere around 10 degrees static and around 30 - 35 degrees by the 3,000 rpm range. However, the advance curve won't get you quite there without modification. You can compromise with it stock but you won't get full advance until about 5,000 rpm and it will be a bit too much advance if static is set to 10 degrees. I found that setting the max advance at 5,000 rpm around 35 degrees and living with a bit less static advance was a reasonable compromise until I recurved it.
 
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