'72 Datsun will work fine with leaded.
SU carburettors are like a high maintenance mistress. they will work amazingly well if pampered, but need regular 'reassurance of your emotional attachment towards them". So if you gonna rebuild your carburettor, get it done by an expert, who is at least 50 yrs ('coz that age group would then have plenty experience, as later these carburettors were phased out in Japanese cars), else it would cost you an arm and a leg in break-downs and visits to the work-shop.
You would especially need to change the jet and rebuild the "Butterfly Valve", and this will need boring and fitting, as a good fit is not sold over the counter and needs work specific to your particular carburettor.
And then starts the story of tuning, where the float setting is also a negating factor, 'coz if its not just right, it would either flood or starve the engine.
A good alternative would be to swap the SU with a Solex, which is almost a straight swap, with just a change in the inlet manifold, to change the angle, so that it is flat parallel to the ground, facing upwards. Use a jet size 15% less than the original Carb, and you are good as new.
These carbs are reliable and will not flood or starve, and need almost no maintenance, comparatively.
Compression: this loss could be attributed to the boring of the piston sleeves, And the valve seats. Since this car uses leaded, there is a build-up of deposit around the valve seats which obviously lights up with every spark inside the combustion chamber, and over time, slowly eats away at the metal, thus not allowing the valves to firmly sit in their seats and so letting the compression escape.
Re-sleeving of the valve stems and grinding new seats into the head would solve this issue, and is a cheaper job than re-boring to an oversize piston. but this can only be determined after the head is taken off, and pitting around the seats is visible.
Once this is done, use an additive, either "Red X" or any other alternative for at least 500 miles, so as to build up the carbon deposit around the piston rings again, which had been washed away by the sea foam.
After that clean your carb jets, both main and idling, and you should have decent compression.
Remember that adding carbon back to the cylinder is a temporary SOS kind of repair, and does not replace the actual re-boring of the cylinder.
To actually determine if the engine needs oversize boring; When you do the compression test, with the plugs off, take the reading as you would normally.
Then taking off the hose from the spark-plug, add 2 spoon-fulls of fresh engine oil into the chambers from the spark plug hole, and let the car sit for 10 minutes.
After that perform the test again, and see if you get higher readings.
If you do, then re-bore.
If you don't then its the carburettor or the valve-seats and valve stem sleeves.
All this is done of-course, because, you say the engine turns over and does not fire.
Did you check if there is a spark from the coil?
Lemme know how it goes.........
