Hi, welcome to the forum
The revs dropping with electrical load makes me suggest checking that the battery is fully charged and that the terminals are tight. But then again, what does it start like? Cranking over quickly?
So instead maybe there is just not enough power from the engine when idling to drive the alternator, in which case things are in a bad way, aren't they... There's usually some audible drop in revs though, say up to 100RPM.
A common fault with most Unos is the vacuum advance diaphragm on the distributor (round capsule with rubber pipe attached). The rubber pipe needs checking as well, for cracks. I pull it off the carburettor, give it a quick clean, and suck on it (really!) to see if there's an air leak through a split diaphragm. If it's OK, there will be a sense of movement inside the distributor and you won't be able to draw any air through it.
After checking/replacing the diaphragm, adjust the ignition timing. Take off the vacuum advance pipe and plug it with something (e.g. a small screw). Then connect a timing light to the No. 1 spark plug and take off the rubber cover on the clutch bellhousing. If you point the timing light at the flywheel you should see a notch somewhere near the four notches in the bottom of the window. Turning the distributor body should cause the notch on the flywheel to 'move'. You're aiming for ten degrees I think. It's difficult to be exact because this model (1372cc carburettor) doesn't exist in the rest of the world!
Actually I'm a little worried that your car could be fitted with 'type 2' electronic ignition, where the distributor does not serve any timing function. These types have a crank angle sensor, and the rubber pipe will be connected to the ECU instead of a vacuum advance capsule. You can check the timing anyway, but it will probably be correct.
As you probably know, the mixture adjustment screw is in the base of the carburettor, usually pointing towards the right side of the car. If you unscrew it, it leans the mixture. Usually you unscrew it and the revs increase drastically (because as you say, someone has enriched it to compensate for an air leak). Basically, you undo it until the revs reach a maximum, then put it back in one turn for good measure. Go around the back and listen to the exhaust - if 'chuffing' then it is too rich, if there is the occasional 'miss' then it's too lean.
Idling on FIAT engines is never particularly good, but you should be able to get it better than it is. Some fluctuation in RPM is normal, and it will always idle a lot slower when cold than hot. Also check the spark plugs - I have read that the idling is better if the electrode gap is WIDER - up to the limit of 0.8mm or so.
Your fuel consumption is probably mostly a product of the ignition timing being off, but also check the air filter (not always as critical as some people suggest, but there are some really neglected cases out there).
Cheers,
-Alex
