Hello Juan
Welcome to the forum and congratulations on the purchase of the car!
To help with your problem, it would be best to know some more details, as Michal mentions. I would also like to know the mileage on your car.
It sounds like you have either a genuine Murena S
or one that has had the kit fitted. A few things can go wrong with sidedraught cabs, particularly tohe Solex'es, which are unfortunately not as well built as the similar Dellortos or Webers.
But I think your engine is sucking false air in one or more cylinders.
To find out which, I suggest you do the following: Adjust each idle mixture screw alone. Give it a 1/8 turn out and check if the engine reacts to that. If it speeds up, give it another 1/8th of a turn. If it slows down, turn it back 1/8 so it speeds up again. Do that until idle speed starts decreasing. Do that for each cylinder in turn. If idle drops too low or too high, adjust the idle screw so it's back around 1000 rpm.
When you feel it's running correctly on all four cylinders, shut the engine off and turn each idle mixture screw down to the lightly seated position. Count the number of 1/4 turns carefully, and turn it back again to the position you found it in. A 1/4 turn difference between the cylinders is not uncommon, but if there's more on one cylinder, then you know where the problem is.
If the problem is related to one of the two outer cylinders (where you have vacuum pickoff's on the manifold), try blanking them off and repeat the adjustment above. You could have a small leak in one of them.
If there is a leak, it could be the manifold gasket that has been sucked in. Replacing it will not harm anything, but you could try tightening the nuts first as removing the carbs completely is a bigger procedure.
Another possibility is for the brass bearings in the throttle spindles to have been worn down. I think these carburettors have brass bearings (and not ball bearings like the Webers and Dellortos), in which case they will wear down with age and use. If that's the case, then carbs are difficult to repair, unfortunately
Another forum user suggested pouring a small amount of thin oil around the suspected vacuum leak - it will be sucked in, and this will initally seal the leak, and the engine speed will change (probably drop). After a while, the oil will all be gone, and the leak will be reestablished, and the engine speed will be like before.
Third possibility is for the carbs to be in need of a good cleaning. If the car has been standing for a long while, the fuel will have dried out. Petrol leaves a chewing gum like black substance, which can block the little passages in the carbs and jets.
I suggest you do some obvious checks, and then go for a complete stripdown and rebuild of the carbs. If you find that they have been wornd beyond repair, I suggest you take a look at eBay for replacements - Solex ADDHE's are frequently offered (with sellers usually claiming they are in good shape) and sells quite cheaply. You can then move jets, emulsion tubes, chokes and aux venturis over from the original carbs.
I hope this helps, please let us know about your progress! And I'm sure we'd all like to see pictures, and know more about your car and you.
Best wishes,
Anders