Now, Alfasud this is a bit difficult to explain. The thing is that you have screw for air on each carburator (if double then you have two screws per carburator). Not many poeple can do this right. First you turn all of the screws to closed position (top right) and then unscrew them for half turn on each screw. Now the difficult job, take one screw and ignition cable from that cylinder, unplug it, if the sound changes then it is close to OK, so the thing is that you have to walk around every cylinder and check every cable and screw together. If you unplug some cable and nothing changes, you have to unscrew it a little more, be carefull not to twist it to much (half a turn by check). Don't do this when your air filter is not on, cause you will tune it up, put the filter on and car won't work good again. This is the game that you have to play few times because it is hard to do it at first. I hope I helped (sorry for bad English).
Regards,