One thing at a time. Do just the idle. Total later. If you have timing tape or a dial back light, you can measure and plot the advance curve. But that will come later.
Plug both vacuum ports for this portion of tuning and testing.

Not doubting that. Just offered some possible causes and a good reason to take action carefully. The area below the blade will be the fuel restriction and the area above will be the air restriction. Length alone is not the only factor. As its an older version, widths are probably the same, but these are things to look for.

Sure you can. You can adjust the screw a little more or less as long as the throttle plate is in the range of .020 to .040" on the slot.



A. This is why I wrote that it is important to measure and write down the amount of slots shown for each 1/4 turn of the rpm screw. (aka idle position screw or throttle stop screw). With the information written down, you will always know where the throttle blade is without removing the carb. This is also handy when using the screw to hold the blades open for testing other things like timing.

B. With increased initial timing, the rpms at idle will increase.
Yes, you may have to start and warm up the engine with the throttles more open. Then increase the timing a couple degrees before reducing the rpms with idle stop screw.
Next readjust the mixture screws for highest vacuum or best rpm depending on what you have. Then set each a little richer (1/8 turn) since under load it will want richer. It is likely they will not be equal as the tranfer slots are unequal and also the distribution on a manifold is rarely equal.

This procedure above may have to be repeated to get a strong idle.

1) There is no magic number or magic shape. 0.025" or square is a good starting point. Over .040 will be a problem because it effects the off-idle driving. You must know that the "idle circuit" provides fuel for most low speed driving and even some cruising up to 2500 rpm or more.

2) If the engine still will not idle strongly without opening the throttle blades more, and you don't want to increase timing any more, then it is appropriate to let more air in. This can be done by any one of several methods; drilling the throttle blades, making an adjustable airbleed in the center of the body casting, cracking the secondaries open a little, or changing the PCV valve restriction.

I personally like drilling the blades. It provides air right next to the fuel coming out of the idle ports so should help mixing. As Dan pointed, this method was even used by Holley on some new carbs.
However, especially because you are in Spain where parts may not be as readily available, my suggestion is to test out with a reversible method. Either the PCV valve or the opening the secondaries a small amount will be good tests.

see above

Robert mentioned the fast idle cam screw. On the choke mechanism there probably is a fast idle cam and adjustment screw. This screw will change the relationship of throttle to the choke. It may be of some help in getting the choke to work better for you. Try setting it so the throttles are open more when the choke is only part open - the situation when the engine has just started running and you have reduced the choke.

Much older vehicles had seperate manual throttle and manual choke cables. Go back far enough and there was also manual timing...

Urich and Fisher's illustrations explain what happens when the throttle is not in the currect range on the transition slot. google urich Fisher wild camshafts if the link above doesn't work.

Now the first time I drilled the throttle plates (on a 3310-3) it was long before I knew about the timing. Heck I hardly understood how the carburetor worked. It did idle better, but had no power to get up the hills on my drive home. None, zero, zilch. We experimented with some different size wires in the idle air bleeds until it worked well again. Every situation is different, but if you do have to add air at idle, then you may need to make the off idle a bit richer the same way I did.