Hi there and welcome!
Your English is good, where in the world are you?

The carb manifolds do break up when they get old, try and get a new one, Tyga can probably help if no one has one spare.

With the carb screws you may be able to cut a slot (with a hacksaw) in the screw and get a flat blade screwdriver in there or use a reverse tap or stud remover to get them out. Or use some very good mole grips. If not maybe a local engineers can help?

As for the "yellow plastic", no it's not a restriction, they're the rectifiers, HRC make slightly more open ones but they [original or HRC] are required.

The RC valve motor only moves if it has to, not just as it's been powered up, if you unplug the TPS (wires coming from the carbs, go into a plug then into the loom, unplug this plug) then turn the ignition on it should move. Details on checking the RC valves position here (down near the bottom of the page).
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