Modern Vespa

Can this be the cause for the air leak?
The idle screw does not work, the engine remains accelerated.
Why the black line on the gasket? Is it normal? Not a stain, it's thick

Thanks

Cant say thats your culprit, but Ive seen leaks from below the carb box before. Other sources are the box holding screw loosening up, the carb base gasket inside the box (below the carb), the carb itself, the slide itself, the engine seals, the cylinder exhaust stub loosening, the head, a stripped out spark plug thread. Start investigating in that order and clean, replace and re-torque and see if the easy stuff solves your problem.

That black line is from the channel your oil is pumped through. On the bottom of the box is actual channel. Oil is pumped from the front of the box to the back and then into the carb. You want that clear of debris and clean! Dont reuse that gasket unless you really need to.

The screw on top of the carburetor that regulates the idle, does not work. When I turn it, the idle remains the same.

the idle speed screw?

if so, then something is borked up.

I'm guessing that the carb is tightened down too tight and the body warped causing the slide to jam up. so now when you go to adjust the speed the slide isn't moving.