Modern Vespa

hello everyone, my vespa holds back a bit the first 2 seconds when you accelerate, after that it pulls on well. I have already done the following: cleaned the air filter, cleaned out the carb completely, new spark plug; after riding I checked it and it was coffee brown-black, checked if the cylinder was tight, put in a new exhaust gasket, adjusted the valves, air screw is now 1 3/4 off. what else could be the problem? i would like to hear it.

Put it on the centerstand, start the engine, and rev the engine slightly. Lock the throttle into position with a zip tie around it and the brake, so it stays slightly elevated RPMs.
Now get a rag, and a glove that is good at handling heat, and force the rag against the tailpipe so no exhaust is escaping.
If the engine keeps running, even stumbling, without shutting off, you have an exhaust leak. If it does die, check the intake manifold for leaks or a bad fit at the base, and make sure you are idling off the idle circuit and not the pilot jet. From the hesitation at take off, and having no idea how to adjust the air screw while the engine is running, it's likely this is the culprit, if the engine does die. So when you shut off the throttle, it's not completely shut off, and air is still getting in.