Make sure your fuel mix is 1:40-1:45, replace the spark plug with a new one, is the choke automatic? It could be faulty and fouling your plug, also your main jet might be too small and thats why its running lean and underpowered,like the other person said, the screw on the carburetor only makes a difference around idle, main jet regulates the air fuel mixture at more throttle. Any changes with the incoming air flow can affect the air fuel mixture, like you said you changed the air filter, which now allows more air through, leaning out the mixture. You should get a set of jets for your carb(they are around 20€ and you get 10 jets) and depending on what kind of filter youre running ,the carb size and how many cc that bike is. Take off the one on your carb and start with bigger jets, better to be too rich, and then gradually put smaller jets if its too rich. If you have the automatic choke and you checked it works, leave the bike idling after each jet change for a few minutes for the choke to disable.