Air fuel mixture set-up help

[Smugger_69]

Started when i noticed the bike is off like stink down hills or when momentom is on my side but as soon as i THROTTLE it up at hill orr on a flat it makes a WEERRRR nosie and bogs out and if i dont let off and keep her bogged she die's

Been told on hear its the MIXTURE air and fuel
Can any1 tell me how to set this up?? and witch screw i should be playing with?

Stomp 140 lifan engine

Please help much appreciated