aerox yq50 running problems

[yossp]

hi. my aerox has started to slow right down.when i pull away it seems fine(0-15mph) then it seems to bog right down max 15-20 mph. it seems to struggle to get fuel. start up and tick over seem fine but when i try to give it more power it is really sloppy. on way home today felt like fuel starvation at times(coughing and spluttering). please help.all bike is standard.apart from new standard size cylinder and piston ,4.2grm rollers . it is a 2006 yamaha yq50. could it be need new carb? please please advise. shops charge the earth just to have a look.