I'm no expert, but, from what I've read you should check the spark plug to see if your running lean or not. You can find picture guides online for what to look for. You can buy a jet pack with various sizes. You can then change jet, go for a ride, check the spark plug and then change to another jet if necessary until you find the correct size needed.
Hope this helps, theres plenty of people on here who will be able to say if I'm wrong or not, and they will be able to give more info.
Brads