how can i fix my dna 70 ?

hi can any one tell me how comes my dna 70 rev to 5 be4 pulling away i got a new drive belt in it but i dont know a lot about the dna and i got about 3 weeks ago and it only dose 58mph but the man i got it off told me he got 75mph out off it can any one help me with this it smok,s a lot to ?????