You should be getting at least 85mph out of your bikes. Sounds like you´ve got something going on. a couple of easy things would be to clean out your carbs really good. take them off and take all the jets out and clean it really good. they are very very simple carbs, so it shouldn´t be difficult. just be sure not to lose any parts and don´t forget the spring that goes in between the throttles. it´s easy to lose or forget. It doesn´t look important, but it is. you can also check the big rubber gaskets on the top end of the carbs when you take them appart. if they have holes in the, which most of these bikes do, then that will hurt your performance. the bad thing is that they cost $200 a piece to replace. I have put cylicone on them and they ran really well for a while, but it´s really not a good permanent fix... but better than spending $400 on an $800 bike. Let me know if this helps at all. The bikes are so simple that it would almost have to be something with your carbs.