bikes are malfunctioning

I have two motorized bikes, both of which are acting strangely. It seems like I can't get the carb setting and/or choke setting right. usually the bikes start just fine but when they start picking up some speed it feels like a governor kicks in and slows the bikes down. i could only imagine that my air to gas ratio isn't correct but my adjustments have failed to remedy the problem. i just replaced the magneto on one of the bikes since i couldn't get it started but it hasn't been acting the same. any ideas?