You weren't exactly clear about what's happening but i'll take a guess.
Engine RPM is not determined by road speed if your variator is working properly.. The variator automatically adjusts engine RPM when you start to climb a hill or hit a head wind or for other reasons..
Both the front and rear variator units must be free to widen and narrow so "gears" are able to change from lowest to top gear ratios. If either one is rusted stuck or has other mechanical difficulties, road speed will be limited and/or engine RPM will be affected.
Aside from that, there's a big bolt on the side of the muffler. Remove that and right inside there is a small baffle tube. All your exhaust must flow through that tube. The reason for the bolt is so you can stick something in there and clear that exhaust baffle tube (which, being so small, is the first thing to get clogged up). Also check the cylinder exhaust port for carbon build-up.