Trying to restore a 1981 SP500. The bike will start with one kick and only when I hold the throttle open. Once it's running I have to hold the throttle open for it to continue running. If I let off the throttle the bike dies. If I open the throttle all the way up the bike maintains the same RPM and very slightly increases but does not go past 2000-3000 RPM. I have taken apart the carburetor a dozen times and checked and rechecked everything making sure it is to spec and working. What would cause this issue and what are some possible fixes? I live at 4500' and everything is stock: jets, airbox, air filter, carburetor, etc.