I've had this same problem and if it makes a rattling sound it sounds like its the catillitic converter.
Cylinder misfire. Possibly failed or failing engine mounts.
a/c kicks on
check your oil lol
650-700 rpm.. If any lower it tends to shake and vibrate to much.. and anything higher is unnecessary.
Ah yes the mysterious shake... As far as I can tell, its the torque converter... Though if that's not your problem, it could always be your idle sensor... Could it possibly be the U-Joints as well?
Check the idle, crankshaft and camshaft sensors.
Check the motor mounts.
what does idle funny mean? Does the whole car shake? (upper engine mount collapsed) Or does the engine shake/misfire? (Intake manifold gasket)
Tune up
Idle speed is computer controlled and is not adjustable. If the speed is incorrect you could have a sensor or actuator malfunction. Other causes include a vacuum leak.
because that's kind of a big engine
twist the top of the choke, there is no idle screw on the 1999 yz250