Clean the carb completely , and check the float height , if you havent got a manual you can probably get one , it all helps.
Be sure that the throttle cable is in ok condition , and that it has a little free play.
The crankcase seals at both ends of the crank could be leaking. At the ignition side , air will be drawn in , and at the cluth/gearbox side oil will enter the crank case , if they are leaking. This will also make the bike hard to start , and could be the reason for a higher and/or more unstable idle than normal. I would think they can be changed without splitting the cases....
You can learn alot by trying to fix this bike yourself , but dont spend too much money on it , you will never get it back !
Also , Eric Gorrs site ( Forward Motion ) has some excellent articles on different kinds of engine rebuilding , you could get a lot of good , general knowledge from reading these articles !
Good luck !