This has happened to me before, and i know just what the problem is. Under the fuel tank, the throttle cable goes into a plastic cable housing, and sometimes the cable guard gets stuck out of the housing, causing the throttle to be constantly open. Just take off the tank, follow your throttle cable to find where it's stuck and release it.

An air leak will cause this type of problem. I assume the carb slide is moving up and down freely. Try tightening the carb manifold bolts and the carb mounting bolts. The carb and manifold gaskets may need to be replaced. Spark plug tight? Cylinder head tight? > any oil stain on the cooling fins where the head meets the cylinder = bad head gasket or the head isn't tight. Valves set with correct clearance? Find the air leak and fix it. :)

Please rate this solution when the problem is fixed. Thanks!

Chances are the piston that is inside your carb is sticking. This happens when vehicles sit and dirt, grime, whatever gets built up. This piston has to be able to move completely unrestricted. I would recommend removing the carb and completely disassembling it, cleaning it which would include lightly rubbing the cylinder wall that the piston moves up and down in to remove anything that may cause the piston to hang up. I hope this helps and please do not forget to rate me.