Is there an idle-air screw to adjust the idle-fuel/air mix on that carb?
If so you should be able to use both the air screw and the idle speed screw to slow it down.
What you would do is back off the idle speed screw until the engine is about to die and then adjust the air screw 'till the engine speeds up a little. ( It will speed up when the air-fuel ratio gets better. )
Then slow it down with the speed screw and do the same thing again with the air screw.
Piston-ported intake 2-strokes do not idle well. At low RPMs there is enough time for the mixture already in the crankcase to go backwards into the carb and load up with even more fuel. This causes misfires at idle, commonly knows as 4-stroking.
You may also have air/vacuum leaks that are preventing a smooth slow idle. Check this by spraying Starter Fluid (auto parts store) around the carb joints. If you hear the engine speed up you can be sure some of that Starter Fluid (highly flamable ether) is getting sucked in there.. and you have a leaky gasket someplace.
Vacuum leaks will make the bike run kinda crappy at all speeds. To me it feels like the engine never really warmed up, has some trouble reaching top speed or climbing hills, and it tends to die when it comes to a stop.