Take apart the carb and clean it. Then clean it again. Clean it a third time even. It has to be spotless, especially all the tiny passages.
When you're taking it apart, screw the two adjustment screws in all the way, and count how many turns it takes. Write this down!! You can set the screws back to this after you've taken them out and cleaned them, and the carb should be close to right.
One screw is for the throttle stop, and the other is for the idle air mixture.
Before you try to adjust eather of them after the carb's back together, there's a few other things to do.
1. Make sure you have a new-ish, clean air filter.
2. Make sure you also have a new air filter element in the bi-starter (auto choke) chamber.
3. Make sure you have a new-ish fuel filter, or the carb will clog up again.
And most importantly, 4, make sure the auto choke is cleaned out and working right.
The auto choke is bolted to the side of the engine head, and as it heats up, it stops feeding in extra fuel. But it tends to rust and gum up with age, so you should take it off and clean it out. But you can't take it apart, so you have to spray WD-40 or similier down the passages, then flush it out with alternating hot and cold water to force the thermal thingy to move.
Once all this is done, start the bike and ride it around for 10 minutes or so to make sure it's absolutly fully warmed up. If it doesnt idle properly with the adjustment screws back where they were before you messed with it, then you can try adjusting them.