I have a Honda 1981 650 custom, The idle seems very slow and it tends to shut off when sitting at lights. How do I adjust the idle up on this?

I would like to know how to adjust the valves in engine B20B

sounds like the idle air screw needs adjusted out on the carb, to slow the engine.. did you just get the vehicle, and trying to fix it up, or is this something that just happened??

The only way to know if the jetting is correct is to see how well it runs - and check the spark plug color.
You can start by warming up the engine and adjusting the idle mixture. Do this by turning the idle mixture adjustment screw (there are only two adjustment screws on the carburetor: one is the idle speed adjustment, the other is the idle mixture adjustment). Turn the idle mixture adjustment screw in or out until the fastest idle speed is attained. Then adjust the idle speed with the idle speed adjustment screw.
Now ride at full power and see how well it performs. If the engine cuts out or surges at full throttle, that means the main jet is too small. If the engine produces black smoke from the exhaust and doesn't seem to produce enough power, then the main jet is probably too big. Confirm the main jet correctness by looking at the color of the spark plug: white or light tan = too lean (main jet too small). Dark brown or black = too rich. Dark tan to brown color = perfect.

First off, check and adjust the valve tappet clearances. Under normal use, the valve tappet clearance tends to decrease. With this comes lost compression and a poor idle.

Next, check the air filter (clean as needed or replace), and drain any water or other sediment from the carburetor float bowl.