A repair manual will have the procedure. The jarlef z650 site has a factory manual for the 81 650 - the procedure should be applicable to your bike (pg 6-11 & 6-12).
Before dong anything, with the carbs off the bike turn the Idle thumb screw in & out and watch the slides move. Look at the gap of each slide from the engine side. Using a wire feeler gauge, check the gap of each slide.
Remove the carbs from the bike & remove the carb tops. Loosen the throttle valve adjusting screw locknuts. Set the slide gap on the engine side to ~1mm by turning the slide adjusting screws. You could eyeball it or use a wire gauge. Tighten the adjuster locknuts without allowing the adjusting screws to move.
Note: before doing the adjustment, be mindful of the Idle (throttle pulley) thumb screw's position - it shouldn't be turned all the way in or out, otherwise you may not be able to adjust the idle properly afterwards.
