Complete novice with KZ650 Throttle/Clutch issue

So I'm having some throttle issues, and as it is that I'm incredibly inexperienced in motorcycle repair, I was wondering if I could get some help. Below is a terrible video shot with my potato of a phone camera, but it gets the main points across. Basically, on my 1979 KZ650 B3, when I pull the throttle, it just revs up and stays there, and the only thing that brings it back down is pumping the clutch lever a few times. I can also fiddle around with that large screw (what is that thing? Terminology?) that goes into the middle of the carbs, either opening it up or closing it, and that changes things too, but messes up my acceleration curve. I bought this from a guy who put pod filters on it, and said he took it to a shop to have it re-jetted to what it needs to be, but I'm wary. Maybe I just need to tear into the carbs, I dunno. Thoughts?

Jetting may be way off, but are you sure it is even firing on all 4 cylinders? If you pull one spark plug wire off the spark plug, is there any one that doesn't change the idle? Ed

All four exhaust pipes are hot when I fire it up - I could try the spark plug thing, but I think they're all firing. If someone could help me understand the underlying cause of why this is happening, i.e. why it gets stuck revving until I push the clutch lever in, that would be awesome.

I cant get your video to work for whatever reason.
So ill just start with the simple questions.
Is the bike being ridden when the throttle issues are happening?
Im having a problem understanding how the clutch would be affecting it unless of course its unloading the motor and putting revs through the roof.
As far as the idle adjustment screw on the carbs goes let the motor warm up before you start messing with it.
Once the motor warms up see if you can get it to idle remotely close to 800-900 rpms.
Im guessing your bike has cv style carbs which totally suck to try and tune without stock airbox, id find a stock airbox myself.
Post some pictures

Try this: (2 people makes it easier): With the engine not running, open the throttle while watching the linkage on the carbs. The linkage should move. Now release the throttle. The linkage should return to its previous position quickly. If it moves back slowly, or not at all, then you need to concentrate on that area. Areas to look are are: the linkage itself, the cable, and the throttle assembly on the handlebar.

like kk said, start with the easy stuff. With the engine off, open the throttle and let go. If it immediately snaps back, there is likely not a mechanical problem. If the return is sluggish, properly lubricate the throttle and cable, and retest. If it fails again, check to insure that the return spring is in place and not broken. If still sluggish, there could be problems with the carb linkage.
This problem can also be caused by a severely lean mixture. Old, cracked manifolds,or manifolds that are not completely sealed agains the head and carbs, is a common culprit. Cheap pods may block crucial air intakes, and a poorly done reject can also be to blame.
The most fun of all is having a random combination of causes.
If throttle-cable-spring maintenance does not cure it, you may want to consult a few of the 7.4 billion forum posts on tuning with pod filters.