Right, this is a little strange! Had a few sets of carbs kicking around for a while and the ones I sorted for my winter project bike worked fine but cosmetically looked crap. Yeah, yeah I know their hidden but I wanted a cleaner set on.
Anyway, I did all the carb mods as usual. 118/120 mains, drilled slides, extra washer etc etc but decided to use the diaphragms that were in the carbs as they looked ok, and I couldn't be arsed to swap them for the ones out of the previous set. A bit dry and very slightly shrunken but thought I got them to fit ok. Back on the bike, float bowls filled and away she fires. 4K rpm on choke for a bit, slowly off choke but revs stay put. At this point I just let it warm up to see what happened but the revs still stayed put. So i'm thinking induction leak or maybe I have simply done something wrong (even though I know the carbs quite well).
Off came the carbs again. Had a look at the diaphragms and sure enough I hadn't seated two of the four in the groove. So I swapped them for the flexible 'wet' set and assumed that was the problem. Turns out I still have the same thing. Revs hanging at at least 5K RPM and staying there. If I blip the throttle they go even higher and hang.
Im stumped and, even though this is a set of carbs I got off ebay a while back and are unproven (at least to me), what else is there to go wrong?
I know I have them mounted correctly on the rubbers. Sure the diaphragms are now seated correctly and I haven't made a schoolboy error like bad throttle cable adjustment.
Any ideas before I bin them?

Air leak from somewhere.

Don't bin the carbs!!

Unless the castings are broke theres never usually anythign that can't be fixed.

Spray wd-40 around the rubbers when its running and see if your rpm's drop.

If you press the sliders up do they all return down properly? Your butterflys are also closed? (you can see them when you lift the sliders up)

Choke not sticking on?