Typically, an engine isn’t going to rev high unless it’s being given enough fuel to do so. Though you’ve tried two carbs, I would think that you’re supplying more fuel than you think. I know that you stated that there is no tension on the throttle cable, but is the slide seated all the way down in the carb? If there’s a hang up anywhere in that assembly (which is pretty easy to achieve if you’re not careful), you could be effectively gassing it more. Since you still have the airbox off, just take a look.
Of course, if you’re still having chronic issues, my Rotax works perfectly and is still for sale. :)