Get yourself a can of carb cleaner or starting fluid, and start the quad and do what you gotta do to get it to come down a bit so you dont blow it up. Then just spray the fack out of the carb, and the tube leading to the carb, all around. If you hit a spot thats got an air leak, you will either make the idle go up, or make it bog depending. If you find a spot like that, hit it a few times and confirm it really changed.
Do that all over the carb/intake area and see.
If you dont find any spots that are changing the behavior of the idle, you dont have an airleak and your problem is with the throttle cable, or pilot circuit (idle screw), or something inside.
Try my test and report back.