It may be a mechanical problem. The servo tray may have come loose or the throttle tubing that the push rod is in. A quote from another thread on this same problem " Yes, I have seen the screws fall out before.(carb screws) Had a guy out the other day and we just could NOT get it to idle down no matter what we did. Throttle closed and even with a finger carefully placed over the intake the silly engine kept running, quite fast at that. We looked at what we thought was everything and finally noticed one (1) of the 2 mounting screws had come out. this allowed air in and the the carb supplied fuel as needed to run. No spare screws on the field that fit so he had to park it for the day but at least we had figured it out.
[8D]