Check all of you lines to make sure they is no air leak. Check all of the screws/bolts on the enine to make sure they are not loose. If you have an inline filter between the tank and the carb, try removing it and using a piece of copper tubing as a temporary measure and restart the engine and see if it changes. Remove both needles and clean them.

Your low speed needle may be too lean which will also cause this problem

The little screen on the pump side of the carb may be clogged as well. It might also be time to replace the diaphrams on both sides of the carb (both pump and pressure.

Also, another thing to try is to open both needles two full turns. NOw lean the low end a little at a time until the engine no longer transitions to full throttle. Do not worry that you are not getting full rpm just yet because you are very rich. NOw lean the high speed until you get max rpm then back off 200rpm. This way the engine does not go lean in the air.