We have an older |Scag 36" WB with the 12.5 Kaw FB460V engine. The entire machine only has about 300 to 400 hours on it at most. 2 months ago I started it just to let it run a while because it hasnt been started in a couple months. It ran perfectly fine. Now the other day we go to start it again to get it ready to sell and it is now surging like crazy. At first it was doing it all over the throttle range. I took the carb apart and cleaned it up and blew it out. After that it was surging all over the place except at idle. Today I took it all apart and put a complete carb kit in it and cleaned it out again with carb cleaner and compressed air. Now it surges at anything above 1/2 throttle and under that it runs fine. Any ideas what is going on with the engine?