HI
YOUR problem is a common oneOS knows of this problemif you contact OS repair at TOWER hobbythey will send you FREEa new back plate-needle valve assembly and mount , for the asking heat and vibration from the current back plate location ( one piece ) is the cause of the problema poor design of some of the OS fx series of engines is having-some 46's also -,been there -done thishope it helps get your engine running as it should-OS has been replacing many internal parts on the 91 FX if you have done damage to the engine as a result of getting air into it and therefore running it leanwhich is the end result ENJOY BEST RESULTS TONY