Wouldn't give up my OS 91FX in my corsair fer nuttin! Ran like a champ before I screwed up the piston ring by not getting enough after run oil in the cyl. then the ring froze to the piston. Took the engine apart cleaned it,put a new ring on reassembled forgot to hook up exhaust pressure line which caused it to die on idle return from full hook up pressure line final adjustments and its running like a fine swiss watch!! Frankly I think the 61fx and the 91fx are some serious work horses!!