I have 2 of these engines 1 is a 91 that I got on a edge 540 the other is a 52 that I bought at a swap meet for 15 dollars.Neither engine was running both looked new.After putting a new manifokd o ring and setting carb back to factory specs the 91 fired up.The idle was poor but a fox miracle plug cured that.Now it runs great.The 52 was pulling fuel in and pushing it back to the tank on compression.After checking the valves I went to the cam.The cam was out of time.The guy I bought it from said it had 4 flights on it and quit but I dont see how with a cam out of time After figureing out where the cam should be it ran near perfect.The 52 is a chinesse copy of the os 52 .I dont know about the 91 as I don`t have a os 91.I haven`t flown them yet but bench runing they seem great