i have 2 ys 91ac's. i have had problems with both. one is very used but in good cond., just runs bad. so i bought another. raced it about 20 flights. now it idles well and the mid-range is good but top end sucks. almost acts like its too rich. so i tried to adjust it, it leaned out. i adjusted the valves, the same thing happened. i got in a habit of using marvel mystery oil after running. but after reading alot of the above symptoms, it sounds like that wasn't a good idea. please tell me if this sounds like my problem, and what i need to order to fix it. thanks