My 91AC drips a bit but runs beautifully. I certainly am not meticulous, but I would say it has been one of the best running engines in our club if not the best and I picked it up used. I would love to run only YS based on my experience and will probably purchase more in the future. Have you tried a few different fuels? Mine wouldn't run on 2 stroke 15% nitro 18% oil and was I frustrated. It would throw the prop. On 4 stroke 15% nitro and 17% oil it is phenomenal.