I have a YS 91 inverted in my UCD. No problems starting or running. I spin 15X6 APC well over 10,000. I put the engine 1/8" back from where they tell you because I knew I wanted to be tail heavy. I also mounted the tank over the CG. The YS91 is more than enough power. Even with several ounces of lead in the tail I hover with less than 1/2 throttle. If you use an inferior brand of 4 cycle, you will probably have to go bigger.... :devious: