Sounds to me like it got hot.Im converting a 22cc echo[21.2actual] and starting out with an APC15x8.If the 3 blade your running is any smaller than a 12 inch i bet your rpms are to high.Richen both needles up some and try again.I would also put a nippel on carb cover and run a line behind the engine and attach to firewall.But i would try prop and needles first.If ya have to lengthen the gear for better prop clearance or move the main gear forward some,use longer mains,put a tail wheel on it,make it look like a Storch!