Installed a new spitfire on my 4 stroke 115 and thought I had,have a motor problem. The motor runs great trimmed down and trimmed up for speed but would shake like a miss in the motor at midrange. Changed oil and installed new plugs thinking maybe I got a bad plug, and had 10 hours on the motor but changed nothing. Then I read and watched a video in the mercury section and the poster was told it was the prop cavitating at a certain rpm, trim. My 115 is mounted in the second hole with a pad to prop of 4 inches. What’s your thoughts...thank you