Took my recently bought 83 mercury 9.8 out with my buddies tonight to do a trial and error on how it runs. Cleaned the carb changed the lower gear oil. Put a new impeller in. It started and ran great in the garage barely any smoke peeing correctly. While we were out it kept stalling and would bog down when opening up the throttle. What could this be??? One thing I haven't done and should of done before I left was change the spark plugs. The motor before I got it running had been sitting In a garage for about 4 years. Please help!!