2003 johnson 25 bogging midrange

[south_ga_boy_1970]

first things first...this is my first boat of my own so im really new to what i need to keep an eye on and what needs attention at what times. but my concern is i have a 2003 johnson 25 two stroke and it runs like a sewing machine at idle and full throttle, but on low rpms the motor seems to bog or clatter...almost like a car with spark knock but the clattering only happen at midrange to low rpm. i can go from stopped (idling) in gear to full throttle and motor dont miss a lick shoot the boat straight to planed out. is this something i should worry about? also the motor is on an aluminum 14', 36" wide bottom boat im roughly 235 lbs. and with maybe another 75 lbs or so of gear in boat, my buddy says the boat should run 35 mph (roughly). but the best i can do by myself in boat is about 25-26 mph up river is this about right for my boat and motor? thanks for any help or opinions on situation, also any things i may need to know about motor also appreciated.