That sounds very similar to a stuck valve i had in a Cessna 172. Best RPM i could achieve was about 1600-1800RPM.I immediately turned toward forced landing airfield while bleeding energy to best glide speed. I then ran the steps that the previous poster mentions(Carb Heat, Mags, try different mixtures, try different throttle settings, Fuel tanks, primer, etc...) i realized based on the heavy vibrations that i either had a valve failure or i lost a prop tip. made a relatively uneventful partial power landing.