it also has a new engine
Do you know the type ? I train on a C152 with a Lycoming 110bhp (O-235-L2C) when i apply carb heat i get a drop of 75rpm, if you got a drop of 200 maybe you can check the engine compartiment if nothing is blocking the air intake before it get's air warmed ?
Also what is rough running ? do you feel it's missing some ignition strokes ?
You are right when you say your 150 is weak for the carb icing, better use it to much than to less, if you get engine faillure or REAL rough running i hope one of the first things you do (after flying the plane and selecting a landing spot) is apply carb heat OFF
