I have a 1973 C-150, I recently noticed (after riding in a friends 150) that my RPM drop when applying the carb heat doesn't drop like it used to. Normal RPM drop used to be 100 RPM, now a 25 RPM drop would be generous. I believe this problem happened slowly over time because I never noticed it, even after flying the plane 200 hrs since the first of the year.

I checked the cable at the arm of the heat box, it extends fully in each direction. I removed the air filter and checked the flap, it fully opens and closes. The orange hose is connected from the box to the baffle. I operate the plane from a 900 ft MSL airport and do my run up on a "full rich" mixture.
Thanks in advance for any advice!