A C150 instructor reported losing power during a practice approach. He declared an emergency; then noticed carburetor heat had not been applied. After doing so normal power returned.

Narrative

My student and I were practicing IFR approaches in a Cessna 150 in VFR conditions. We noted some slight engine roughness; but the engine was still producing full power. The engine [then] lost power; going from 2;400 RPM to around 800 RPM. The engine would stay at 800 RPM then return back to 2;200 RPM and repeat this process. My student and I immediately established best glide and gave full throttle; full mixture; and checked fuel and magnetos. Nothing [helped]. We decided to continue the approach and maintain altitude as best we could. I took control of the airplane and declared an emergency to local traffic. This declaration of emergency caused a regional jet to have to execute a go around and orbit ten miles to the east. We were in constant contact with the regional jet throughout the ordeal. At ten DME and 6;500 FT I decided to go through one last check and noticed that we had not pulled the carburetor heat. I immediately pulled the carburetor heat and the engine came back to full operating power. We notified the regional jet and we landed without incident. Our emergency declaration caused minor delays on the ground. We also caused disruption in the landing of a commercial airliner. Communication was maintained with all parties and pilots involved.