C152 ENG RPM DECREASED AFTER TKOF. WHEN ENG PERFORMANCE DID NOT IMPROVE; PLT DECLARED AN EMER AND CIRCLED FOR AN EMER LNDG.

Narrative

ON THE DAY OF THE INCIDENT; I WAS TEACHING A SOFT FIELD TKOF WITH MY STUDENT PLT. AS ACFT (C152) LIFTED OFF THE GND; I NOTICED THE RPM DECREASING SLIGHTLY. AFTER A QUICK TROUBLESHOOT I REALIZED NO ENG IMPROVEMENT. I CONTINUED WITH THE TKOF AND LIFT-OFF AFTER REALIZING THERE WAS NO MORE RWY. AT THAT POINT MY DECISION WAS TO DECLARE AN EMER AND MANEUVER TO CIRCLE BACK AND RETURN TO THE RWY. I STARTED THE MANEUVER AT APPROX 400 FT AGL AND COMMENCED A HARD R TURN TO AVOID THE RAMP TO THE N OF THE RWY. MY FIRST THOUGHT WAS TO BRING THE ACFT ON THE GRASS TO THE S OF THE RWY. AFTER DECLARING AN EMER; I HEARD TWR DEVIATE A CESSNA ON FINAL TO TURN TO THE S TO CLR THE RWY. UPON COMPLETING THE TURN; I REALIZED I COULD MAKE THE RWY AND CONTINUED TO DO SO. I MADE THE RWY WITH NO INJURIES OR DAMAGE. UPON RETURNING TO THE OFFICE; I REALIZED MY STUDENT AND MYSELF FAILED TO INSPECT THE MAINT LOGBOOK (SQUAWK BOOK) AND THE ACFT WAS WRITTEN UP FOR QUITTING DURING STARTING THE ENG. FAILURE TO COMPLETE A PROPER PREFLT AND INSPECTION OF DOCUMENTS RESULTED IN THE OCCURRENCE.