What was found
AIRCRAFT WAS ON TAKEOFF ROLL, NO INDICATION OF PROBLEM DURING RUN-UP OR INITIAL POWER APPLICATION DURING TAKEOFF. ABOUT 750 FEET DOWN THE RUNWAY, THE RT ENGINE'S RPM SURGED. THE PILOT THOUGHT MAYBE A CABLE BROKE. AS ACFT SLOWED DOWN, PILOT NOTICED THE RT SPINNER WAS WOBBLING. ACFT EXITED THE RUNWAY, PARKED, AND SHUT BOTH ENGINES DOWN. UPON FURTHER INVESTIGATION, FOUND THE BROKEN COUNTERWEIGHT CLAMP.