PIPER COLT HAS RPM DROP; MAKES EMER LNDG AT CLOSED ARPT.

Narrative

WAS ENRTE ABOVE BROKEN CLOUD DECK; BEGAN EXPERIENCING RPM DROP; SEEMED PROGRESSIVE AND GRADUAL. FULL APPLICATION OF CARBURETOR HEAT DID NOT REMEDY. LOCATED ARPT 3 MI BEHIND ME PER GPS AND WAS ABLE TO GET DOWN THROUGH HOLE AND MAKE SUCCESSFUL LNDG EVEN THOUGH THE ARPT IS UNDER REPAIR. AT PATTERN ALT I COULD MAINTAIN ONLY 1500 RPM. ACFT NOW GNDED PENDING INSPECTION OF ALL ENG CTLS; PARTICULARLY POS OF CARB HEAT BUTTERFLY. RECOMMEND: WHEN CHKING OUT IN ACFT REQUIRE 1 HR PREFLT INSTRUCTION ON SPECIFIC ACFT WITH QUALIFIED A&P IN ADDITION TO CURRENT PRACTICE. SINCE CARB HEAT CABLE FEELS SECURE AND TACH INDICATES RPM DROP I FELT OK TO GO BUT ACTUALLY I HAVE NO KNOWLEDGE OF BUTTERFLY POS IN RELATION TO KNOB POS. MAYBE PUT REFERENCE MARK UNDER ARM? VISIBLE DURING PREFLT? CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES THE ACFT WAS A PA22 CONVERTED TO A PA20; A TRI PACER TO A COLT; TAIL DRAGGER. THE PROB WAS THE CABLE FOR THE CARB HEAT HAD SLIPPED AND WOULD NOT OPEN ALL THE WAY. ONLY PARTIAL CARB HEAT WAS AVAILABLE. THERE WAS ENOUGH DRAG ON THE CABLE TO GIVE A PROPER INDICATION ON THE RUN- UP. PROB WAS ATTRIBUTED TO A VERY SLOPPY ANNUAL IN WHICH THIS LOOSE CABLE WAS MISSED. RPTR SUGGESTION FOR A MARKING WOULD HAVE MADE NO DIFFERENCE IN DETECTING THIS PROB.