AN INSTRUCTOR AND STUDENT FLYING AN ERCOUPE EXPERIENCED A SURGING ENG AND LANDED ON AN INTERSTATE WHILE PWR REMAINED. A THROTTLE CABLE WAS FOUND LOOSE.

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IT WAS A NORMAL TRAINING FLT WITH ME; A FLT INSTRUCTOR; AND A STUDENT WHO IS TRAINING FOR A LIGHT SPORT PLT CERTIFICATE. THE ACFT IS A 1946 ERCOUPE. WE WERE APCHING ZZZ WHEN THE ENG BEGAN TO LOSE PWR. THE ENG WOULD SURGE. IT WOULD RUN WELL FOR A SECOND OR TWO AND THEN IT WOULD QUIT. THIS SURGING OF PWR KEPT REPEATING. IT APPEARED AS IF SOMETHING WAS BLOCKING THE FUEL FROM FLOWING STEADILY TO THE CARB. WE WERE AT 5500 FT MSL (1500 FT AGL) WHEN THIS PROB BEGAN. WE WERE NEAR AN INTERSTATE AND STAYED NEAR IT AS WE BEGAN TO LOSE ALT. WE TRIED CARB HEAT TO NO AVAIL. THE OUTSIDE AIR TEMP WAS AROUND 90 DEGS AND THE HUMIDITY WAS LOW; SO WE DIDN'T REALLY EXPECT CARB HEAT TO HELP AND IT DIDN'T. WE TRIED RUNNING ON L MAGNETO ONLY AND ON R MAGNETO ONLY AND THAT DIDN'T MAKE MUCH DIFFERENCE EITHER. WE TRIED MOVING THE FUEL VALVE FROM ON TO OFF AND THEN BACK TO ON. THE OFF POS DID SHUT FUEL OFF; BUT MOVING IT BACK TO ON MADE THE ENG CONTINUE SURGING. THE ACCESS ROAD TO THE INTERSTATE HWY WAS LONG AND STRAIGHT AND WE NOTED A GAP IN THE AUTOMOTIVE TFC ON IT SO WE ELECTED TO LAND. WE BROUGHT THE PWR BACK TO IDLE AND MADE A STRAIGHT-IN APCH TO THE ROAD. THE ONLY ADDITIONAL EXCITEMENT WAS CAUSED BY POWER LINES STRETCHED ACROSS THE ROAD. WE SAW THEM JUST IN TIME TO 'DUCK' UNDER THEM AND LAND. THE LNDG WAS NORMAL. THE MOTORISTS BEHIND US STOPPED AND HELPED US PUSH THE ACFT OFF TO ONE SIDE OF THE ROAD SO THAT TFC COULD PASS BY. THE FAA SENT A DESIGNATED AIRWORTHINESS INSPECTOR OUT TO LOOK AT THE PLANE. HE DETERMINED THAT A BRACKET SECURING THE THROTTLE CABLE WAS LOOSE AND THAT THIS COULD HAVE ALLOWED THE THROTTLE ARM ON THE CARB TO MOVE AND THUS CAUSE THE SURGING WE EXPERIENCED. WE ARE GOING TO HAVE A MECH SECURE THE THROTTLE CABLE AND DO A THOROUGH INSPECTION TO SEE IF HE CAN FIND ANYTHING ELSE THAT COULD HAVE CAUSED THE ENG TO BEHAVE AS IT DID. WE ARE ESPECIALLY GOING TO HAVE HIM EXAMINE THE HEADER FUEL TANK AND THE FUEL LINES TO SEE IF THERE IS ANY FOREIGN MATTER PARTIALLY BLOCKING THE FLOW TO THE CARB. AS OUR ACTIONS; WE FELT AS IF WE DID THE RIGHT THING LNDG WHERE WE DID AND WHEN WE DID. WE COULD HAVE GONE A BIT FARTHER; BUT WE SAW A CLR WAY TO A SAFE LNDG AND WE TOOK IT. NO ONE WAS INJURED AND NO DAMAGE WAS DONE TO PERSONS OR PROPERTY IN THE AIR OR ON THE GND.