2 CFII S PRACTICING INST APCHS HAVE ENG PROB. EMER LNDG AT MIL ARPT.

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DURING AN IFR TRAINING/CURRENCY FLT; MYSELF AND ANOTHER GENTLEMAN; BOTH CFII'S; HAD JUST COMPLETED THE NDB RWY 24 AT SGH AND REQUESTED AN ASR APCH TO DAY. HOWEVER; DAY WAS UNABLE TO WORK US IN AND SUGGESTED THE ASR TO FFO RWY 23R TO THE 'OPTION' INSTEAD. WE ACCEPTED THE APCH AND CLARIFIED THAT IT WOULD TERMINATE AS A LOW APCH ONLY. THIS WAS IMPORTANT BECAUSE BOTH OF US KNEW THAT CIVILIAN ACFT SUCH AS OURS WERE PROHIBITED FROM ACTUALLY TOUCHING DOWN AT FFO; WRIGHT-PATTERSON AFB. EVENTUALLY WE SPOTTED THE RWY 23R THRESHOLD AND WERE INSTRUCTED TO CONTACT FFO TWR. WE DID SO; THEN COMPLETED THE LOW APCH AND SWITCHED BACK TO DEP AS WE APPLIED PWR FOR THE CLB. AT THAT POINT; THE ENG BEGAN SUDDENLY TO RUN ROUGH AND LOST PWR. APPLICATION OF CARB HEAT AND SUBSEQUENT ATTEMPTS TO LEAN THE MIXTURE FAILED TO CORRECT THE PROB AS WE CIRCLED OVER FFO. WX AT THE TIME WAS CEILING 700; SKY PARTIALLY OBSCURED; 1 MI VISIBILITY IN FOG AND RAIN. UNDERSTANDABLY; WE DID NOT WANT TO CLB BACK UP INTO THE LOW CEILING AND DECIDED THE SAFEST COURSE OF ACTION WOULD BE TO LAND AT FFO AND SORT OUT THE PAPERWORK WHEN SAFELY ON THE GND. I WOULD LIKE TO ADD THAT CARB HEAT HAD BEEN USED THROUGHOUT THE INITIAL ASR APCH; APPROX 3 HRS FUEL WAS REMAINING; AND NO MAGNETO TROUBLE OR FUEL CONTAMINATION WAS FOUND DURING THE PREFLT INSPECTION AND RUNUP. IN FACT; THE ACFT HAD PERFORMED FLAWLESSLY UNTIL PWR WAS APPLIED FOR CLB AFTER THE ASR TO RWY 23R AT FFO. DAY APCH SWITCHED US TO FFO TWR WHO CLRED US TO LAND; ROLLED THE EMER EQUIP; AND HAD US ESCORTED TO PARKING. LATER; ONE OF THEIR MECHS EXAMINED OUR ACFT AND DETERMINED THAT WE HAD EXPERIENCED A FAILURE OF THE L MAGNETO. THROUGHOUT THE ORDEAL; THE FOLKS AT WRIGHT- PATTERSON WERE VERY PROFESSIONAL AND UNDERSTANDING.