A PA24-250 TRAINING FLT IS CUT SHORT AFTER THE FIRST APCH AND MISSED APCH AFTER THE ENG STARTS RUNNING ROUGH DUE TO CARB ICING. CREW STAYS ON GND FOR A BREAK IN THE WX AT ISP; NY.

Narrative

I WAS FLYING WITH THE OWNER OF THE ACFT DURING AN INST PROFICIENCY CHK. A PRECAUTIONARY LNDG WAS MADE BECAUSE OF ENG ROUGHNESS DUE TO CARB ICE AND PROBABLY SOME SPARK PLUG FOULING FROM 2 HRS OF NON LEANED OP. THE OWNERS (2) OF THIS ACFT REQUEST INST PROFICIENCY INSTRUCTION APPROX EVERY 6 MONTHS. IT HAD NOT BEEN UNUSUAL FOR THIS ACFT ENG TO DEVELOP CARB ICE IN THE PAST. IT IS EQUIPPED WITH A CARB TEMP GAUGE. ON THIS DAY; WE WERE OPERATING ON EASTERN LONG ISLAND IN GOOD VFR WITH ISOLATED SNOW SHOWERS ALOFT TO THE S OF LONG ISLAND AND TO THE W OF ISP. AFTER SOME INST AIR WORK; HOLDS AND APCHS; WE DECIDED TO DO 2 ILS RWY 24 APCHS AT ISP AND THEN PROCEED TO FRG. ON THE FIRST MISSED APCH; THE ENG BECAME ROUGH. EVIDENTLY THE LOWER PWR SETTING ON THE APCH ADDED TO THE CARB ICE. FACING A ROUGH ENG AND EITHER FLYING APPROX 20 MI FOR ANOTHER APCH OR 20 MI TO FRG WHILE NAVING AROUND SNOW ALOFT; I ELECTED TO MAKE A PRECAUTIONARY LNDG AT ISP. THE CORRECTIVE ACTION FOR THIS DAY WAS A CUP OF COFFEE; GND INSTRUCTION AND A DELAY TO LET THE CARB ICE MELT AND WAIT FOR THE SNOW SHOWERS TO PASS. ON FUTURE FLTS IN THIS ACFT; WHEN ENG ROUGHNESS OCCURS; A BREAK IN TRAINING TO APPLY CARB HEAT AND LEAN THE MIXTURE TO AVOID FOULING WILL BE THE PROC. SUPPLEMENTAL INFO FROM ACN 571578: WE INTENDED TO DO 2 APCHS AT ISP. AFTER THE FIRST; WE WERE ON THE MISSED APCH WHEN THE ENG STARTED SOUNDING ROUGH. WE DETERMINED THERE WAS CARB ICE AND; APPROPRIATELY; APPLIED CARB HEAT. SINCE THERE WERE SNOW SHOWERS IN THE AREA; WE DECIDED TO LAND AT ISP AND MADE SUCH REQUEST. THE ENG BEGAN TO RUN SMOOTHLY AGAIN SHORTLY AFTER APPLYING THE CARB HEAT. THERE WAS NO PROB WITH THE ENG AFTER THAT.