What was found
(CAN) A/C ENROUTE AT FLIGHT LEVEL 210, DECLARED AN EMERGENCY. AIRCRAFT TURNED BACK. PILOT REPORTED ROUGH RUNNING ENGINE & MISFIRING BADLY A/C LANDED SAFELY. PILOTS EXPERIENCED POWER LOSS ON NR 3 CYLINDER FIRST & THEN ON BOTH NR 3 & 4 CYLINDERS. ENGINE IS EQUIPPED WITH CYLINDER HEAD TEMPERATURE PROBES FOR ALL CYLINDERS & COULD SEE 3 & 4 AS COLD CYLINDERS. BASED ON THE INVESTIGATION PILOTS ARE CONFIDENT PROBLEM WAS ICE CRYSTALS IN THE FUEL. FUEL IS FEED TO THE CYLINDERS FROM A DISTRIBUTION HEAD ON THE TOP OF ENGINE. A SHIELD THAT IS DESIGNED TO DEFLECT COLD AIR FROM THE DISTRIBUTION HEAD AND THE LINES COMING FROM IT, REQUIRED BY AN AD.