CORP ACFT HAS ROUGH ENGS; HVY ENG VIBRATION AT CRUISE.

Narrative

IN THE INTEREST OF FLT SAFETY; I WISH TO RELATE DETAILS OF AN INCIDENT INVOLVING THIS ACFT FOLLOWING A DEP FROM PORT ANGELES; WA; (CLM) TO SPOKANE (GEG) ON AN IFR FLT PLAN. THE ACFT DEPARTED IN CLR WX ON CLB TO A VFR ON TOP ALT OF 15500 FT DIRECT TO GEG. THIS ALT WAS ACHIEVED OVER PAINE FIELD AND THE ACFT CRUISED SMOOTHLY AT THAT ALT. THE CTLR RPTED THAT ANOTHER PLT HAD ENCOUNTERED CLOUD TOPS AT 15000 FT XING THE CASCADES NEAR SNOQUALMIE PASS; S OF MY POS. TO AVOID AN INADVERTENT CLOUD ENCOUNTER AND POTENTIAL ICE; I PROCEEDED TO 17500 FT AND LEVELED OFF. ABOUT 1 MIN LATER 35-40 MI E OF PAE; THE R ENG BEGAN TO MISS AND THE EGT GAUGE DROPPED ALL THE WAY DOWN. I REQUESTED CLRNC TO PAE; RECEIVED IT; AND TURNED AROUND. THE CTLR ASKED IF I WOULD DECLARE AN EMER WHICH I DECLINED AT THAT POINT WHILE TRYING TO SORT THINGS OUT WITH THAT ENG. SHORTLY AFTER THAT CONVERSATION THE CYLINDER HEAD TEMP ON THE L ENG DROPPED ABRUPTLY AS DID THE CYLINDER HEAD ON THE R ENG. BOTH WERE AT THE BOTTOM OF THE GAUGE AND BOTH ENGS WERE THEN RUNNING ROUGH. THERE WAS NO CHANGE IN MAP; RPM OR OTHER ENG INSTS. AT 1 POINT BOTH ENGS WERE MOVING IN THEIR MOUNTS SUFFICIENTLY TO NOTICEABLY SHAKE THE INST PANEL. THIS SEEMED TO BE CORRECTED SOMEWHAT BY ADJUSTMENT OF THE MIXTURE CTL. I BELIEVE THE TEMP ON THE 1 WORKING GAUGE WAS 1400. THE USE OF LOW BOOST HAD NO EFFECT. I DID NOT USE ALTERNATE AIR SPACE SINCE THERE WAS NO MAP LOSS NOR INDICATION OF MOISTURE. THE OUTSIDE AIR TEMP WAS ABOUT MINUS 30 DEGS CENTIGRADE. SINCE WE HAD A LONG SUSTAINED DSCNT WITH COLD ENGS; I ELECTED TO MAINTAIN MAP AT 25 INCHES; RPM AT 2250; AND TO DSND SLOWLY. DURING THE ENTIRE 15 MIN PERIOD; THE ENGS WERE RUNNING ROUGH. I EVENTUALLY DECLARED AN EMER NEARING PAE BECAUSE IT SEEMED POSSIBLE THAT WE COULD LOSE BOTH ENGS. AT APPROX 8000 FT WE CROSSED PAE. AT 6000 FT BOTH CYLINDER HEAD TEMPS CAME UP AND THE ENGS SMOOTHED OUT. I COMPLETED A VISUAL APCH AND NORMAL LNDG ON RWY 16; WENT BACK TO THE APCH END; AND PERFORMED NORMAL ENG CHKS INCLUDING A FULL PWR TURNUP. I THEN DEPARTED FOR CLM WITH NO FURTHER PROBS. AT THE START OF THE DAY ON AN EARLIER FLT TO HQM; ALL TANKS HAD BEEN DRAINED FOR MOISTURE WITH NONE DETECTED. HOWEVER; AT THE CONCLUSION OF THE FLT A SMALL AMOUNT OF WATER DROPLETS WAS DISCOVERED PARTICULARLY ON THE R ENG; PERHAPS 1/4 OF AN INCH IN THE SIGHT GAUGE. WE CHKED ANOTHER ACFT FUELED OUT OF THE SAME PUMP AND THE FUEL WAS ABSOLUTELY CLR OF VISIBLE WATER. MY ASSUMPTION IS THAT THE SUPER COOLED FUEL HAD CREATED ICE CRYSTALS IN THE FUEL WHICH; WHEN INGESTED BY THE ENG; CAUSED THE ROUGHNESS. IT'S MY VIEW THAT THIS WAS A SERIOUS INCIDENT. THE VERY PURPOSE OF PRESSURIZATION AND TURBO CHARGING IS TO IMPROVE THE OVERALL PERFORMANCE OF AN ACFT AND TO AVOID WX RELATED PROBS. THE ACFT WAS DOING WHAT IT WAS DESIGNED FOR; YET THERE IS AN APPARENT WEAKNESS. I'VE DISCUSSED THIS WITH OTHER KNOWLEDGEABLE C340 PLTS. FROZEN FUEL VENTS IS A POSSIBILITY BUT SEEMS UNLIKELY SINCE THERE WAS ABSOLUTELY NO VISIBLE MOISTURE PRESENT EXCEPT FOR A SLIGHT FROSTING OF THE WINDSHIELD ON THE INSIDE ON EITHER SIDE OF THE COCKPIT. 1 PLT TOLD ME OF A MODIFICATION THAT RTES WARM OIL TO THE SPIDER ON TOP OF THE ENG WHERE FUEL IS DISTRIBUTED. WE'LL LOOK INTO THAT AS WELL AS THE USE OF PRIST. IF NECESSARY; THE LATTER SHOULD BE A STANDARD REQUIREMENT WHEN REFUELING IN THE WINTER; NOT SOMETHING THAT IS ONLY CASUALLY OR OCCASIONALLY USED. IF OTHER PLTS HAVE HAD SIMILAR EVENTS IN THIS ACFT; IT WOULD BE A GREAT HELP TO HEAR ABOUT THEM IN ORDER TO DEVELOP PROCS THAT WILL AVOID A REPETITION. ENGS CONTINENTAL RAM CONVERSION -- 325 HP TYPE: TSI0-520--NB. TIME: SMOH 215. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES COMPANY MECHS HAVE BEEN REFERRED TO CESSNA TO DETERMINE PROCS FOR RTING THE WARM OIL AROUND THE SPIDER AREA. PRIST IS A TRADE NAME FOR A PRODUCT USED IN FUEL TO AVOID THIS KIND OF PROB. IT ACTS AS ALCOHOL TO PREVENT FREEZING OF WATER CONTENT. RPTR ALSO SENT THIS SAME RPT TO THE TWIN CESSNA FLYER ORGANIZATION AND RECEIVED AN IMMEDIATE RESPONSE. THERE WAS A 1973 SVC BULLETIN ISSUED REGARDING THIS PROB. THIS BULLETIN HAS NOT HOWEVER BEEN INCLUDED IN THE HANDBOOK. IT WOULD BE HELPFUL FOR NEW OWNERS TO HAVE THIS. IT WAS ALSO SUGGESTED BY THE ORGANIZATION TO USE ISOPROPYL ALCOHOL INSTEAD OF OTHER SUBSTANCES.