PVT PLT OF A C340 DECLARED AN EMER; AFTER EXPERIENCING A 'ROUGH' RUNNING ENG AND LOW CABIN PRESSURE; DURING IFR CRUISE TO RECEIVE EXPEDITED ATC HANDLING TO DEST IN VFR CONDITIONS.

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IN LEVEL CRUISE AT 11000 FT MSL; STRONG VIBRATION STARTED AFTER CHKING OUT OP OF L AND R PRESSURE DUMP LEVERS (NOTICED CABIN PRESSURE DIFFERENTIAL LOW; 3 VERSUS 4.2 PSIG; ACFT JUST OUT OF ANNUAL INSPECTION). IMMEDIATELY TURNED ACFT TOWARD DEST IWS AND BEGAN TROUBLESHOOTING PROB. LOCATED PROB IN L ENG WHICH WHEN REDUCED TO IDLE/NEAR IDLE CAUSED VIBRATION TO GO AWAY. DURING TROUBLESHOOTING; I ADVISED ATC OF PROB AND THAT I WAS CHANGING COURSE DIRECTLY FOR DEST. THEY ASKED IF I WAS DECLARING AN EMER AND I SAID 'YES' IN ORDER TO GET ATC TO AGREE TO HEADING CHANGE AND DSCNT TO VMC (9000 FT MSL). ONCE ESTABLISHED AT 9000 FT WITH L ENG OF IDLE/NEAR IDLE; PROB WAS MANAGEABLE. LANDED AT IWS WITHOUT INCIDENT. TROUBLESHOOTING OF PRESSURIZATION PROBS IS PROBABLY BEST LEFT TO MECH ON GND. ATC COULD ALSO BE MORE HELPFUL IN FIRST FEW MINS VERSUS ASKING NUMEROUS QUESTIONS AT AN EXTREMELY BUSY TIME AND BASICALLY 'FORCING' PLT TO DECLARE AN EMER BEFORE PROB HAS BEEN ASSESSED. ON THE GND; THE ENG PROB WAS DETERMINED TO BE A CYLINDER NOT FIRING ON THE L SIDE RESULTING IN VIBRATION. THE PROB WAS UNRELATED TO PRESSURIZATION PROB WHICH WAS DETERMINED TO BE THE L SIDE PRESSURIZATION DUMP VALVE WHICH WAS STUCK OPEN. BOTH PROBS WERE REPAIRED BY AUTH REPAIR STATION; ACFT TEST FLOWN AND PLACED BACK IN SVC.