EMER DECLARED WHEN SMA PLT SENSES ROUGH RUNNING ENG DURING STEEP L TURN.

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DURING A L STEEP TURN; THE ENG RAN ROUGH WITH DECREASING PWR. I WAS IN CONTACT WITH ATC AND DECLARED AN EMER AND REQUESTED AN IMMEDIATE LNDG AT THE ARPT DIRECTLY BELOW. ATC PROVIDED THE FREQ AND I CONTACTED RIVERSIDE (RAL) TWR. RAL CLRED ME TO LAND ON RWY 9. LANDED AND TAXIED TO TRANSIENT PARKING WITHOUT INCIDENT. TANK FUEL GAUGES INDICATED UNEVEN FUEL USE AND LOW FUEL LEVEL IN THE R TANK. I BELIEVE LOW LEVEL CAUSED FUEL EXHAUSTION SYMPTOMS IN TURN. WHEN LEVELED WINGS AND MADE STANDARD RATE TURNS; ENG PERFORMED NORMALLY WITHOUT ROUGHNESS. FLT WAS A PLANNED 5.5 HR VFR FROM JACKSON HOLE; WY; (JAC) TO CHINO; CA; (CNO). DIPSTICK MEASUREMENT AT JAC INDICATED FUEL LEVEL OF 10 GALLONS R AND 20 GALLONS L. ADDED 20 GALLONS TO EACH TANK FOR A TOTAL OF 70 GALLONS. ARRIVING FLT FROM TORRANCE; CA; USED 60 GALLONS IN 5.8 HRS FOR 10.3 GPH. SAME FUEL USE SHOULD PROVIDE SHORTER DISTANCE FLT TO CNO WITH A RESERVE OF 13 GALLONS. PLANE WAS PARKED IN RAL ON AN INCLINE SO AN ACCURATE DIPSTICK FUEL MEASUREMENT COULD NOT BE MADE. FUEL WAS PRESENT WHEN TESTED THE LOW POINT DRAINS. A FEW DROPS OF WATER WERE FOUND IN THE R TANK. FLT FROM JAC INCLUDED VMC AND IMC LIGHT TO MODERATE TURB FROM N OF LAS VEGAS TO LOS ANGELES BASIN; WITH A LITTLE RAIN. MAY HAVE BURNED MORE FUEL THAN ORIGINALLY PLANNED AND MAY HAVE HAD WATER BTWN DAMS IN TANK. PLANE IS WET WING AND DOES NOT HAVE A RUBBER BLADDER. ADDED 60 GALLONS OF FUEL AT RAL AND CONTINUED FLT TO TOA. TANK DIPSTICK TEST AT TOA INDICATED 31 PLUS 33 GALLONS AFTER THE APPROX 30 MIN FLT. BELIEVE DECLARING EMER AND LNDG WAS PRECAUTIONARY AND COULD HAVE CONTINUED FLT TO CNO; BUT GLAD I LANDED.