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I have a Yamaha 9.9HP two stroke serial number 9.9FMH 682C S156 916 W which I was told had been uprate to 15HP when I bought it 2nd hand. My local dealer can run it at wide open throttle in his test tank but when I get it on the water powering a 10 foot Zodiac inflatable it will not run above 1/2 throttle at which point it appears to suffer from fuel starvation. The reed valve has 6E7 stamped on it and the carb number is 63V00 KJ29, can anyone confirm if the carb is for a 9.9 or a 15HP and has anyone any idea what my problem may be? I was wondering if I may need to get the card re-jetted for altitude as the engine was bought at sea level and I am now living at an altitude of 4000 feet.