I recently changed the oil in a 2004 honda BF20 after 20 hrs of use (motor is new). After 15 min of running at full throttle, the engine started billowing white smoke out of the idle exhaust port and also the prop exhaust and cavitation plate exhaust. The green oil light and water stream were still ok. When stopped and tilted the motor up to investigate, I noticed an excessive amount of oil leaking out of the airbox overflow drain and down all over the lower unit. Also, there were new black oil stains around the prop exhaust and caviation plate exhaust. These stains were black, burned oil. The oil leaking out of the airbox was clean. <br />Of course, when I took the motor to the shop, there was no smoke, and the mechanic said it was "tilting the motor up" that causes this problem by having oil leak into the carburetor. But that doesn't explain the fact that this motor had not been tilted since before the oil change, and the smoke/leak did not start until after 15 min of full throttle running. As well, the motor was previously kept tilted up as the boat is moored, without any problems. <br />The oil change was conducted exactly as per the honda manual - oil allowed to drain and refilled with exactly 1 litre of oil (as specified when oil filter not replaced). The oil level was checked correct prior to running. <br />The shop thinks there is no problem with this motor... any advice/comments would be appreciated as I don't know what caused this problem.