Yamaha T9.9 4 Stroke Gas in Oil

[brittanydog]

I just returned from a week long fishing trip. My buddy and I have the same 1997 and 2000 Yamaha T9.9 *****ing motors. He was having problems with the motor about 2 days into the trip, the motor would run from 30 mintues to 1 hours and just die, It would restart and run for 30 minutes to 1 hours and die again. We change all filters and the problem continued. He checked the oil, and the oil was overflowing from the dip stick hole. He changed the oil (motor starts, runs and idle smooth for another day or so and the crankcase fills again. We believe the crankcase is filling with gas. I check mine my oil and the mark was above the full mark (just change oil prior to trip).

What can be causing the gas to get into the crankcase?

[boats.net]

It can be caused by a number of things. Check the compression, if the rings never seated ,are worn or carboned up(have the engine decarbonized) the fuel can get past rings. Make sure it has correct spark plugs, check thermostats may stuck open, a cold running engine can cause it. May change prop to a smaller pitch, if engine is under constant load from over propped it will cause it to make oil. Check the fuel system a carb flooding can cause it to make oil.