Why is gas getting into the crankcase of your Yamaha 90hp 4 stroke outboard engine?

How do you use your boat?

Gas getting into the oil is, strangely enough, a normal thing so long as it's not excessive. All four-stroke engines do it. Your engine has gas above the pistons, oil below them, and a set of sealing rings that keep the two apart. When you first start out with a cold engine the rings aren't sealing yet and the combustion isn't complete yet, so the unburned gas has to go somewhere and a lot of where it goes is into the crankcase. They call this blow-by.

If you mostly use your boat for fishing, this is what you do: You put the boat in the water at 6 am. You start the engine and drive for five minutes to get to your first fishing hole, then shut down. After a couple hours you wonder how they're biting at a different spot, fire up, drive for five minutes to get to the next place, shut down and fish. By now you have a few nice perch and sunfish and the fish have stopped feeding, so you crank up and drive for five minutes to get to that great little beach with the grills. You have a great fish lunch and sit around swapping fish stories with the crews of the other five boats doing the same thing as you. Now the sun starts to set and it's time for the fish to start biting again. Everyone drives eight minutes to their favorite fishing holes and catches their limit on trout, then drives another five minutes to load up and go home. You had a fantastic day, but your engine never got a chance to warm up...so you blew gas past the rings the entire time you were out there. If this is what's happening, change your oil twice as often as the manual says and you should be fine.

That's the good news. The bad is, your rings might be worn out and if they are, you need to overhaul the engine to fix it.

How to tell is very simple: google "Blackstone Labs." This is a company that analyzes motor oil. You can order a sample kit from them for free. When the kit gets to you, follow the instructions in it and send the oil with a check for $25 to the address they give you. You will shortly receive a letter telling you everything that's wrong with your engine - if your rings are wearing out, the metal will wind up in the oil and they'll find it.