1990 90hp-Mercury Oil mixing in fuel filter

Hello all,

I am new to the forum, I've been searching for a while for someone with my issue but I cant find anything related due to the generality of my problem keywords. (Oil/Fuel)

I have a 2 stroke 90hp Mercury outboard. It stopped working a couple of months back, the motor would turn but not start, at some point it started and died right away. So, for a few months it just sat there, recently I noticed that the fuel filter is full of blue-ish fuel. Which means gas is mixing with the oil in the fuel filter, last time I did the oil/fuel mix I added the correct ratio, however, apparently oil is draining from the motor into the fuel filter. The boat does not turn or start now. I changed the plugs and made sure they get power, however, it does not go past that. I'm not a mechanic, however, buddy of mine who is (a car mechanic) told me its an electrical issue and that someone might have added oil to the gas tank, we drained the fuel filter and pumped some gas into it and the gas is fine, so no one added oil to the gas tank. (public storage..) However, he was unable to explain why it was draining backwards, the oil is supposed to remain in the main compartment. The oil started draining towards the fuel filter about half an hour later after pumping the new fuel.

So... I'm thinking of draining everything. Gas, Oil and start new, however I would like to know if someone can enlighten me on regards to the oil leaking into the fuel filter. (no use on draining everything if its just going to happen again) Would it be a bad gasket? (if so, where should i check?) And is this the root of my problems or just a part of it?

Thank you for any help in advance.

It sounds like the oil tank has built up pressure in it due to a stuck check valve. Witch is pushing oil into the carbs and down your fuel line and mixing with the fuel. Try removing the drains on the carbs and flushing them out. If you see a lot of oil in the carbs replace the check valve in the oil tank

Thanks, will check that and keep you posted.

Sorry for the long response time. I finally got around to open up the boat. However I don't know where the check valve, is, I'm attaching the pictures of the engine (hopefully good quality) could you point out in what general area would it be located at? or where i should i start dismantling?
PIC 1 -
PIC 2 -

I realized that I understand less about this motor than I originally thought...

the check valve is located in the second picture it is right next to the oil pump. It is brass and has one small oil line going to it and the other end goes into the main fuel line. You may also want to replaced the vent cap for your oil thank it is missing the top of it. It is in the first picture on the back of the tank near the stater at the top.

Thanks for the help. Just to make sure I am not taking off the wrong thing, please let me know if this is are the parts you mentioned.
<--- Check valve?

<----Vent cap? ( I believe it never brought it)

yes the parts pointed out are them.

Thanks a lot for the help, I got around to it the past few weeks, it exactly what you said. The carbs were full of oil, drained them, pumped some gas and some starting fluid and it turned on with no issues, turned it off/on a few times made sure it wasn't just the starting fluid working. Engine works great even with old oil and gas which is surprising. I let it sit for a week and was a little hesitant to start but it did, apparently the check valve does have an issue or maybe the old gas, anyways I was waiting on the valve to arrive. However, typical boat... last week water pump went out, now I have to replace the impeller on the pump this weekend haha, the vicious cycle continues... Thanks for the tip!