Engine keeps locking up. 30HP 2 stroke Mercury 2005

Hey guys, Im new here but since I put the fishing boat up for sale, I'm really worried about this issue...
Its a 2005 30 HP mercury 2 stroke

So heres the back story:
I owned this boat for a year myself and the first half year was perfect, went out fishing and flushed the motor everytime. Never an issue.
Then I didn't use the boat for a few months and we decided to go camping on an island, so I use the boat to get us there and go back fro a second trip. so after having already had the boat running for 40 mins earlier without issue, while I'm out in the water on mid throttle, the engine locks up and stops. the pull cord won't budget and I'm stuck. I mess around with it for a while and 10 mins later the pull cord starts moving again so I turn her on, and then after 2 mins, she locks up again. we get towed to land and i put the boat away for a few days.


I finally go to check it out, pull cord not budging so I start to trouble shoot. The prop moves fine in neutral (my friend says you should be able to move it with a finger... but i think its a bit stiffer than that)... so it can't be the leg....I remove the cover and pull cord mechanism and the mechanism spins and works fine. I remove the spark plugs to look inside. I think I may have sprayed some wd 40 in there since I thought that if the pistons were stuck it could help....
I then attach a wrench on the top crank nut and surely it spins fine.... so I put it together and it starts... no issues...
I feel relief and blame the fuel/oil mixture being old and maybe not lubing the engine enough.
fast forward 6 months to me wanting to sell the boat since it never gets used anymore. I can't turn the motor on but the pull cord is fine. Changed spark plugs.. nothing... then remembered the full was oil so I changed that and it worked.
Now a week later I go to turn it on for a potential buyer, and the pull cord is jammed again. Once again I removed the cover and the pull cord mechanism, the spark plugs, spray minimal wd40 inside, and i grab the crank wheel with my hands and manage to force it to turn, each turn gets easier. Put it back together and she started fine.

Im looking for your advice guys but some ideas I have are:
- something is making the pistons create a vacuum and get stuck after sitting for a while
- the pistons or rings or something bad is screwed...
- lack of lube in the old fuel made it originally get jammed... but unsure why it was stuck the other morning when the fuel was a week old...

Im going to have to get a mechanic to look at it, but since i am handy enough, I was wondering if you guys can use the above clues to put together any ideas of what I could do myself since Im just trying to sell this and want it to work normally again.

Thank you in advance!

Tell us in exact numbers as to how much oil you are putting in the gas.----Exact numbers.

For every 20L of fuel I add 400ml of oil. It's a 50:1 mix

I just went to turn it on and it's fine today. It's been 2-3 days since I had to in-jam it last (as mentioned in the long story).

Hi
I've just joined the forum as I'm having problems with starter motor but I did have a problem with the motor locking up when the magnets under the fly wheel came loose and jammed the motor. This might not be the case with yours but worth a look.

regards
Richard

If it was for lack of oiling, the engine would be locked up permanently. What oil are you using in this engine? Low grade oil leaves behind byproducts that will clog up the rings and stick them to the cylinder walls, when it gets hot. I would get my hands on a bore scope and take a look at the cylinder walls. If they are scuffed, game over.

I don't have that tool but will try to look into it. I hope its not stuffed. I will have to check the oil type, I just know its 2 stroke valvoline outboard oil. But will find further details later.

Water getting in?

Jeff

not sure how water can get in.... i don't think that likely. The engine wasn't submerged nor have any waves crested over it.

Check the lower unit grease as gears may have went south....