70 hp Yamaha four stroke issues

I have an 1997 yamaha 70 hp four stroke. might be off a year or so. was on the water Saturday and everything was normal. Then suddenly the motor started struggling to get 10 mph. I took the boat out and looked at it on the ramp. Some how the prop nut was slightly loose. So i took it off and redid it. put the boat back in the water and it did not help. I just rebuilt the carburetors a month ago. I decided to start on the lower end. I drained the old oil. There was not much of it. I did not notice shavings and it turns smoothly. i refilled it and ran the motor on a hose and drained it and refilled it. I took the boat back to the lake yesterday after looking into the lower end. it still has an issue. I replaced the water pump a year ago but it does spit water out just not a lot. i am asking for suggestions. I plan to take the carburetors off and clean them and reinstall them. not sure but thinking something in the lower end is messed up. I cant figure out why the prop nut was loose.

Yes , do the carburetors again.-----Always seems to help.------Did you check motor oil level in the engine ?------How much oil was in the lower end ?------How did you refill the lower end ?----How much oil did you put in the lower end ?

well i misspoke. it is a two stroke. I checked the compression all three were 80 psi. there was about half a quart of oil in the lower end, and it was black. i filled it half way up and ran the motor to try to clean it out. Drained it and filled it with new oil from the bottom plug till it came out the top plug. it almost took a entire circle jug. sorry i dont remember how much that is.

Hi, 2 Stroke, are you sure you had oil mixing with the gas? Compression sounds low. Also sounds like lower unit had insufficient oil. Does engine rev up to high rpms like 5000? If engine gets to high running rpms but boat not moving, I would say issue in the lower unit. I would order or download the service manual for more troubleshooting info. Checked my service manual for 50 and 60 Hp. Compression should be 139 psi min. Sounds like issue is in the cylinders. If auto oil injected maybe the oil injector failed. Ouch.

The issue is resolved. The water pump was bad and needed rebuilt and the lower unit was low in oil. there was no water in it because i replaced all the seals a year ago. i rebuilt the carbs, water pump and rinsed the lower unit out and filled with new oil. Everything seems fine now. Thanks for the advice.