2004 Yamaha 15hp outboard motor thumping/ knocking


New member
I’m New to the forum here and looking for some advice/ help. I have a 2004 Yamaha 15 hp 4 stroke on my 15 foot jon boat. While on the lake the other day the motor started to make a thump/ knocking sound only at full throttle. The sound does not appear until I hit full throttle or just below full throttle. Someone recommended I change the lower unit oil as well as the motor oil. (I just purchased the boat and motor a little over a month ago). When I changed the lower unit oil it was jet black as well as the motor oil. I also replaced the oil filter. After that was replaced I hooked it up to the hose and let it run idle to get the oil flowing through the engine. 2 days later I decided to take the boat out to test it again. Same thing happened, started up just fine, idles just fine, shifts from forward neutral and reverse just fine. And is completely fine as I give it throttle until I hit full/ almost full throttle then the knocking comes back. Does anyone have any idea what this is before I take it into a shop and they charge me an arm and a leg.