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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.
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I think it was wishful thinking changing the oil.....anyway, explain the knocking a bit better, does the motor miss/shake or is it a mechanical noise? -
I am thinking timing too far advanced or bad gasComment
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The motor doesn’t seem to necessarily shake, I guess missing would be appropriate? It really just sounds like a rapid thumping sound. I have a video of it but not sure if I could even post that on here.Originally posted by ausnoelm View PostComment
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spark happening before piston gets close enough to top dead center, causing piston slap
just a thought on what could be a knock as you said.
timing light will show if this is happening when the problem is thereComment
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Pull all plug wires. Rock fly wheel back and forth. Go all the way around like in 360 while taking your time. Pull plugs and repeatComment
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