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Loud knocking noise

J

Jetmech1

New member
Loud knocking noise

moved this post from wrong tread section I posted it in previously.


Let’s start from the beginning:
I was losing lower unit oil from my monitoring tank and after troubleshooting to my lower unit I removed and replaced the 2 o-rings on the engine coupler yoke.
After re-assembling the outdrive to the boat I ran the engine in the driveway and no leak was found.
The first time back on the lake the engine started making a loud knocking noise at high RPM and before outdrive is trimmed up. The noise goes away after boat is on plane but returns when I increase the throttle.
I re-pulled the outdrive after some suggestions it was the yoke u-joints were worn but the noise is still there after replacing the universal drive shaft, seals and bearings except the drive gear assy since it looked good. The noise wasn’t there before pulling the outdrive so I’m hoping it’s something someone has encountered before.
 
Not sure, one minute I think it’s the outdrive than the next the engine. I feel if it’s the engine than it would make the loud knocking always and something would’ve broken already.
 
you can try pulling off the drive again when its out of the water, hook up a hose to the thermostat housing and run the engine to check if the noise is coming from it. How about the gimble bearing? You can check it when the drive is off. Should spin very very smooth by hand.
 
The boat has to be under load and usually just before I get it on plane. Don’t think taking the drive off again abs rumbas you suggest would work in this case. Going to pull plugs and do a compression check to see how the engine is performing.
 
An engine noise would be tied to RPM and be fairly quiet at idle and get louder with increased speed (rod knock from ruined bearing) A bad lifter would make the same noise level at all RPM. Since it wasn't making a noise until you removed and replaced the drive it makes most sense to assume it is coming from the drive.
 
What about a engine stethoscope. I bought one from Harbor freight 5-6 dollars, haven't used it, so not sure if that would help, worth a try. good luck
 
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