2006 RXT Knocking noise at 3000RPM

[thzpcs]

Hi all, I wanted to see if I could get some help figuring out this weird issue I'm having with a 2006 RXT. I inhereted it from my father with a basically unknown history as he's never been big on any sort of maintenance, and it also sat down in FL uncovered for close to 2 years, so I went through it the best I could, replaced oil filter/coolant, plugs, though I stupidly didn't drain the gas tank of the old gas. However, it has started right up every time, no weird hesitation or anything else, and happily cruises at 7800 RPM and I've seen it rev to 8200 when doing sharp turns and such.


Now, one thing I've noticed is that it has this really odd stumble/"knock" at almost exactly 3000RPM and nowhere else. Best way to describe it is like a couple of golf balls bouncing around inside the hull. Nothing weird at idle, and the moment you're above like 3200RPM it goes away.


I recently rebuilt the supercharger, since it was last done in probably 2016 at best, and figured that might be causing the issue, but the same issue is still there. Also worth noting that even though it's a 2006, the engine is from ~2014, as I believe the original engine was trashed by the dreaded ceramic washers.


As I said, it really hasn't seemed to affect anything, and it's happily been running with the issue for 2 seasons now, so it hasn't really been a huge concern other than just keeping it out of that range when I'm riding it. I've always filled with 93 and used fuel stabilizer in winter as well, so it really doesn't seem like your traditional knock/ping from running low-octane fuel in a forced induction/high-compression engine.

Hopefully that's enough background detail, but for the real question is: Any idea what it might be?

 

[MacSeaDoo]

Does the sound match-up with a stumble in power delivery at that speed?

When were the plugs last changed? What do the plugs look like?

 

[thzpcs]

Does the sound match-up with a stumble in power delivery at that speed?

It generally feels fine from what I can tell, and doesn't really feel like it's missing bad at that speed. Maybe a slight stumble, but if you work it up from 0-100% throttle, it doesn't really hesitate or anything. If you're riding it, you'll only notice it when you're specifically cruising along and holding the throttle right in that speed range. Otherwise, it behaves 100% normally.

When were the plugs last changed? What do the plugs look like?

They were changed when I got it ~2 years ago, but I did a full workover of it this last weekend. New plugs, oil/filter, coolant, and that's when I did the SC rebuild. Aside from the SC sounding much happier, basically nothing changed in terms of the weird stumble.

 

[dcm0123]

"Now, one thing I've noticed is that it has this really odd stumble/"knock" at almost exactly 3000RPM and nowhere else. Best way to describe it is like a couple of golf balls bouncing around inside the hull."

There is a knock sensor which should prevent pre-ignition if this is the problem. Unfortunately it (#31) is under the intake manifold.
If someone replaced the engine and did not clean the sensor mounting surface properly or torque the bolt correctly, it could be your issue.

 

[thzpcs]

Weird question, but is there a chance that the noise could be cavitation? I took a look at the wear ring, and yeah... It was so worn in one section that it actually sawed it in half and came out in two pieces. Haven't gotten it in the water since replacing it, but just realized that might have been part of the issue too, and might explain why it was so bad only at that one specific speed?