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I was riding 2 up the other day with a friend. I weigh 180, and they weight around 150. So we weren't too heavy for the bike. The motorcycle also only has about 4600 miles so fairly new. While I was riding the motorcycle on the freeway, I was in 6th gear on the throttle around 8000 RPMs going 75ish when randomly the RPMs dropped to about 7200-ish, and then that rattling noise started happening but I didn't notice to really as much until we got off the freeway due to the noise and wind. It doesn't seem like there is any loss of power and no service lights have turned on. When in neutral and off the throttle, it doesn't seem to really make the noise but when accelerating on the throttle the noise comes back. I got home checked the oil level and saw it was empty. I checked how far I rode and in total, I went about 50 miles. Today I did an oil change and refill. There was about 1/3rd of a quart of oil in there (nothing I know.) But to my surprise and liking, there were no metal bits or flakes in the oil that I could see or find. I turned on the engine in neutral allowing the engine to lubricate up and let it stand in neutral for about 10 minutes. The rattle noise seems to have dampened significantly after putting in the oil, however, I can't help but notice that same rattle coming from the bike when I go a little bit on the throttle. it just doesn't sound exactly like the same bike it was. I decided to take it around the neighborhood (only up to about 5.5k revs) and it feels like the same bike it was. Didn't feel any loss of power but when I start to rev the bike up I feel that I can hear the noise a bit louder again. Does anyone know what the issue might be? I have a feeling it may be something to do with the valves, but I really hope not.
 

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Pretty sure it's piston slap. I'd bet that running without enough oil caused some burnishing damage one one side of the cylinder and piston. If I'm right, there's almost certainly some galling blocking a little bit of the oil passage space in the piston rings, so the most damaged side of the piston will be getting the least oil from now on.

It will continue to run, but will likely get progressively worse over time, probably at an accelerating rate. If not dealt with, it will eventually result in a catastrophic engine failure.

How to deal with it? Pull the engine and replace the piston, piston rings, and cylinder. It's easier that you'd think....I actually made a video series showing how I did it on my cbr300.
 

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haha...yeah, sorry about that, was on mobile earlier.

There's a link to the playlist in my signature, but here's the handful that are probably most useful for you:
  • Remove the engine:
  • Engine teardown part 1:
  • Engine teardown part 2:
  • Clean and inspect (what is piston slap):
  • Reassemble the engine:
  • Reassemble the bike: CBR300R - Reassemble the Bike
There's more videos before removing the engine that give details about when i got the bike, how to pull off the fairings, and the throttle body and yadda yadda. I also tore my engine all the way down to where i had the crank case in two halves, with the transmission out and crankshaft in hand....with 20/20 hindsight, that effort didnt change the outcome.

Hopefully this is helpful to you!
 
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