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Discussion Starter · #1 ·
I thought for sure it was in my fairings so I took them off. First ride with no fairings and the rattle is still there and maybe even more noticeable. The source is definitely not from within the fairings. Next mod will be the PAIR valve block off and removal of the baffle and valve. Block off plate ordered yesterday. In the meantime I'll be riding it as street fighter with some front blinkers rigged up.

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I believe I heard that was the culprit on these motors. One youtuber, Jake the garden snake, built a CRF300 and I think he had some degree of success in getting rid of the rattle. I'm not sure why but mine never had this as an issue, but I never really noticed any rattle before or after I blocked my PAIR valve for the EJK kit.
 

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My 300r rattled (more of a corseness or harsh noise) but my 250r didn’t have it. I put it down to the original 249cc motor being stroked from its sweet spot to get the 286cc.
 

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Discussion Starter · #5 ·
The 5k rattle on my bike is literally across the rev range from 4,600 rpm to 6,400 rpm where it intermittently and predictably occurs all the time. I hope it is that pair valve removal that is the fix and not the headlight cluster or the cam chain causing which I'll believe only when I eliminated everything else.
 

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The rattle being described here appears to be the same rattle as described in one or two other forums since 2016 which has generally been a timing chain rattle. My own CB300R 2020 model has this same sort of rattle of which I am convinced it is the timing chain tensioner just not properly doing it's job. The dealership people can never hear it but it bugs me most of the time till I try to tune it out. At only 10,000 klm I find it incredible that both manufacturer and dealership mechanics are not aware of a common problem like this which has been complained about now for over 6 years. by many honda 250 and 300 single owners but the head in the sand option suits them best. The solution which I have seen most information on involves work to or replacement of the auto timing chain tensioner. I hope it is not wrong to link this 6 year old you tube video in the hopes that it may still help out people currently having this noise issue.
If it does not help anyone I do not imagine it can hurt anyone either. The rattle I experience and believe this video may help people with occurs mainly in 5th and 6th gears cruising at around 5,000 to 6,000 or as wide a range as 4,500 to 6,500 rpm heard when not accelerating nor decelerating. Just flat road constant cruising. I have occasionally also heard it on other bikes with this engine being ridden on you tube.
 

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I have a 2021 cb300r and i have that "rattle", i have always chalked it up to the type of sound a single cylinder makes. it is in the 4500 to 6500 rpm range when cruising. I also have a 2020 honda pcx 150cc single and it makes the identical noise, thats why i always thought it was how a single sounds...
 

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I have a 2021 cb300r and i have that "rattle", i have always chalked it up to the type of sound a single cylinder makes. it is in the 4500 to 6500 rpm range when cruising. I also have a 2020 honda pcx 150cc single and it makes the identical noise, thats why i always thought it was how a single sounds...
Hi Dudestuff11. Sadly though not uncommon the noise is definitely not normal and any noise which is not normal has a risk of sooner or later causing something to go wrong or wear. As the gentleman in the video mentions he had 2 or 3 failures of the part before he put in a manual adjustment which resolved his problem perfectly and so far permanently. I see that manual adjustment as the likely eventual fix for me but at the low mileage of my bike will wait a while longer and monitor the noise level increase. It is likely I will attend to the noise when I do my 1st valve clearance check at between 20,000 & 30,000 klm.
I also own a 2015 PCX150 and it makes zero noise of the type mentioned. Make sure you replace your drive belt by 17,000 at the very latest. That is also the service book replacement time. The belt will appear in good condition to look at but they go poof in a big way if ignored. You will notice the maximum speed becoming just a few klm lower before the rev limiter cuts in nearer the end of the safe belt life.
 
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