Motorcycle Safety & Driveability: Yamaha R15 V2 engine vibration, engine vibration, wind shield

Question
QUESTION: Hi,
I own a Yamaha R15 V2 (150cc, 17bHp) bike, its 2 months old and completed 3000Kms with 2nd servicing done.
Recently I am facing a strange problem. When engine RPM reaches the mark between 3000-4000RPM, I feel some kinda noise other than engine sound. When RPM goes above 4K or drops below 3K everything seems good.
I checked that with Service dealer, he said its wind shield vibration sound. Which I bet its not, because if its wind shield sound then should come when engine starts. Why only in that specific RPM?
I am bit worried as the bike is new. Could you please advice what needs to be done for the above concern?

Thanks in advance!
Sachin

ANSWER: Every bike, every engine size, every engine configuration emits different "harmonies" at different rpms. Even two engines of the exact same size and configuration -- even assembled side by side by the same person in the same factory -- will have slightly different sounds and vibrations because their exact machined sizes and tolerances will be slightly different ... even if only .001 mm different.

What you're experiencing is not necessarily a flaw but just a quirk. The weight and balance of something in the engine (crank, cam, gear, counterbalancer, who knows what) is such that it gives off a vibration when spun at a certain frequency (rpm). The windshield, being mounted where it is and how it is and being that it's a relatively flimsy piece of plastic material (rather than solid steel) picks up on this vibration and amplifies it -- like a megaphone.

You can probably solve the problem of the noise you hear by removing or replacing the windshield, but the vibration will remain. Whether you feel/hear it or not remains to be seen.

In the end, I wouldn't worry about it. Every bike has vibes at some rpm or other. We bikers call that "character."

Pat

---------- FOLLOW-UP ----------

QUESTION: Hi Pat,

Thanks for your information.
The noise/sound that I am hearing was not when the bike was brand new and till I got 2nd servicing done. So I am not sure whether to ignore it? As the problem started after 2nd servicing?

I would ignore that noise, but its strange because early morning when I start the engine, it sounds normal. Once I hit road and travel couple of KMS. I would hear that noise. Moreover while traveling sometimes that sound/noise goes off. So I don't know whether its a wind shield or something related to the Walls of engine which needs to be tuned?

Thanks so much for your quick advice.
Sachin

Answer
It could be anything. As engines break in from new, their parts wear -- particularly between new and the time of first service. Theres so much wear the first 600 km or so we change the oil right away, to get all that excess metal out of the lubrication system.

It could be that just re-torquing one particular bolt or fastener was all it took to notice the problem.

You notice it sometimes and not others probably due to the engine temperature. When cold, the parts all fit together differently than when warm, or hot.

Again, try removing all the windshield hardware and see if that solves the problem.

It is possible that a technician made a mistake during the first service, but I doubt it. Your bike is just developing it's own character. Small engines can be very buzzy. But every engine has an unpleasant vibration somewhere.

Pat