Disturbing squeal around 4k RPM

Hello, my second season on my beloved VFR 750 RC36-1 from 1990.
My second season on wheels at all..


My Viffer have developed a disturbing squeak / squeal?

I can hear the cam gear alongside. And I don’t think they are connected.
The sound is hard. And “planted” through my body.. like it’s coming inside my neck or head.
Opening the visor I can hear it from the bike.
Oil level is at max, looks good. and changed in february.
I jump around the rpm to provoke. Don’t take the driving seriously.
Chain and sprockets changed 1000km ago, ish.
But I doubt that’s the issue since the sound is still present when parked.
The sound Can disappear after waiting for a red light. And then appear again after 10 secs.
ONLY around 4K rpm.
No other symptoms.
No oil light.
Only rpm has an effect.
Clutch lever has no effect
My thoughts were vacuum leak at the carbs???


I changed the rubber boots below the carbs, they were like rocks, and the sound went away, for about 2-300 KM, and is now creeping back.
I haven't tried re-tightening them yet, and i will try that, but i doubt that's the solution, they can't just go loose?

When i start the bike, No weird sounds.
when i go for a drive, it drives perfectly especially after the boots were changed.
After about 10-15km i can provoke the sound again, heat related? but still drives perfectly, no power loss or "unevenness"
easiest way to provoke is to engine brake for a long time, and getting slightly on the gas again.



The videos are BEFORE the 4 carp rubber boots were changed. (the boots between intake and carbs.)

Now the sound isn't as loud, or consistent, but still creeping worse and worse.



Another thing to note in the troubleshooting, the Front right cylinder have not been firing correct the last couple of years, when i got it i thought it ran a little uneven, and rough.
Still on all 4 cylinders, but it turned out the spark plug cap was dead, had a loose internal connection, and it was changed a couple thousand km ago. Ran WAY smoother and have more power after 4 new plug caps.

I am 99% sure it is vacuum related, or atleast air related.

for the most provoked example.: https://youtube.com/shorts/_e50QwNIBzU
This video it is still there, but less consistent.: https://youtube.com/shorts/NdVvlilWJsI

 

usually, the boot clamps have a small spacer on the screw to prevent over-tightening (i guess) that can be removed for firmer clamping with a proper JIS screwdriver. for your running problems you could experiment with idle mixture screws 1/4 turn in or out.

but, wait ! you ride without earplugs ? try some.


---------------------
say good-bye to greenland , buddy, if the evil imperialist trump wants it.

 

I have earplugs but i want to hear the V4 <3 <3 <3
i use plugs on longer commutes, but rarely.

It's running great ever since replacing spark plug caps, and carp boots, no running issues.

The sound is just nerve shattering, undrivable, bystanders can hear it...
I will look into the spacers if i decide to tear it apart again, but i will wait for more suggestions from experienced people first...

Trump can go dig a deep hole and jump in it for my sake, in most of denmark we have painted him as a clown
In my eyes he's a orange baby, dropped to many times
no translation og subtitles, sorry
No offence... but that guy...

 

Det er nok godt, det samme ellers en rammende opdatering af en klassiker.
Welcome! How does the bike idle? Typically leaky manifolds/inlet will manifest in unstable idle running. Simply because when idling the vacuum in the inlet is high and the flow is smallest. If leak in that area is obvious it could also be the connections for vacuum gauges. That aside it seems a nice "civil" sounding VFR. Has it been imported or was it originally made for northern Europe? That may affect some details of the air filter box.

 

Tak!

The bike idle's good, or the same as the past 10.000 km... Rougher than a injection motor, but alright for a 35 year old karp...
I checked all 4 screws where the vacuum gauges are supposed to attach, screws, not nipples, they are tight.
(RC36-1 came with screws in place.)

Low RPM and High Vacuum are my thoughts exactly.
The carps are coming off tomorrow again, to check if something is out of order...
After changing the boots, the chirp went away for 300km isch, and crawled back!

After this i am close to giving up on it to be fair, sad but i can't ride it with this noise, headache....
I'm not gonna throw 2-3000$ into someone rebuilding the carps and the issue might still be there.
Going THAT deep is out of my expertise and spare time...

As far as i know, sold in denmark from new.
Airbox is as following: Carps > lower plastic housing, filter, upper housing... no gaskets, just plastic ridges that slot together.

The weird part is that the issue can disappear shortly and reappear seconds later, especially after being stopped for a small period time, 10 sec - 2 min.
and ONLY around those RPM in the Video. 99% certain.

akrapovic exhaust
(No exhaust on the bike at all still have the squeal/chirping... had to check )