what is my noise?

[hondarider]

1980 750 C with a windjammer fairing, 5th gear going between 55 and 59 mph, i hear what sounds like a chain slap, only between those speeds.  I used to have a windshield on it. called the shop and they think i always had this noise, but now hear it with the new fairing. could it be the cam chain or drive chain, or am I just goofy? what do you guys think?

[mmorri]

I've got the same noise on my 81 Custom.
No fairing. It does it in all gears between 5800 RPM and 6400 RPM and then is smooth all the way up to redline.
As it's right at cruising speed for the freeway while in top gear, I ofter just drop it to 4th and stop worrying.
Top end looks good as for tolerances and lash. Chain is adjusted.
It sounds for all the world like a main bearing; But, that wouldn't have such a limited range of presentation.
It'll do it in neutral, also; (But never when I specifically want another person or bike mechanic to listen to it.)
It's been doing it since I got the bike at 20k miles and i have 35k on it, now.
I'm thinking it must be something in the clutch.
Any ideas?

[LukeM]

IF it's a cam chain noise, it would do it in all gears and neutral, assuming you're able to hold the throttle at the exact RPM  that makes the noise. You can also check this by getting up to the target speed in 5th, then downshifting to 4th, and revving up to get to the target speed. If it does it ONLY at 55-59 MPH, then there's a good chance it's drivetrain related (like the drive chain slapping or somesuch).  Maybe a cable that's flapping in the wind?

Hope this helps.
Luke M

[mmorri]

Unless it's an input shaft issue on the transmission, it's not drivetrain related.

When I spoke about road speed it was just to illustrate how I deal with the problem when I'm on the freeway.

It makes the noise in all gears and in neutral with the clutch in or out.

Again, I've checked and adjusted the timing chain and checked the clearances in the cam/valvetrain.  All of the valve springs appear to be good.

I can't believe it's a rod bearing as it'd make the noise for a much wider rpm range and frankly, the engine would have died long ago if it were a bad rod bearing.

I was hoping someone had found that the pipes had a baffle that can break free and resonate at the right rpms or that some ignition/timing glitch could cause it.

It's a definite knock and not the kind of sound a slapping chain makes.  It sounds like a bad rod.  It's too loud to be wrist pin slap.  I'm kind of at a loss.

On Thu, Aug 14, 2014 at 5:50 PM, LukeM [via Honda CB750'S] <[hidden email]> wrote:

IF it's a cam chain noise, it would do it in all gears and neutral, assuming you're able to hold the throttle at the exact RPM  that makes the noise. You can also check this by getting up to the target speed in 5th, then downshifting to 4th, and revving up to get to the target speed. If it does it ONLY at 55-59 MPH, then there's a good chance it's drivetrain related (like the drive chain slapping or somesuch).  Maybe a cable that's flapping in the wind?

Hope this helps.
Luke M

Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.

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[LukeM]

If you can take a video of the bike when it's making the noise, that would help the diagnosis.
This is presuming that it's not a drivetrain thing, but something in or around the engine.

Luke M