Topic: What the Clack? CB550F Rattle

[RodPlunger]

Hi All,
Can someone take a listen to my bike (see video) and maybe tell me what the rattle might be coming from? It seems to disappear when under throttle and is only present at idle. The video is shot when the bike is cold, and you will hear me rev it up to show the difference when it is above idle. It does get a little better once it is warm, but is still present. The bike idles at around 1100, but seems to bounce when there. Stock pipes, stock carbs, stock airbox. Thanks!

https://www.youtube.com/watch?v=muv_wlVYpxQ

[dave500]

don't let it idle low and slow when cold,hold a steady faster idle,when were the carbs vac synced last?this can smooth out that primary chain rattle,if you have the throttle lock screw use it to hold a smooth faster cold idle as you gear up for a ride,dont let these idle too low,the tachos aren't exactly accurate either yours might be 750-900 idle rpm who knows?

[RodPlunger]

Based on other reading, I had read that this could be due to the carbs being out of sync. I have not vac synced them. I was kind of waiting till I got a replacement snorkel for the airbox (which I now have) and swapped out the NOS exhaust for one that doesn't have holes (which I did).  What's the best tool for determining what the idle is actually at?

[dave500]

your ears and general feel,dont worry about a precise idle rpm its not important,not like a Harley where they seem to have a competition to see who can idle the lowest?

[RodPlunger]

Thanks Dave... Just to sum it up. You are saying it is the primary chain that is rattling? So, sync the carbs and adjust the idle. Do I need to readjust the cam chain tension?

[flybox1]

your ears and general feel,dont worry about a precise idle rpm its not important,not like a Harley where they seem to have a competition to see who can idle the lowest?

+1
New from the factory, 35+ years ago, they all idled perfectly at or near 1000rpm.
Its 35+ years later.  age and wear change things a bit, but we can get it close.
my 750 idles nice and smooth at 1300 on my tach.
recheck your cam chain tension.  that along causes a lot of noise if slightly out.
Synching really can smooth things out once all else is in order.
timing, valve lash, cam chain...etc.

[TwoTired]

What you are hearing is transmission gear clack.  You will likely notice a change in timbre or intensity when the clutch is actuated.
The power transmission gears are always meshing, even though they are spinning on a shaft when not engaged and passing power.  They still have mass and inertia, so they change tooth contact sides fore and aft when there are any speed changes of the crankshaft.

If each cylinder's power pulses aren't exactly the same, the crankshaft changes rotation speed between cylinder firings, causing the gears to clack as they change tooth face contact.  Higher RPMS smooth the inertial pulses/frequency.  And adding or removing clutch mass with engagement/disengagement changes the pitch or harmonic frequency.

The answer is to make all the cylinders fire with the same intensity.  A thorough and precise tune up, as well as a vacuum sync will usually clear up most of the clack at idle.  You can also fine tune the pilot screws to minimize.  But, that is a pretty fiddly opperaion.

The clack itself does not indicate any impending mechanical failure.  But, it really should be avoided as it does increase wear what with all the parts bashing each other instead of sliding gently.

My 77Cb550 F will make that same noise when it is tune up time (In fact, all the Cb550s will).  Precise ignition timing and a good tune-up per the 3K schedule quiets it down spectacularly.

[RodPlunger]

Thank you TwoTired for a very clear description! 

What you are hearing is transmission gear clack.  You will likely notice a change in timbre or intensity when the clutch is actuated.
The power transmission gears are always meshing, even though they are spinning on a shaft when not engaged and passing power.  They still have mass and inertia, so they change tooth contact sides fore and aft when there are any speed changes of the crankshaft.

If each cylinder's power pulses aren't exactly the same, the crankshaft changes rotation speed between cylinder firings, causing the gears to clack as they change tooth face contact.  Higher RPMS smooth the inertial pulses/frequency.  And adding or removing clutch mass with engagement/disengagement changes the pitch or harmonic frequency.

The answer is to make all the cylinders fire with the same intensity.  A thorough and precise tune up, as well as a vacuum sync will usually clear up most of the clack at idle.  You can also fine tune the pilot screws to minimize.  But, that is a pretty fiddly opperaion.

The clack itself does not indicate any impending mechanical failure.  But, it really should be avoided as it does increase wear what with all the parts bashing each other instead of sliding gently.

My 77Cb550 F will make that same noise when it is tune up time (In fact, all the Cb550s will).  Precise ignition timing and a good tune-up per the 3K schedule quiets it down spectacularly.