The Low RPM Rattle

[Grussell]

        I started out in February with a 1980 CB750 K bike in really bad condition.  Six months (and who knows how many hundreds of hours of work) later I've got a 1979 K engine in a 1980 frame.
        Here's the problem... at low RPMs (say 500-600) I can hear a rattling.  It sounds strongest on the right hand side of the engine (the side opposite the starter clutch assembly).  The rattling goes away as soon as you get the RPMs up just out of that range.
        Here's the question... Is this rattle normal for one of these older DOHC bikes, or is this a sign of certain doom for my rebuilt engine?
        Any and all help is of course much appreciated.

[oldbayman]

Could it the cam chain? I've  got the same thing in mine - 1981 750CB F SS. I've tried adjusting the cam chain tensioner but it doesn't take away the noise. I think that this is probably because there may be no adjustment left.

[Grussell]

        The cam chain could definitely be the culprit.  Its way up there on my list of suspects.  The chains I put in the Bike were used, but well within tolerance.  The chain guides weren't too worn but they were pretty old.  I couldn't even find new ones that were reasonably priced.
        Do you know if that is and engine threatening condition, or if its just going to make a bit of sound?

[dylansdad022102]

from what i know your rpm idle should not be 500 it should be up around 1000rpm's +-. turn your idle adjust screw. as for the noise some times its the clutch basket. the rivets become worn and the basket rattles around a little. basically its normal.


but definatly get your rpm's up.

[Grussell]

Thanks, I upped my idle RPMs to just over 1000 and it does sound like a much smoother running bike.  180 miles on it now.  I'm still breaking the engine in, but its getting there.  Thanks again for the help.

[Curt750F]

I have an 1980 cb750F SS with about 32,000 on it, I think I kinda get that rattle but it goes away around 700-1200 rpm, i think its just normal...or at least i hope so

[gonebiking-2]

For starters, the idle speed on those engne is around 900 - 1000 rpm. The rattlle you talk about  is the clutch. Mine started rattling at around 7000Km and 20 something years later, its still rattling. NORMAL for that engine. So dont worry about it. You can rebuild the clutch, but the rattle will come back after a few thousand Km/ Miles.  Whe you pull the clutch lever in to engage 1st gear, you should notice that the rattle almost goes away. But get the idle speed up to arounf the 900-1000 rpm and the rattle will not be so bad

[fernandodefina]

I have a 1981 CB750F SuperSport ugly clutch rattle @ less than 1000RPM idling in neutral but shifts great, talked to a Honda mechanic today and he seems to think its normal even in the newer models.

[Rick V]

Syncronizing the carbs will usually help with the clutch noise.If the carbs are out of sync the firing pulses are uneven causing the clutch to rock a bit , causing noise.

[vstanford]

Yes, the carb sycronization will likely fix the problem.  A friend had this rattle coming from the engine on his 750.  He took the carbs off, cleaned them, reinstalled them and sycronized them.  The rattle went away.

[lcochran]

Adjust your idle up to 900-1200 rpm and make sure your carbs are sinked as best as possible.
I have a little rattle in mine  partly cam chain partly clutch.
Oh well
:)

[BlakTy]

Hi everyone, new here. Working on a 79k and have the same rattle when in neutral at idle. I was gonna replace the clutch but I think ill try cleanin the carbs and a sync. Any info on gettin the engine rebuilt would be great, leads, recommendations and tips would be greatly appreitiated. New to this and want to avoid rookie mistakes. Also selling a strong sabre v45 if anyone is interested.