The International CBX Owners Association

‘79 with 45,000 miles.
I am an easy rider so was surprised today when I accelerated harder than usual and there was a very distinct knocking - not pinging. This was at 3/4 WOT. I could replicate the noise on demand. I use Chevron premium (probably 91 or 93 octane).

My experience with other engines is pinging seems to effect all cylinders, this knocking was louder than pinging and I sensed I could feel it in the bars and it’s “cadence” was slower than what pinging would be.

The carbs were recently rebuilt by OldSchoolCarbs and they seem to be solid.

Bike starts, idles, and accelerates fine.

Where should I begin in my troubleshooting efforts?

Just a guess, but the valvetrain runs at half speed to the crank, so maybe its something in there??
Or possibly loose engine mounts...check them all.

I’ll check and post back. Is there a way to differentiate between pinging and detonation?

I’ll check and post back. Is there a way to differentiate between pinging and detonation?

Pinging IS detonation. Is the knock from deep in the bottom, or definitely from the top end? A bottom end knock under load is a big-end rod bearing. Not unheard of, and if so, you don't want it to fail. So there's no unusual noise at idle? Was the bike running reasonably well with no knocking before the carb rebuild?

Dave

Engine mounts are tight. I don’t believe the noise I am hearing is coming from the valve train. I would think valve train noise would be constant - like a noisy lifter. The knocking which has me concerned is “accelerating related”. I am starting to think something like detonation; pinging would be more broad based (all cylinders). I will pull the plugs and see if I have any debris present which could facilitate detonation. Timing is per spec.

Could be wrong gas , I mean bad gas ?

Bad gas? Always a possibility but I fill up at a busy Chevron.

The knocking which was the genesis of this thread is....gone. The more I think about it, I believe it was detonation of some type. I could not replicate it today; no pinging or otherwise.

The only noise which is of a concern (today) is the damn clutch. The growling significantly subsides with the clutch pulled in - and resumes when the clutch lever is released (I never know which people refer to as clutch “engaged”...does this refer to when the clutch is pulled in?). The growling is much less noticeable once underway, whether because the growling stops or other noises overwhelm it

Not familiar with a "growling" noise from the clutch.
A clutch rattle or knock is a common CBX noise. There are many threads on the subject.
It can be lessened with a carb synch or may require replacing the rubbers inside the clutch.
https://www.sixcenter.nl/onderdelen/det ... 3&start=31

I would strongly suggest you gauge sync your carbs, since they were just rebuilt. They were bench synced by the rebuilder, but that's only close (usually). Your bike will quiet down considerably with a proper sync.

Dave

“Growling” may be the wrong word, yet “rattling” doesn’t really nail it.

I have read a carb sync will quiet the old girl down and I should quit bellyaching and just do it (I have a 4-port sync gizmo which works great when it isn’t collecting dust...which is what it is doing presently). I have been putting off syncing the carbs because the bike idles smoothly and as I understand it, once over an idle, any out-of-sync issues are no longer in play. Is this correct?

Correct, Dave. Carb synch at 3/4 throttle won't have any affect.

It might seem to idle smoothly, but you'll be amazed once you sync them. Of course sync has little to do well up on the needle, but it does effect smooth pulling power off idle.

Dave

Carbs were pretty close and now even closer. Not a discernible difference in the rattling although since I spent the time balancing the carbs, I will say the rattling is less so.

Now I have to make a decision on moving forward with the rubber bushings are investIng in better ear plugs.

Buy the bushings and use them as ear plugs until you feel "motivated" to change the old ones.