Help diagnose clacking and rough idle?

I've uploaded a video with a decent audio track here:



This 77 KZ650-C1 generally runs excellent. It seems to sound fine at 1500RPM but starts to act up when I try to set the idle around 1000 (by the dash gauge the accuracy of which I haven't confirmed). Acceleration is smooth from the curb straight through the gears and all the way to freeway speed. There are no hesitations or galloping.


Some history:

4-1 exhaust was replaced with a stock set of 4-2's. Carbs rejetted to 97.5/15/3rd.

The carbs have been thoroughly cleaned and adjusted this week with the exception of the float level which I didn't change. With help in a thread in carbs, I finally found the missing puzzle pieces of what to do with the fact I have both air and pilot screws and now have those set properly.

Valves were done 500 miles ago, while the 4-1 was installed. Have not been done since.

APE manual cam chain tensioner was installed a year or two ago. I've doubled checked it this week. Loosening the tensioner doesn't increase any noise. Tighter and the engine produces a "whirring" noise.

New battery this week.

Cleaned all electrical contacts I could find.

Coil mod done and wiring rechecked this week. Have not checked voltages but didn't notice any change when the mod was originally done a year or two ago.

Has dyna ignition. Coils are black and don't appear to be marked so far as I can tell.



It has the light switch and at idle, there's a notable difference in engine sound when I turn the lights on. Engine speed reduces very slightly and actually sounds like it evens out a little (not that I'm on a witchhunt or electrical problems but thought it might be relevant)

Any thoughts or directions to look in would be greatly appreciated!

On the right side of the engine is a CD sized cover. Under it lives the mechanical ignition advancer. Check to make sure it will return after giving it a twist.If it is sticking,it will affect low rpm running.

Some light oil like sewing machine or 3-in-one will work fine for lubrication.Apply the oil to the pivot areas of the advancer and where the springs attach.

New sparkplugs,sparkplug caps and wires all will help in low rpm problems.

In your video clip, noticed your idle is set under 1k!

Adjust your idle between 1000-1200 and confirm the difference!

I have the same noise on my bike. I can't help you with the idle issues but from what I have read on here the noise is coming from the transmission. There are some springs in your tranny that are loose causing the noise in neutral. If you have the clutch pulled in you should no longer get that noise. Is that correct? If so there is a thread on it somewhere here or at wgcarbs. I will search for it and find it.

That sound of "digesting nails" is common when the idle rpm is lowered enough.

Proper engine tune (ignition, valves, camchain tension, and dynamic carb sync, etc.) should allow the lowest possible idle rpm before getting to the crunching sound.

Good Luck!

You may need to sync the carbs, I know at a low idle if the carbs are out, you get a cyl unbalance and it causes a cam chain noise/ rattle.
I think you may be trying to idle it too low also, the tach's are not acurate although you said you checked it, Ilde it where it runs the best and does not bog when you take off

Check your carb holders. I had exact same symptoms before I replaced mine on my KZ650C1, no problem since.

My 650 which same as you I removed 4-1 and installed nice stock system. I'm using 102.5, 15, 4th slot, along with all new carb holders, boots, Dyna S, float levels, etc. My bike sounds about the same as yours. I idle mine at about (according to highly accurate tach!) 1200-1300 rpm. I still need to check valves and sync carbs (did a very careful manual sync). I believe noise is coming from primary chain (engine to trans) which when slightly loose will make noise idling with uneven engine pulses. It idles best at 1200-1300rpm and is rough below that. My bike runs great and pulls hard to red line.

OMR, how did you "freeze" a pic from the video??

"OMR, how did you "freeze" a pic from the video??"

Simple... Pause the video on the frame you want(scroll back and forth) then DON'T use cntrl/alt/print screen keys on your keyboard... Just Print Screen Key only!

Then paste into paint or other imaging programs.... Whola!

try not to let the bike idle below about 1200 or 1300 like the rest of the guy's have said, let the bike decide what it wants it knows best old air cooled gsxr 750's had a tach that did'nt even have any numbers below 3000 rpm.

Digesting nails is a good way to describe it!

I did put some gauges on and synced the carbs (cheap set of dials off eBay). The bench sync surprisingly got it extremely close and I only needed some minor adjustments to get balanced. While diagnosing, I also put in new plugs but have never changed the wires and caps which go straight into the housing of the coils.


I've just gone out to check. I can confirm that the advance appears to return nicely but I gave it a little oil just for good measure. The sound doesn't appear to change with the clutch or gearing. Visually the holders looked okay but I did a quick spray test just to be sure and didn't notice any behavioural changes.

Raising the idle very carefully, I can say that it's just barely over around 1200 or so (eyeballing by the dash gauge which I have no way to verify) that the engine finally evens out and the sound pretty much goes away entirely which seems to be expected so I think I'm going to take that as my cue to relax.


Thank you everybody for your knowledge and input. My fairly mechanically green mind had visions of pieces of loose parts shredding around, waiting to ruin my day.

It's very reassuring to hear from people who know the machine well!

APE manual cam chain tensioner was installed a year or two ago. I've doubled checked it this week. Loosening the tensioner doesn't increase any noise. Tighter and the engine produces a "whirring" noise.

The whirring is normal, and closer to proper operation.

Sounds to me like cam chain...

I have to adjust my tensioner every season. Several times in three months after putting in a fresh cam chain.

With a manual tensioner, it should be turned all the way in by "finger pressure" only, with the engine at TDC.Turn the engine over, then repeat till tensioner will not go in farther. Remember, light finger pressure only! Then turn it out only about 1/8th turn, then tighten lock nut.

Hope it isn't somethin else serious!

Cheers!