77 KZ650 Clunking at Idle

Greetings,

I apologize for the long post but I didn't want to miss anything that might be helpful.

I have a KZ650C which had a tune up service at a local shop about a year ago. This was the first service it has had since it's supposed overhaul before I purchased it a little more than a year ago. It's been a daily rider.

At the time of service, I was told that it had been "fiddled with quite a bit" over the years. Has a Dyna ignition which the shop simply stated was "an aftermarket electronic ignition which was notorious for failure", had generic car spark plugs in it, a MAC 4 into 1 exhaust and parts of the carbs had supposedly been bent to try to rectify certain problems. Rich exhaust would flow forth out on startup and it was getting about 30 mpg/hesitating significantly under load then surging forward.

After service (carb kits, rejetting, proper plugs, oil, it is certainly significantly more responsive but I'm still seeing some items that are concerning.

Upon getting it back, after coming to a stop, it would idle fine for a moment, then the tachs would drop slowly as it made an ocasionall clunking sound until the engine stalled.

Putting a higher heat range plug solved the stalling problem but it currently still makes a sporadic clunking sound that can be felt through the frame. This sound accompanies a "stumble" in the engine where the tach shows a sudden drop momentarily. It's almost the same type of sound that the bike makes when shifting to first at a stop but lighter and not so pronounced with an almost unnoticable "grinding" quality. This only happens at idle and generally doesn't happen when I've just gotten off the freeway, largely occuring only in town and happening at seemingly random intervals every few seconds. Throttling up to around 1300rpm's from the 1100 that I have it idling at seems to stop the clunking noise. At about the same interval during low rpm riding (less than 2000rpm), I notice a slight roughness or sporadic surging but given sufficient throttle, the problem goes away entirely.

To try to describe the sound, it's similar to a rough tuned lawnmower. It's generally even but with what sounds like an occasional weak cylinder?

"Purrrrrrrrrrr, pur purrrrrrrrrrrrrrrrrrrrrrrr, purrrrrr, pur, purrrrrrrrrrrr"

On the freeway, I don't notice this unevenness, it's a pretty constant, smooth sound, with just a slight waxing and waning/pulsating of tones that makes me suspect my carbs need adjustment. Roughly, it sounds like:


wuuuuuuuuuuuuaaaaaaaaaaaaaaaaaauuuuuuuuuuuuuuuuuuuuuuuaaaaaaaaaaaaa with each uuuaaaa cycle taking about 3 seconds at 4000rpm in 5th gear (Imagine me sitting at my desk trying to make those sounds and it doesn't seem to silly to read. )


Pulling the plugs, I see that they're all consistantly sooty, but in good shape.


My immediate plans were to try to adjust my carbs, then do WG's coil circuit mod as my admittedly limited experience says that sounds logical based on some of the things I'm hearing/feeling/seeing but the clunking noise still bothers me and I can't help wonder if they're all related. Speaking again to the shop mentioned earlier, I'm told that this is because the 650 uses a chain vs a gear and that there may be some slight stretching and slack being taken up but that it was normal and nothing to worry about. But honestly, it doesn't feel normal and it's enough to choke the engine at times if I've been idling long enough. It's especially pronounced when the engine is warm and I'm constantly fiddling with the idle set (moving it higher when it's warm to compensate for the fear that the engine will stall.


Yeah, so I think I've covered everything adequately. Those of you still awake after all this may be able to point out some piece of information I've neglected to include, however!

Any help or suggestions would be greatly appreciated!

I would put the correct plugs for starts, I am not a big fan of changing heat ranges on bikes, use the NGK plugs.
I would next pull the valve cover and check the timing chain, and then check the valves.
dont know about bending things to adj, sounds funny to me, george is the carb guru,
if the carbs are out of sync my 650 gets a rattle in the top end.
last thought, the dyna systems have been very good with minor problems in my opinion, have it in both of my running bikes

How many miles did it run??

I do have the same sound in my z650. It can be the camchain which is not tensioned enough or your primary chain. The sound is a sort of clatter only present at low rpm's. To get ride of this sound I already got it idling at 1400rpm
This chain is powered by the crank to an internal axel. This chain will wear out in approx 35 to 40.000 miles

The bike has currently almost 36K miles, with the last 8K being put on by me in the past year. Has been doing this the entire time I've had it but not so pronounced as when I first got it back from the shop at which time I could not come to a stop without a stall.

Immediately riding back to the shop after just a couple of blocks, it was declared that I'm not riding at a high enough rpm for the heat range. While it's true, the one hotter plugs did keep the bike running, it was not clear to me if the heat range specifically was questionable, the plugs themselves or some other issue.

I'll get a set of the correct heat range today and report back. I've just moments ago followed my manual for retensioning the cam chain and don't notice any difference. My original thought was a loose cam chain or tired guide but this shop (which itself may admittedly be questionable) insisted it was normal.

I'll report back shortly, thanks for your thoughts!

Clunking at idle makes me think primary chain.
Sometimes if the idle goes to low you can here a clunk clunk clunk coming from the bottom of the engine.
At proper idle you won't hear it.


my $0.02

are you sure that the adj is not stuck?
I would still pull the valve cover and look for myself.
that is total BS on the plug reason the shop gave you, if the plugs are black, then its running rich and they dont know how to fix it.

Try this test for the erratic idle: after warming up the engine set the idle as low as it will go without stalling the engine; spray carb cleaner around the area where the black rubber carb holders mate to the head. Do this at all four carbs. If the carb holders are not sealed tightly against the head, the carb cleaner will be sucked in and change the idle - probably stalling the engine. If this is the case, replace the carb holders and idle will improve. Ed

I have had the same noise appear when ever the engine wasnt running smooth. The miss causes the the chain the slap and hence the noise. I have had this happen with bad intake boots, dirty pilots, carbs not syncd, bad ignition, even leaking enrichening valves, any thing that will make it miss.
I am not a big fan of changing heat ranges spark plugs unless adding a turbo, supercharger, nitrous or major engine rebuilding with high comp pistons.
You need to make sure your carbs are set good and you posted that things have been bent or modified to make it run better. If your not good with carbs I would send them out. Then check your valve clearences and do a comp check.

Also I havent hear about dyna initions being bad at all.

As already suggested, would check the valve clearances.
Also check ignition timing with a strobe light (and operation of advancer unit).
And would not rely on the bike's tachometer for rpm reading at idle.

So I found this thread and its very informative. i was wondering what the original poster found and what solved his clunking. After 9+ years i doubt hes still around, but its worth a shot.