Kz650 engine sounds like a diesel at idle

About 31k miles. Bought it with about 23k on it. Ride it hard every summer.

Developed a tiny head leak on the left side about 2yrs ago. Kept it topped with Lucas heavy duty oil stabilizer. Has always started with ease (1st or 2nd kick), idled, revved and run perfectly. No noises except a backfire on high rev engine braking every now and then.

Rode it last week over the hill and it had a different "pop" ...twice....while engine braking. Probably over 5k rpm. Hit the market, started up fine, sounded ok, rode home.

Cooled down...sat over the weekend...fired it up and it was so noisy that I thought the exhaust was loose or something insane. Did tension the cam chain (which I do twice a season) before too.

Here's the deal: Rev it up into the 3's and low 4's...and the noise goes away. Bike sounds like a 70's inline 4. It STILL starts, idles, runs and rides like 100% normal. 

Wild, right? Really feels like valves - all of them! Lol. But you guys tell me: Am I on the brink of something catastrophic? Orrrr....coax it approx 25min down the hill in low revs to my shop. 100% not setup for a head job here at home. No garage....and hardly even a place to wrench. Carb work is about all I can do with my space here...

If it’s a cam chain idler etc breaking up running it at all is a “ lottery” and I doubt anyone will advise riding it to your shop - do you have a buddy with a van etc

I believe you, man, have nailed it:

Didn't mention: Before start up this evening...I adjusted the cam chain. I could hear a small "adjustment" in there when I loosened the bolt that 1/8 turn to let any slack out.

I have a truck...uuugghhh...gonna be a bear getting it in the back. I've played the cam chain roulette game before and LOST. My gorgeous '81 CB900F had its snap on Mem Drive in Boston about 14yrs ago. Took the whole engine with it. Parted it out with tears in my eyes. 

Ok - my bud has a trailer....sounds like the only smart option.

Also....just doing the mental gymnastics here...

Is there even a part available for that without a donor engine? Haven't gone down that wormhole at all....

bqeberle wrote:

Rode it last week over the hill and it had a different "pop" ...twice....while engine braking. Probably over 5k rpm. Hit the market, started up fine, sounded ok, rode home.

Cooled down...sat over the weekend...fired it up and it was so noisy that I thought the exhaust was loose or something insane. Did tension the cam chain (which I do twice a season) before too.

Here's the deal: Rev it up into the 3's and low 4's...and the noise goes away. Bike sounds like a 70's inline 4. It STILL starts, idles, runs and rides like 100% normal.
 

Does the noise go away or is it simply drowned out by the engine sound?

Here's what I'm thinking: something broke, and there is one valve that's causing the noise. When I was a kid, my bike had a valvetrain issue that caused the valve clearance to become massive. The rocker was flying across the gap and basically slapping the valve open, rather than pushing it as it's supposed to do. The very excessive clearance also meant the valve was being slammed shut by the spring instead of following the rocker. Overall it sounded diesel-like.

I think you have one cylinder acting up in a similar manner: it's still running, so the riding experience is mostly the same, and the other three cylinders are masking the issue.

Possibly a broken valve spring?

Oh wow - 

Yep. VERY possible! That one side with the head leak probably just finally let it the valve go after running hot for 2.5yrs of the leak.

The sounds goes AWAY once you rev over 3500. Like...it's smooooooth. Engine runs perfectly. Wild. 

Only sounds awful at idle and super super low rpm....

Man - you guys are THE BEST. This is why I love the forum! Either way...sounds like I'm calling my friend with the trailer....

Starter clutches are known to come loose also.  

I had my 750 sounding like an old farm tractor with a bad exhaust a couple of years back. The racket at low rpm was dreadful but disappeared at about 3k. The cause was my manual camchain tensioner. The lock nut had come loose and the tensioner bolt had backed out almost all the way. Realised this when I removed the valve cover and saw the chain slack. Checked that the cam sprockets had not skipped any links. Re adjusted cam chain tension and all was good. Big relief. Phew!


 

Sometimes the primary drive chain get very noisy if the carbs are out of sync.or the chain slack/strech is beyond the service limit.

bqeberle wrote:

Also....just doing the mental gymnastics here...

Is there even a part available for that without a donor engine? Haven't gone down that wormhole at all....

Pretty much all the key parts for the motor are available

Just peeked at parts....yep, all good. 

I'm still on a valve on cylinder 1 had "something happen". Makes perfect sense...and the noise reaaaalllly sounds like an upper engine rattle. Love all the suggestions, guys...but I did service the drive chain and check the slack as well as dial in the tensioner before firing it up. My starter works when cold, but slips when hot.

Top end! Eh...it was time for a valve job anyways...and to address the tiny head leak.

Cheers, everyone. Just bummed that it could be a minute before my shop can get to it.