Engine rattle above 3500 rpm

Pulled the plugs and checked the color, they were a nice light grey.

Um, they're not supposed to be gray, they should be tan to light brown. Could you post a close up pic of one of the plugs?

Well, I put the bike on the center stand and warmed it up. I went through all the gears, up to 4500 rpm, and no knocking or rattle. I then rode to the hardware store and the rattle returned around 3500 rpm. Is seems to only rattle with a load on the engine/drivetrain.

That's a classic description of detonation right there, only does it under load and starts around 3500 rpm. I bet if you take it up to 7k to 8k rpm under load the noise goes away.

Usual suspects:
Wrong spark plug range or type
Fuel Octane to low (not likely on stock KZ)
Lean mixture (including vacuum leak)
Timing advanced
Sharp edge in combustion chamber
Cam timing retarded
Restricted exhaust (yes, that is a possible cause)

The gray on the plugs could very well be aluminum, caused by detonation, and that is a "serious" problem.

You could easily prove me right, when it starts knocking around 3500 rpm open the throttle wide open, if it gets louder then it's detonation (spark knock). "BUT", if I am correct (and obviously I think so), you stand a real good chance of blowing a hole through at least one piston :woohoo:

KZ1100 has roller rod bearings

Forgot another common cause of detonation, air/fuel mixture contaminated with oil mist.

steell wrote:

Pulled the plugs and checked the color, they were a nice light grey.

Um, they're not supposed to be gray, they should be tan to light brown. Could you post a close up pic of one of the plugs?

Below is a picture comparing a properly operating spark plug (the one on the left) and one that is light gray in color (the one on the right). The following post has what the piston looks like with the light gray picture.

Do worry, no KZ were harmed in the motorcycling mis-adventure!:silly:

Spark Plugs

OH, the carnage!

My bike has a similar rattle at a similar rpm... I failed to use small orings that go over the caliper shafts to keep them from vibrating. The problem does no harm other than the noise being a tad irritating... Try lightly applying the front brake when the rattle occurs. If the rattle goes away, it will be the front brake. My rattle definitely sounds like it comes from the engine but it is definitely the front brake.

IF that doesn't pinpoint the rattle, try using one grade higher gas next time you fill up.

If that doesn't fix the problem, tell me about your cam chain tensioner stuff... did any get changed when you did this valve stuff?

Thanks for the comments. I have never seen a spark plug colored the brown as shown in the picture. I have changed out the plugs twice, with a set of NGK and Champions. The bike runs better on the Champions. All the Z bikes I have ever had run better on Champions. I need to pull the cam cover and check the valve timing. I have been guilty of installing the cams one tooth off.

I did some testing last night on my trip to the gym. I could not get any rattle by luging the engine, trying to get it to knock. The rattle is rpm specific. The bike runs great around town, starts and idles easily. Oh well, I guess it is time to get the wrenches out. I had hoped to prolong this because I would like to replace the rings and valve springs this time. Thanks again....chris3

If it runs to rich the plugs will be black.
If it runs to lean the plugs will be white.
If the mixture is right the plugs will look like the pic on the left.
If the plugs look like the pic on the right, then the pistons will end up looking like the one in the pic.

I have never seen plugs in the color shown in the pictures, ever. It might be the photo. I don't know how to post a picture on this forum so I cannot show you.

I can get a photo to photobucket, how do I get it from there to here?

Below 3500 rpm, the bike runs just great. Some days the rattle starts above 3000 rpm. I will give the valve timing a look and tinker with the carbs this weekend.

One other thing, I am getting a fine spray of oil on the pants leg on the left side next to the number one carb. I just can't tell where it is coming from. Maybe I am getting a head gasket leak in the upper rpm ranges and that is what I am hearing. I have a spare head ready to install. I will order a gasket set and swap heads to see if that cures the problem. I want to look at the rings too because I need to replace the base gasket. It is weeping oil on the front side......Thanks again for your comments....chris

Below the pic in photobucket there are four boxes, Email, Direct Link, HTML Code, and Image Code.

I just copy the Direct Code link, click the Img box right above the text box in the Reply to post screen here, then paste the link between the two Img tags. Change the size=150 to size=500 in the img tag.

Here is the bike in question. Now that I know how to post pictures, I will take a photo of my plugs and let you see.....chris3

Only the hole might be a lot larger

Ever figure this out? I am having the same problem and would love not have to rebuild the whole motor
Thank you!