engine knocks when hot!

I have recently acquired a new crummy problem- my kz650b knocks at all rpm ranges only when it becomes warmed up to hot and loses power. When I begin riding her for the day, she is perfect- plenty of power, no noise, smooth, etc...

I suspected perhaps piston knock, but that doesn't seem to make sense. Wouldn't it knock more when cold then become less of a problem when everything heats up? I recently checked chamber pressure and got a pretty even 120-125 psi on all cylinders.

I have searched the forum several times for related articles with no avail. It's not the chain knocking noise, as it will do this at idle. I don't think it's valves, as I can hear the light tapping of a valve sometimes at idle. The knocking is more pronounced.

Does anyone have any idea what would cause this after my bike gets warmed up?

-Confused in Colorado

maybe its your oil thinning out when it warms up so your clearances are looser and making noise. a rod bearing or main would be my guess since 650s have bolt on rods and bearing halves like a car from what ive seen of them.

First off, has anything been done to your bike or have you done anything out of the normal recently (using fuel from a diff gas station than you normally do, changed spark plugs or reset timing, using a diff brand or weight of oil, etc) that may have contributed to the problem?

Since you said you've only recently started experiencing this problem, and only after the bike has reached normal (and above) operating temps, I'm inclined to believe it's going to be a timing or fuel/air related issue.

Perhaps it's as simple as you got some bad fuel, the carb manifolds or airbox boots are leaking, the points are worn and/or not gapped properly, or the ignition advancer mechanism is hanging up and in need of being lubed.

I'd check the simple stuff above first before even considering tearing into the engine to check for bearing wear.

I don't think it's gas- been happening longer than that. I tuned my bike with a timing light recently, so I don't think it's the timing. Interesting thought with the oil- will check for carb drainage into oil next time. Could my kz550 coils have anything to do with it? They are 1.3 ohms and designed for electronic ignition and my bike says 4 ohms. Just a thought:laugh:

If it's knocking in the head, it ould be preignition from carbon buildup OR bad gas.

Otherwise...

Oil is too thin or clearance is excessive somewhere.

knodking in block, piston slap
knocking in case, likely a rod bearing.

Without hearing and listening around, it;s hard to say.
Could be something totally different than my WAGs here

Does this bike have a vacuum petcock?
If so, smell your oil.
If it has even a hint of fuel smell, it's been thinned out with fuel.


Use a broom handle or piece of metal rod and hold it to your ear and to different parts of the engine fto learn where the noise is oming from.
-Duck

Post edited by: Duck, at: 2006/10/10 23:40

dont get that piece of rod anywhere near a plugwire,dont ask how i know! saw a guy using one of those jc witney stephoscopes years ago and found a bad plugwire the hardway!:sick: :S id say try a piece of 3/4" pvc pipe you can hear pretty good through it and wont conduct joltage!