Tiger in the cylinder.........

[DrMojo]

Question from a new KX 500 owner......  I noted that at idle the cylinder area has a noticalbe growling sound....sort of a muffled box of marbles...sort of.....  Is this the valve gear/mechanism at work keeping the beast breathing?    The bike is only a few years old (99) and in very good shape.

Thanks!

[Hogwylde]

Sounds like classic piston slap to me.  Does the tone change when you squeeze the clutch?  If it does, could be a worn clutch basket letting the plates bounce around a little.

Pull the exhaust pipe and stick your finger in the port and see if you can wiggle the piston.  If you can, you have too much piston/cylinder wall clearance.  You might be able to get away with just a new piston.  Don't push your luck too long though.  That piston slapping around can quickly turn into a grenade and you'll be picking pieces of it out of your crankcase.

[MarkNZ]

I've often wondered where that Noise comes from..... At Idle it Ding Ding Dings, then Behind that is a very mechanical gear driven whirring Sound, Quite Audible and slightly Disconcerting....Is that the Noise you mean ?? When you rev it, the Noise increases in Freq and dissappears.I think its something to do with the Drive for the Waterpump, The KX500's were'nt designed to Idle like a 4-Stroke and maybe thats Your Big KX saying " Lets go for a Ride !!"

My 97 has made that noise since Birth, and my CR500 makes a Similar
Noise, though a lot Quieter.

[DrMojo]

That is it.  I spoke to the original owner and the motor has about 40hrs on it and it is stock (no rebuilds).  Compression is great and there is no piston play.  

The sound is almost like lifter tap in a four stroke so I assumed it was the valve mechanism.  The original owner told me that the KX's make this noise.  And yes, when you rev it, the noise goes away.  It is just very noticable at idle.  

I wanted to get more input before I started to crank-up the big boy and blew it up.  

Thanks.  Any one else have any input?

Al

[vmxguy25]

I have been reading here for a while but this noise post prompted me to become a registered user since my 90 model has this same noise.  So far, all explanations given could have been the possible cause, but let me add a couple more.  First, if your engine has a lot of time on it, the noise could be coming from the upper exhaust powervalve.  If this is worn a bit, it may rattle with the larger exhaust pulses produced by the KX5.  Another possibility for the noise also stems from the big motor's strong power pulses at idle.  This has to do with the straight cut gears in the transmission.  All modern motocross bikes have straight cut gears(as opposed to helical cut) for strength, but they can be slightly noisy.  You dont hear this noise on smallbores, but you can hear it in the big motor because every time that piston fires, it is rocking those gears back and forth against each other.  This second explanation is what I always believed the noise in my KX5 to be since the topend is new(I put it there in 1994 ), and the powervalve assembly is in good order.  The noise has always been there for the last 9 years and it always goes away if the rpm's are increased just slightly.

   This was quite long for my first post, but just wanted to contribute a little.

                       Gary

[demographic]

First, if your engine has a lot of time on it, the noise could be coming from the upper exhaust powervalve.  If this is worn a bit, it may rattle with the larger exhaust pulses produced by the KX5.

I was going to post a question about this exact noise as I have a fresh piston in mine and the crank bearings are good as is the piston to bore clearance but I know there is some wear on the powervalve rack and pinion gear.
I now think you have cleared up what the noise is

It was a very good first post, Ta muchly

Regards, Scott

[DrMojo]

Gary,

Thanks for your comments.  The info is very helpful.  By the way....welcome to the KX crew........

Al