Not your tipical engine noise

Joined
May 23, 2000
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#1
Help! I just bought my son a 91 KDX and I'm having trouble trying to sort it out. It was supposed to be in excellent mechanical condition, but I've had to redo quite a few things that the previous owner repaired. He supposedly put new rings in last year, but there is a constant "grinding" sound coming from the engine as opposed to the normal kips sound. The bike runs ok, but it doesn't seem to have as much top end as my 1990 KDX. I'm wondering if he didn't put something back together right. Any idea's?
 
Joined
Jan 29, 2002
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#2
Was the grinding sound there when you bought it?

Is the sound always there, does increase decrease with RPM?
Where approxiametly does the sound come from?
I know this is a lot of questions but there is a multitude of directions to go to for answers.
Does the motor have compression?
I would take the pipe off and look into the exhaust port with a flashlight.Hopefully you will see a piston that looks real nice, IE no scratches , marks or blow by on the rings .
A constant noise can be piston damage or could be clutch basket, waterpump bearings ETC.
Piston, rings would be my choice given that someone was in there recently.
After you've checked the cyclinder check in again, there are a lot of experts here.
You really need more info!
 
Joined
Aug 21, 2003
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#3
does the sound come from the clutch. does it stop when the leaver is pulled in if so this is normal. most 2 strokes i have herd make some sort of odd noise.
 
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#4
The noise was there when I bought it. I new it sounded louder than my bike but one of the problems I had to correct was an exhaust leak at the pipe. I installed new o-rings and eliminated the leak. What I'm left with is a grinding type of noise that is similar acting to the standard "kips" noise that most KDX's make. The bike has plenty of low end power, but not much top end. It starts easily and really doesn't run too bad overall.
 
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#6
Need more info.
First off, try to eliminate the clutch, does the noise change when you engage clutch?
Go to motor.
Easy to do, look in the exhaust port. Do you see scratches etc.
Take the top end off.
If anything is amiss you'll see it, for all you know you might have a c clip thats wedged against the side of the piston riding up and down for the ride. Believe it or not engines can continue to run okay with pretty bad damage, it just takes awhile then it gets bad.
There are so many things this could be I would definitely take the top end off.
If you have not done this it is not as hard as it seems. Start with a clean bike go slow, if you have problems email here and anyone of several guys will get you through.
1) check piston, wear
2) check connecting rod bearings.
3 ) check lower end bearings.
4) check KPS, make sure everything moves smoothly, clean while your in there.
5) check cylinder for obvious wear.
If you do not have the tools bring piston+ cyclinder in to a shop to measure clearances.


If everything in the top end is fine I would then move to the water pump bearings . It is hard to diagnos with out hearing the noise.
What oil and ratio do you use???
The noise is not normal, all the above procedures should be in the archives.
Good luck
 
Joined
Sep 11, 2002
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#7
Maybe the kips is stuck in the "closed" position. That said you would have good low end but when revs pick-up its not opening, and not producing good top end pull. Good luck with it.