engine vibration

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May 31, 2001
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#1
I finally got my 1991 Kdx200 going after almost a year of trials and tribulations and 3 pistons. We had the cylinder sleved after the first seizure and have made some bonehead mistakes since. But I thought I had it nailed this time. The bike ran great for about 8-10 hours of riding over 4 days in the woods of northern Minnesota. On the fourth day it developed a nasty engine vibration and a moderate loss of power. Also makes a ton of engine noise down low. I pulled the top end and found a groove 2-3 mm wide in the bore and a normal amount of scoring also. It does not seem to have any up and down play on the rod bearing. I am thinking there is a crank/rod bearing problem that is causing the vibration and cylinder scoring. Wiseco did the sleve and I am running a .60 over pistoon.
 

smb_racing

Master of None
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#2
check the obvious things first, make sure the engine mounting bolts are tight. The loss of power could be explained by many things, perhaps you need to repack the silencer. Did you rejet for the top end rebuild? Might be running a little bit rich. Just some thoughts.
 

MADisher

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#3
uhm, Check and make sure you have both Circlips still.

It appears there was a problem with some piston pins being a few thousandths long preventing the clips from sitting properly. Someone recently detonated thier top end with a loose circlip.

Perhaps that's where the 3mm groove came from.

-MD
 
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#4
Ugly! Where is this groove? Transfer side? Only above the transfers?
What has gone through the engine? Is the remanence in the pipe? Airbox & reed fittings ok/intact?

First off I was going to say the crank is tweaked out of true which would certainly cause the vibration, but the groove is obviously a problem & can’t see how could have been caused by anything other than debris. Is the rod big end blued?

More information about groove please.
 
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#5
I looked at the rod big end and it is not blued that I can tell. The groove is on the transfer side below the port on the left. Not any debris but a lot of oil/gas mixture. I was running it fat on oil after the last top end rebuild. Both clips were in place and the bolts on the motor are tight.

Joe
 
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#6
Hmm, any blueing at the rod end would indicate overheating but that would not appear the case.

Something has travelled from the bottom end up the transfers & strangely it jumped out as the piston was at the top of its stroke & was dragged down the bore between the piston. (Usually is pushed out on top of the piston).

If it is not still in the bottom end then presumably the second attempt it made it through complete & found an open exhaust port. Usually they like to bounce around in the head to damage that for a while before disappearing. This may all happen in a matter of seconds.

Damage to that part of the bore will cause minor powerloss due to pumping inefficiency but more likely as it should damage/ stick the ring in place.

The noticeable vibration & where the debris came from is a mystery & one I would not like to gamble another top end on. I would pull the bottom end & check the alignment of the crank & replace the mains as a matter of course.

Let me know if you find anything else.