Maintenance & Technical > KX500 Original

1986 / 87 KX500 electrics

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madmcnish:
Hi there,
My KX500 (1987 motor in '86 rolling chassis) has started running rough (blubbering and cutting out) and I think it might be the CDI unit.
The engine has recently been rebuilt and was running fine.
I've checked it over and everything seems OK mechanically.
During the rebuild, the stator and coil were replaced and the plug is also good, but the spark is pretty weak which leads me to think that the CDI may be on the way out.
The stator voltages were all checked-out and were OK after the rebuild.
Does anyone have an idea if the CDI from different years will fit the '87 motor?
Any ideas to what else could be causing the rough running?
Thanks in advance for any advice.
Cheers,
MadMcNish 

Johnniespeed:
 Mad, rather than spending a lot of money on parts you may not need, why dont you ask the members here if they would put your suspect CDI unit on thier good running bike to test your CDI unit ? 
  If a good running bike still runs good with your CDI then your CDI is not to blame.
  Now who has a good running 86/87 ?
  John

Hillclimb#42:
When did it start running bad. Right when you got it back together? or well after break-in? How are you telling it has a weak spark? It was hard for me to test when having a firing issue with an old '85 125, I worked on. The best way I found was to remove plug, put it into the wire, and kick. The compression kept me from kicking fast enough to get a good spark. The spark was never bright and scary like a car's spark, either. I also read, you should always have a plug in it when testing. Without the resistance of the plug, you can damage the CDI. Also, sometimes a weak spark is a result of a weak ground connection. You might remove the Cdi, or coil, clean mounting area and re-install. The reason I asked about when it started messin' up, is that when a motor is getting worn, it usually needs fattened up with jetting. After a fresh top-end, it would be rich. You say plug looks good, so that may not be it, but how does plug reading get affected by weak spark? Where is it blubbering? All the time? Bottom rpm range, mid, top? Have you inspected the reeds?
Don46 would be a great guy to ask. He has reverse Alzheimers and can rattle off a bunch of spec.s on that kinda stuff. I guess he has trouble with yesterday, but is like an owner's manual for any bike he has owned.

hughes:
My 87 did the same thing and I found that my plug wire was damaged. What I mean by damage is that the plug caps threads onto the plug wire and the end of the plug wire was worn out you could almost pull the plug cap off the plug wire by hand. I trimmed about 1/4 off the plug wire and threaded the cap back on. It almost acted like a plug fouling out. Might not be your problem but it's easy to check and see.

don46:
A cdi from a 86, 87, 88, or 89 will work. or you can put a whole 90 or newer ignition system

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