CDI, Coil, and stator . . . what does each one do?

[Jones]

I'm trying to diagnose a problem with a 88 kx 500 that used to be mine, but now belongs to a friend. The bike will not produce power under load, but it starts on the first kick and has good throttle response. As the load increases, it begins to stumble and sound like its running rich. Climbing hills just doesn't happen. I believe it's an ignition problem now, but just to paint a clear picture here's whats been done to try and remedy the problem;
Air filter replaced
Entire fuel system cleaned and checked - including float height
Reeds checked
Piston and rings replaced
power valves cleaned and reinstalled with marks aligned

So . . . the manual says a weak magnet in the stator can come from age. I'm thinking that enough of a magnetic field is there to get a good spark at idle and slightly above (where it runs perfectly) but not enough to generate enough current to run the advanced ignition curve as rev's increase. BUT . . .I only have a dangerous amount of knowledge. I teach highschool science, and get the ideas of how the ignition works, but I don't know where each piece contributes to the whole package. Also, I am mechanical enough to know that the best information for solving these problems is not theoretical, but comes from the guy whose done it before . . .
The stator generates the electricity for the plug, but how does the CDI control ignition and where does the coil fit in? Does the coil just get the charge right for the plug? This is where I get lost . . .

Just think - If you straighten me out, I can help hundreds of dirtbiking kids and we'll have a whole generation of great bike tuners out there!
(school teachers always dream of inspiring the next Einstein . . .or Honda or Showa or Gorr . . .)

Thanks in advance.

Jones

 

[jmics19067]

if you want just enough info to be dangerous then I can help you!

the windings under you magneto flywheel do nothing more than produce electricity preferably at the proper time the magnet passes the windings you will have a properly timed pulse. the cdi monitors the pulses and adjusts the timing for how fast the pulses are<engine rpm>. the coil does nothing more than take your electricy that you already produced and transforms it into a higher voltage so it can jump the spark plug gap.
If you have an ohmmeter and a service manual for your bike take some readings according to the manual of the stator and the coil. if they check good take off your flywheel cover and hook up a timing light. you can use a regular automotive timing light you just need to hook up the power wires to a battery. start up the bike and observe the timing marks with the flashes of the light as you rev up the engine the timing should advance and as you start really wringing it out the timing should start to retard again. If it doesn't and your ohm meter readings are ok you have a good chance of the cdi being bad.

if it was hard starting and a dull spark I would start to suspect either the stator windings or coil windings either not producing or not transforming the voltage high enough. and use the ohmmeter readings to help confirm it.

and the most important thing I ever learned about electricity is if you see a bad wire it is bad if you see a good wire it still could be bad

 

[darringer]

A friend of mine had an 85rm that ran the same as you described. After alot of hair pulling and cussing we went back through everything a second time and found a loose main jet was the culprit. This may not be your problem, but don't overlook the stupid little things. Just a thought...

 

[Jones]

Thanks gents! A new stator goes in tomorrow, hopefully I guessed right - If not, we have to go cdi. The choice was a simple one - a stator is 90 bucks from moose - a cdi has to come form Kawi . . .where a stator is 500 bucks!
Thanks again - especially jmics, I don't know if this is good or bad, but your explanation is just what i thought all the stuff did . . . hopefully we were both right! Darringer - good advice, but if a loose jet is the culprit I'll feel so stupid that I'll sell my bike and get an XBOX with motocross madness so I don't have to go outside where people might see me.

Jones

 

[jmics19067]

Your welcome but that is the simplest of all igntion testing that is so generic it could be too simple to prove anything wrong. When you have trigger coils and throttle position sensors and such it can easily throw a wrench into the system. The best thing to do is to follow the trouble shooting tree in your manual. The manufacture knows what they needed to build it and anything outside of that is a guess. Personally I hate guessing at parts and I might not always prove something is good but I need to prove to myself that something is wrong for me to be comfortable.

 

[KXTodd]

Just a couple simple thoughts. Have you changed the plug and cleaned all the contacts?

 

[Nevada Sixx]

you may can use a smaller gap in your spark plug, clean out your choke on the the carb, and repack silencer.

 

[NAYKID]

Did you check the sliencer/spark arrester for any obstruction, I fell victim to this once.