Modifying the KDX CDII increased the timing cap by 1uf to 5.7uf and when I tested it I was really thrilled because it ran so good all the way through the rev range I couldn't believe it. But this was with a rewound stator coil which produced more voltage than a stock coil. Later I changed the coil to an OEM coil and changed the CDI's pull-down resistor to bring the negative voltage waveform up to where it needs to be for performance. Seeing that it was designed as an enduro bike they probably made the curve so that the spark timing hinders mid range and top end performance for more reliability. (As power in an engine increases, heat does also. Too much heat leads to piston seizure). Now that I've changed the curve, I have more of both mid range and top end power! I think it basically gave the mid range more spark advance, and the high revs more spark retard, both of which increases power on this bike. An interesting twist to the story: I had a rewound stator coil on my bike up until I installed a stator coil from Elextrex in England. The OEM coil should have 304 - 456 ohms according to the manual. My locally rewound coil only has around 250 ohms. The Electrex coil has 400 ohms. I think the rewound coil has wire that’s slightly thicker than OEM which would account for its lower resistance although the coil body is almost the same size as original. When I installed the Electrex coil my bike lost some of the top end power, a fair amount. It just wasn’t impressive anymore. So I reinstalled the rewound coil and shazam, the magic was back! I looked at the voltage waveform with my oscilloscope on the timing capacitor with the two stator coils. The Electrex coil produced more peak voltage at idle than the rewound coil. So with the Electrex coil the pull-down resistor would have to be lower in value to pull the voltage down to the stock levels (which are even lower than that from this rewound coil). Someone on dirtrider.net said that when he installed a new stator coil from Electrex that the timing seemed more advanced and more prone to kicking back when he kickstarted it. That would make sense if the coil was putting out more voltage than the stock coil since that would advance the spark timing. When I received a new timing light (that is made to work with CDI ignitions) and put it to use on my KDX I saw that the timing progressively advances from idle all the way to maximum advance around 3600rpm and then starts retarding (going more toward TDC) more and more with higher revs. The following data is from my KDX with a rewound stator coil which affects the timing, but probably the same amount of degrees all the way thru the rpm range. But anyway this test does show the relative effects of timing cap change on the timing curve. 4.7uf is the standard value of the timing capacitor in the CDI. I added 1/2uf and 1uf to it for the following 3 values of timing cap. The readings (in degrees BTDC) under SR are from a standard CDI for my international version of street-ready KDX which uses a 4.7uf cap but a lower value of thermistor (which is in series with the cap). rpm 4.7uf 5.2uf 5.7uf SR ----------------------------- idle 19 19 19 17 3600 23 21.5 21 21 6000 19 19 18 19 Ideally I should of tested the timing at 9000rpm also but I didn't have the timing light that also reads the rpm. Instead I used a circuit and then a frequency meter which I had to the side and didn't want to hold the revs so high long enough to get a reading. Also it was hard to rev it to 9000 and keep it there. I have experimented with using 5.2uf and on street runs I felt it gave me equal results and probably doesn't let it run as hot. Below is the graphs of the standard curve and the modified curve. It shows my current setup using the 5.2uf cap with the stator plate advanced 1mm (CW), and the standard timing curve. This shows that the modified timing is more advanced at low rpm's and less advanced in the lower midrange and more advanced in the upper midrange to maximum rpm's.
My international model (SR) was designed to have 21BTDC as the timing at 6000 rpm. I think all old KDX's are that way (probably pre-'90). So if you have a modern KDX with 19BTDC timing then you may have to advance the stator plate a couple degrees more. Lastly I didn't like how the "analog" curve did not ease up on its curve at high revs like digital CDI's do. So I created a simple mod to the CDI which allows a "tail-end" curve adjustment which lets it rev out higher. Look at the following chart which shows the digital curve of a RM250 (top curve is throttle closed, bottom curve is throttle open. this circuit changes with throttle position), a stock KDX curve, and my modified curve (#2). My final choice of timing capacitors is a .22uf added to the std 4.7uf, totally almost 5uf. This was better than the 5.2uf for the mid range. The final product is reliable (race tested) and gives a noticeable power increase all through the rev range. Try it, you'll like it!
If you are interested in having these timing mods on your bike then I can modify a used CDI from EBay and sell it to you for $100. I just need to know if you have a standard stator coil or one from Electrex. If you have a rewound one, then I can't help you since I don't know what its negative voltage level is. My email is j57aguar@yahoo.com More of my pages: SR mods, motocross mods, KIPS servicing, tire changing guide, alternative healing methods |
