Food for thought: the cdi has 3 basic functions: 1) Tells the coil when to dischsrge its spark,2)sets length of spark ie. megajewels,3) may or may not set a rev. limit.
Now lets look at #1, the cdi tells the coil when to discharge. The cdi is triggered when the pad on flywheel passes the pickup button. The pad is 8deg. in length out of 360deg., so your cdi can tell the coil to discharge either 4 deg. retard or 4 deg. advance or anywhere in between. This is built into the cdi unless you have an adjustable cdi. Now anyone who has ever degreed their GY6 engine knows that due to variations in manufacturing true tdc is rarely right on the marks, so the built in crank timming can be either advanced or retarded. This is why someone invented the degree wheel so as to find true tdc. Now with this in mind it is nearly impossible for someone to claim their cdi will advance timming 4 deg. Two things are needed to make this claim 1) cdi specifications in which nobody supplies and 2) Knowing the true tdc of a particular engine. So my engine with a particular cdi may have a total advance of 1 deg where as some one else using the same cdi might have 3 deg total advance. The most important spec of a cdi I believe is the megajewel spec. This tells you how long the spark duration is and what the regain capacity is at high rpm. The longer the spark the better the burn which produces more power. Do not mistake spark duration for spark intensity. The cdi does not control the sparks intensity, the coil controls the intensity of the spark. So by using the correct combonation of cdi and coil you can increase both the intensity and duration of the spark which will help create more power by a more complete burn of the fuel. Most stock engines will only completely burn about 60% of the fuel. A good tunned engine with all the right components will burn 85% of the fuel completely. Only an engine running an exotic fuel and super charger will burn close to 100% of the fuel completely. So the bottom line is cdi should be replaced with the coil for optimum performance. Hope this was of help and not confusing.
Tom
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