I've a Honda CG 125 (Pakistan version Euro 2) and on stock it goes to 105 KMPH max (could be 110) but that's the reading from the stock meter. I've recently installed a local CDI unit (which according to the mechanic: does not limit the revs hence more speed).
Limiting revs by design protects your engine
A rev limiter in an ECU limits the RPM's so the end user will not damage their engine by over revving it and having a valve to piston contact event which is catastrophic. As well, an inductive ignition will have a longer spark ensuring proper ignition of fuel in the combustion chamber, a CDI ignition can have a more powerful spark with a short longevity.
Your model of bike was produced from 1976 to 2008. You will need to provide the model year if you want a truly detailed answer.
IF your mechanic replaced an electronic ignition with a CDI unit and fit points at your crank and removed the signal generator I would be shocked. If you have an older model of bike that already had a points/CDI system then upgrading it a newer type would make sense.
To answer your question, you stated;
What role does CDI unit play in limiting the revs?
A CDI does not have capabilities to limit revs. If you now have a higher top end speed because you can push the engine into redline deeper because the rev limiter was removed from the system then you are potentially damaging your engine due to over revving.
You don't have more power, you have more RPM's. This is not good.
IF the bike is running better then perhaps there is an issue with the original configuration that could have been fixed.
I suggest not pushing the engine deeper into redline in order to care for it properly and prevent any physical damage from occurring to it in the future.
CDI means Capacitor Discharging Ignition, which already describes the principle of work quite well.
There is a single capacitor charged to about 300-500V. This voltage comes from the 12V system via a voltage converter, or from special high voltage windings in the alternator.
Upon a trigger signal, the capacitor is discharged via the ignition coil, which can be a separate device, but can also be integrated into the CDI unit.
This needs not more than three electronic parts, but in reality, there is more. For example, the trigger signal sometimes comes from a pickup coil and has to be conditioned first.
But there is more: To improve power, ignition should take place earlier at high RPM and later at low RPM. If a CDI is capable of this, the trigger occurs early and the ignition is delayed by the CDI depending on the current RPM.
The other way around, a CDI can be used to limit the max RPM and so max. speed by delaying the ignition so much, that the motor doesn't gain any power from the combustion. One reason might be to protect the motor from too high RPM, but often, this is used to fulfill legal restrictions. For example, 50cm² vehicles are limited to 45km/h in europe, and in the past, 125cm² had to be limited to 80km/h for drivers below 18 years.
Today the CDI contains a microcontroller in this case, which allows to set the max. RPM very precisely. That scooters run exactly 45.000 km/h.
So, if your mechanic has a "better" CDI, it either doesn't actively limit the max. RPM, or it has a better timing at higher RPM, allowing more power output and hence top speed.
Yes, stock CDI limit the power. not every engine of the same model would provide the same horse power.
The CDI's now have micro controllers which see how fast you are accelerating , and your top speed. and would stop giving out ignition signal to the spark plug. thereby a rev limited CDI.
If you go for after market no revn limit CDI. you have to be a bit careful not to damage the engine. as the rev's do not have a limit and the engine would rev to its physical limit.
if you do not have proper air/oil/water cooler for the engine. the heat generated would damage the engine and the components.
with these precautions in mind you can go ahead with your racing CDI : thumbs up :
Im working on TCI ( TRANSISTOR (IGBT) CONTROLLED IGNITION) my bike stock tci goes max to 12000 rpm. i decided to increase the speed so i looked at racing cdi. People says it dosent lock the rpm after ceartain recommended level. But there is one more thing which take place to limit the rpm and that is Ignition coil. In some days I made my own transistor Ignition system. And i used my dc power supply to feed that system at 14volt. And my rpm increase by 1500. I was happy so i fit this up in my bike and removed the stock tci. I gave my made igntion system supply through the bike battery but when i gone for test the result were exact same to stock TCI. I was unhappy after result because it was working fine. So i realized that i giving that circuit 14v supply through power supply at my home but when i gone for test ride so that time my bateery was providing appropriate 12v. Then i got that the ignition voltage play an important role to increase the rpm. In my case 12 volt supply was enough to reach at 12000 rpm but when i gave them 14 voltage it increase to 13500rpm.