While we have a thread going on the cdi, I thought I would share a little info about them. There used to be a lot of discussion and debate about there being a rev limit incorporated into the cdi. Most were tested at one time and it was found that they did not have a limiter built in but rather depending on what you had, the cdi simply went flat at a certain point. What does make the difference is the timing advance built into the cdi. Some seemed to be better than others, [longer advance curve] Those that advertise "racing" cdi's are just selling a cdi that has a slightly longer timing advance curve that work above the normal rpm range.
99% of the time, a stock cdi will work just fine. Most stock cdi's will go flat around the 8000 rpm range. That is ok cause a basic stock motor develops max power in the 5000 to 6000 rpm range. A stock gy6 won't rev too much above 8000 rpm anyway because of air/fuel and valve restrictions.
Which one to get? That is a good question because there is no way to tell one from the other without trying them.
I personally would use one of two different makes. My first choice is the oringinal cdi found in the stock yerf from '05 till the company folded. They are getting hard to find. These cdi's have a advance curve work well into the 9000 plus range.They are tough and I have never had one fail. I have a couple that are 5 years old and still work strong. With my personal mods, my motors will turn over 10,000 rpms. I once bench run one of my motors to purposely blow the motor. I wanted to know what would fail first. I mounted a motor on a stand and run it it up till there was no more to go and held it there. I ducked behind a large tool box with a remote throttle control in case the motor exploded!
The motor went up into the 11,000 range and the cdi never cut out! The valve train started to fail at 10,000 plus. The valves started floating right at 11'000 which caused the motor to peak out but nothing came apart! After the test, I popped the motor in a buggy and still use it. A good testimony to the strength of these motors. I use the old stock yerf cdi in everything I got, a couple of Dazons and a couple of Yerfs.
My second choice would be a late model, '08 and up hammerhead stock SYM motor cdi. They seem to look and work identical to the early yerf versions. .