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Discussion Starter #1
Ok, so I've read up a but about how people are fitting smaller 2 stroke cdis on larger bikes (mainly cr500 taking cr250 and 125 cdis, and yz250 taking yz125 cdis). Can anyone tell me the difference between the curves? Why would you get more power from a 125 unit, and why wouldn't a factory use these curves from stock?

Thanks!!
 

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Discussion Starter #2 (Edited)
The reason I ask is because I've put an rm125 cdi on my 300. Low down the power is insane, but up top it coughs and splutters where it normally ran clear on the original cdi. I have tried 2 125 cdis and they both act exactly the same, this says to me the timing is the only variable here, brand new stator fitted too. Is it possible there is too much advance or retard making it act like it's running rich? Should I try leaning out the main jet?

There is a lot of evidence of people putting 125 and 250 cdis on cr500s. And the 125 cdi on the 250. I can't understand why I should run into problems?
 

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I maybe miss reading your post.
If it runs fine with the stock CDI, run the stock CDI.

When you swap CDI for extra HP, you really need a dyno. Your only looking at 1 ~ 2 % gain in some spots. That you likely wont feel.
Plus, thats only if the swap works. Most swaps make less HP. The factory is not in the business of in stalling the 2nd or 3rd best CDI they own. All of them make 125 CDI's but the made a CDI just for that bike. Why?

Before you swap anything, you should know what it's going to do. So what was the old timing curve, whats the new timing curve? What effect was it going to have at what rpm?
 

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Discussion Starter #4
300's it seems tend to be downtuned for more mild power delivery in enduro. The original cdi failed due to a bad stator, I replaced it, and 2 years later the stator got wet and died again, taking out the cdi. When diagnosing the fault the first time round I fitted the rm125 cdi, and to be honest the power is brutal, really takes off at low revs.

When comparing cr125 and 250 cdis online you can see the curve starts with a massive increase in advance on the 125, but around 7000-9000rpm there is only about 2 degrees extra advance, I've adjusted this out on the stator plate but it still breaks up at high rpm. And testing 2 cdis getting the same results it can't be anything electrical as I've changed the stator again and the ht coil.

Ordered a 300 cdi from the states, but I love the 125 power delivery, also ordered new reeds, only other thing I can imagine causing these problems... But if retarding the timing has no significant impact.. Could I have 2 shot cdis?
 

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Its hard for me to follow what going on and what part or what point its giving you a problem.
It sounds like the bike had a problem with all stock parts. Now your looking to do two things at the same time, fix the stock problem, and go faster (fab CDI)? THats hard to do if your not 100% sure what the first problem is.

It only misses with the 125 CDI?
Can you get it to run clean with a stock CDI?

You say "years later" you fitted it with the 125 CDI and liked it. So you did have it running good years ago with 125 CDI?

You may still have a bad stator.
 

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Many cdI's stop retarding the ignition timing past a certain RPM which is set to match the engine so it's not advised to put on a CDI from a different sized bike. So if you put a 125 CDI on a 250 you may wind out the engine too far and damage the piston since now the CDI allows the engine to rev out farther.
 
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