98 cr 125 upgrades

Hey guys, just wondering what your doing to you 125s to get more power. My compressions is a little on the week side so I'm gunna rebuild the top end this winter along with a new clutch cause it's pretty jerky. Thought about getting an FMF gold series fatty with a shorty to go behind it. Also thought about getting a 53 rear sprocket and a 13 for the front. I'm mostly riding trails and stuff like that. I know everyone is going to say sell it and just get a 250 but I've had a 250 and hated it. Almost has to much power for what I like to do. Wondering if there is anything else I could do just to get a little more bottom to mid power out of it. And yes I realize that you can only get so much power out of a 125.



My '98 CR 125 was my last racebike back then...i kept it and I raced it until last year.


I actually mounted a Mugen cylinder on it which adds a lot of power. But these are some suggestions:


-change the stock airboot with the same part of the '98 CR 250 ! The 125 version is simply too restrictive.

-a Boyesen RAD valve or Mototassinari VForce3 reedblock will add overall power

-a 38mm carb will boost high-end at the expense of some low. However If you like to ride it wide open a big carb is a must! I had good results by using a 38 mmKeihin PJ out of older CR 250s.

-If you do use a Keihin PJ try a BGJ needle but it will need a massive 185-190 main/ 58 slow. This needle will awaken your engine biiig time!! Regardless of 36 or 38 PJ carburetor.

-lower the head 0,1mm to raise compression

-53 in my opinion is already too short. I'd take smaller steps and first try a 52t. (i personally went even LONGER than stock.My Mugen cylinder adds so much that i used 13/49 !)

-FMF fatty pipes are pretty good. Pro Circuit revs a little higher but has a tad less mid.However i found italian Messico pipes to outpower both of them! A big boost in the middle of the powerband with great overrev.

Edited by nino

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