CR125 Non reving past 11.5k rpm - Please help

[Newtyp]

Hello everyone.

Currently I am running both a 1999 mod and stock CR125 moto in kart racing. My mod moto is fine and operates perfectly with all the works however the stock unit I have which I just rebuilt (top, bottom, reeds, removed and plugged power valves/we do not use in karts and jetted perfectly from my earlier notes) does not rev past 11.5 k or so. It just stops pulling. After plug readings, to want more fuel (looks leaner on the top-end), but I am nearing ranges that are settings for my mod moto, which makes close to 40 hp compared to stock.

It definitely loses peak power which I know before modifying the current mod motor I have never had a problem bringing the rev's up to 12.5k with a 12.8k over rev, with the exact same parts. Any help or recommendations would be greatly appreciated as I have tried timing changes, carb changes, but used same pipe and configurations from a working setup.

Could it really be a stator or ignition system. I changed box from a vertex to stock and its the same thing.

Very perplexing…. :bang: :bang: :bang:

Thanks everyone for any advice or recommendation as I spent all Saturday testing at my home track with no resolution.

 

[motometal]

timing? float level? could it be something with the way you blocked off the ex. valve? An effectively lower exhaust port window could do this...

 

[Newtyp]

Appreciate the input.

The float would not be an issue since we run something called a pump around fuel system eliminating the floats. Timing would and could be an issue, though it was set to stock. I will have to check that again. Lower exhaust port timing would be a factor but the motor is stock with stock cylinder. Electronics are the only thing left I guess...

Any other ideas? Thanks again !

 

[loudandproud]

you didnt change your gas did you. Just the wrong brand of gas will make my 250 rev funny or not reach max rpms.

 

[Newtyp]

Hello,

Thanks for the input. Gas has been consistent I am running Sunoco Purple leaded 110. Have had no problems in the past with a oil mix of 28:1.

 

[motometal]

I wasn't sure by your post if you are using the same carb you used before?

I tried putting a crescent slide carb on a new RM85 (instead of the crappy low tech stock round one), and it simply will not rev, tried jetting all over the place even a totally different carb from ****, as soon as I put that stock carb back on it runs like a top. I give up. Some combinations just don't work.