Throttle Response Question CR125

Good morning. I recently purchased a CRG Road Rebel with a CR125. ran great when I got it but given the unnown hours on the top end I opted to put a fresh top end on the motor and the throttle response on the top end is excellent, though I have a question regarding the low to mid range response. It seems like a dog on the bottom end. Kart will not launch from a dig and doesn’t seem to wake up until 6-7K rpm. The CR125 engine is a 2004 model with the OEM Keihin carb that has the TPS sensor. I’ve broken in the new top end and noticed the plug is reading very clean so the motor is not running rich or lean. The engine has the plugs for the power valve which is not connected and I was curious if the lack of responsiveness on the low and mid range RPM is expected with the power valve not operational or if the carb is the likely culprit and I can see much better responsiveness on the lower end of the RPM range even with the power valve not running if I address the carb. Appreciate your insights as I don’t want to chase the problem with parts if the power valve config is the reason for the lack of low end.



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That’s pretty normal, especially with a kart specific pipe. (assuming you’re running one?)

Nothing really happens until 8500 in the karting application. Gotta keep it spinning. Don’t be afraid of overrevving it on upshifts, you’ll find the engine will just stop accelerating. Make a bit of that RPM (It can change with jetting) and shift before that speed, you don’t want to make it a goal to hit that RPM wall, just find it and work your way back.

Thanks for the insight and quick response! Very good to know before I chase low/mid end messing with the carb when there are no issues. Yes I’m running an RLV R4H pipe.

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There is something to be said for having a narrower carburetor bore to keep intake velocity up at lower engine speeds (Bernoulli principle). The Keihin is a flat slide 34 mm bore, as opposed to the de rigueur round slide 30 mm Del Orto, differentiating mid-high (advantage former) versus low-mid (advantage latter) RPM throttle response. At least, that’s the theory. I’ll only claim that from my limited experience with the CR, those differences seemed to hold true. YMMV.


John Denman advocated for the VHSH30 at the outset of Stock Moto I believe and did a lot of testing with it, but the 34 was kept due to (perceived) cost.

Yeah, I recall reading a discussion about that on ekarting some years back. The entire moto kit wound up being 100% Honda CR125 in its origin. Also worth mentioning that the Del Orto was designed for use with a pulse pump, & avoids overflow issues experienced with the gravity feed designed Keihin, but it is what it is, & tuners got it to work. Honestly, that brand name recognition probably attracted more people into the sport, which was a good thing.

Now it’s in the hands of the kart engine makers, & in the spirit of the moto package, I hope TaG-KZ & SSE motors can sort of carry the reliability torch for KZ karting.