I have a Vortex 125CC ROK GP on an OTK 401R that will only go up to about 13,600 RPM. 11/72 sprocket, 108 chain, 145 main jet, needle notch at the 2 position. I’ve cleaned the carburetor and changed the plug numerous times with no luck. We have three other OTK karts with the same ROK GP engines and setup witch get up to full RPM and speed no problem. What am I overlooking?
How does your minimum RPM compare to the other karts? ie is your driving comparable to the others?
Could it be time for an engine service? Cracked reeds or low compression would cause low performance.
Blocked exhaust is another thing that can catch people out.
Sliding axle or binding brakes will sap power, but generally you’ll notice your brakes are pretty hot when that happens too.
Thanks for the response. The kart/engine is new with about three hours on it, our group are all fairly close driver skill wise.
It seems to pull strong and be equal to the other karts, accelerates well up to 13600 and then just hangs there.
If exhaust blockage could it be the power valve possibly?
Nothing obvious with brakes or any rubbing/dragging.
Check the power valve operation.
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Same model of coil ?
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If there is no evident issue like reeds, power valve, spark plug etc, it is very likely that you are running too rich. Swap the carb with one of the other karts that are running well and if you see the engine going back to normal, you can then focus on diagnosing what’s wrong with your carb (float level, then check the main jet with the proper tools, even if it says 145, it can be much bigger in reality. Like the other guys may be running a 148 real and you may be at 155 real). If it still runs poorly with one of the other carbs, then you know you need to check ignition, followed by mechanical parts
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These issues described above are very likely down to a mix-up that happened somewhere along the way at the Rok factory involving the power valve spring. Some engine’s spring is to heavily sprung and causes the poor performance.
I had heard about this but became more keenly aware of it after ordering a spare spring, and to my surprise (dismay) it was a very different spring than what it was supposed to replace. The video below has all the details and images you need to see, but I can 100% confirm that this is a real issue, as I now own both the springs you see in that video. Not only does he show the measurements of both springs, but he also dyno tests them both and the heavier one essentially breaks the engine performance-wise. I don’t know how there isn’t more official notice given about this on ROK’s own sites at least, but in any case that’s the deal, and the wrong spring might still get shipped to you if you order a replacement part.
After my initial post I’ve been going back and forth with the kart shop I received the deprecated spring from in order to get the right one, finally - and they’re trying to make it right, but now it is quite clear that ROK has never updated the part number, which makes anyone’s attempt at getting the correct spring much more confusing. All they see is that they have a spring, it has the correct part number listed. Hard to argue with, but they’ve also been open to figuring it out. Long story short, I contacted Swedetech, where my engines were built, and this is as an official response as I think anyone is going to get for now: (and should help you be sure of whether you have the correct spring or not)
My email:
Hi, I have two Swedetech Rok GP engines, I am hoping that someone on your staff can shed light onto some controversy over what the correct power valve spring for this engine is (unfortunately I cannot find any official information about it online). The issue is this:
The power valve spring which came in both your engines is different (thinner gauge, slightly shorter) from what came when I ordered a replacement spring, but it appears to be exactly the same “replacement” spring mentioned in this video by Power Republic. I cannot find any official word from Rok about this “updated” spring and the part number for it has never varied anywhere I have ever seen (meaning there doesn’t seem to be seperate part number for separate springs). Yet, there are two different springs out there and the one in my engines seems to be this “updated” one. If anyone could help shed light on this situation, it would be greatly appreciated.
Their response:
Yes, at some point a 13mm longer spring was available, but it’s no longer legal. The correct length spring is roughly 41mm long and 0.80mm gauge.
Hope that helps
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