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Discussion Starter · #3 ·
Got the motor built and it runs great but seems to run out of breathe on top end. I have a 125cc big bore kit in it. T4 exhaust. Open air box. Ported head. 2k clutch springs. Quad runs great out of the hole but quits pulling after 30mph. I have tried lots of different jets in carb. Seem to have it dialed in pretty well. I have been trying different spring and roller weight combos to get more top end. I have 5 gram to 13 gram weights and 1000. 1500 and 2000 rpm contra springs. Race clutch bell. Just doesn’t seem to pull anymore after 30mph. It gets there quick and runs great but that’s it. I even tried a bigger front sprocket to get more top end but still only 30mph. Any ideas? Maybe a no rev limit cdi? I tried one already and quad wouldn’t even run.
 

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Got the motor built and it runs great but seems to run out of breathe on top end. I have a 125cc big bore kit in it. T4 exhaust. Open air box. Ported head. 2k clutch springs. Quad runs great out of the hole but quits pulling after 30mph. I have tried lots of different jets in carb. Seem to have it dialed in pretty well. I have been trying different spring and roller weight combos to get more top end. I have 5 gram to 13 gram weights and 1000. 1500 and 2000 rpm contra springs. Race clutch bell. Just doesn’t seem to pull anymore after 30mph. It gets there quick and runs great but that’s it. I even tried a bigger front sprocket to get more top end but still only 30mph. Any ideas? Maybe a no rev limit cdi? I tried one already and quad wouldn’t even run.
30 MPH is great for a KFX 90, The KFX 90 top speed is only 15 MPH due to the automatic transmission limiting collar...Not sure? if the Kymco 90 is the same as the KFX 90...

The spec's on the Kymco 90 has only 7hp max motor & CVT transmission not sure if you can get any more speed from it. 2014 Kymco Mongoose 90 @ Top Speed
 

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Discussion Starter · #5 ·
Thanks for the reply. Yes the Kfx90 and kymco 90 are the same machine. Kymco makes the Kfx90 and the arctic cat 90. So far top speed has been around 33mph. We do have the limiting collar out of clutch. Just seems like it should have more on top since we added so much more power. My son races this quad in a woods racing series against built TRX 90s. He does very well in the woods but in the open fields they pull away. Just trying to lessen that gap. I am still experimenting with springs and weights in the clutch. And trying to find a no rev limit cdi.
 

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Thanks for the reply. Yes the Kfx90 and kymco 90 are the same machine. Kymco makes the Kfx90 and the arctic cat 90. So far top speed has been around 33mph. We do have the limiting collar out of clutch. Just seems like it should have more on top since we added so much more power. My son races this quad in a woods racing series against built TRX 90s. He does very well in the woods but in the open fields they pull away. Just trying to lessen that gap. I am still experimenting with springs and weights in the clutch. And trying to find a no rev limit cdi.
Here's a thread that might help you... KFX 90 performance.

I also found that you need to know if your CDI is either AC or DC..?
How to tell my style of CDI? (AC or DC)
Your CDI has two plugs, a 4-pin and a 2-pin. The 4-pin side may have 3 or 4 wires connected to it (both AC and DC versions). If the 2-pin connector has only 1 wire going to it, it's a DC CDI. If there are 2 wires attached to the 2-pin connector, then it's an AC CDI.

Note: A very few DC-powered CDI systems will have two wires on the two-pin connector. For these systems, please check to see if your headlights will work while the engine is not running, and if your stock CDI has "DC" printed anywhere on it. In these cases, you need a DC CDI.


And have you tried the unrestricted CDI..?
 

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Discussion Starter · #7 ·
Yeah I ordered a cdi and it wouldn’t even run with it so I went back to stock. I read a bunch of dhall79 posts. I haven’t been able to actually get a hold of him to ask questions directly. I have read the thread you sent. A lot of good info. I am using pretty much same clutch components but he was running 5 gram and I believe I have 9 gram in it now. Gotta do some experimenting with weights and see if it helps.
 

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Got the motor built and it runs great but seems to run out of breathe on top end. I have a 125cc big bore kit in it. T4 exhaust. Open air box. Ported head. 2k clutch springs. Quad runs great out of the hole but quits pulling after 30mph. I have tried lots of different jets in carb. Seem to have it dialed in pretty well. I have been trying different spring and roller weight combos to get more top end. I have 5 gram to 13 gram weights and 1000. 1500 and 2000 rpm contra springs. Race clutch bell. Just doesn’t seem to pull anymore after 30mph. It gets there quick and runs great but that’s it. I even tried a bigger front sprocket to get more top end but still only 30mph. Any ideas? Maybe a no rev limit cdi? I tried one already and quad wouldn’t even run.
Hey by any chance do you know what jets your running just installed a big bore kit on my 90 to make it a 125
 

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I am running a mikuni vm26 carb now. It’s a 150 main, 15 pilot.
Ahhh fellow kfx90 tuners💪.I have a kfx90 with 125 big bore,24mm Nibbi carb jetted @105,42, mid clip,2 1/2 turns out,uni pod filter,300 ex front suspension(diamond j customs makes one as well),qazacky unlimited cdi.camshaft is on the way and is currently in the shop getting port matched.what I would really like to find is a stroker crank.i know these are based off a gy6 so I'm actually quite curious if I could have went bigger piston with a little case boring.there is little to no info on these machines so I am happy to do my part and add what I can.of anybody knows which nerf bars or a stroker crank can fit I would be indebted🤙
 
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