Gwcrim,
I don't know "why", as I am a " put it in and see" kind of guy but with all the mods I've done on my CN250, I found that although the redneck intake or CDI and coil upgrade will cause the idle to go up a bit, a 1/4 turn on the idle screw fixes that. Its the lack of fuel at half throttle and up that causes bad performance. Although I initially went with a #40 pilot jet from the stock #35 when I put on my high flow exhaust, I didn't see any noticable changes in the low speed performance. I have since changed carbs (Still a CVK) and am back to the #35 pilot and a 135 main.
Anyways, I can't explain why but its just the way it is.
I'm sure someone here can explain it in a way that would confuse the heck out of me!

I'm wondering if it has anything to do with the CVT vs a regular transmission.

Typically the size of the main jet in these carbs only affects WOT operation. Partial throttle mixture is controlled by the needle. The closer to WOT you get, the more the main jet size matters. Seems like these buggies run with more throttle opening than a bike with a standard trans. The pilot jet handles the transition from the idle circuit to the main circuit. That's at low throttle, like 1/8 to 1/4 maybe.

I'll say one thing.... even 100% stock the jetting is off. It spits and sputters way too much.

Crim

I'd say most people ride at WOT or close to it.

Just thinking about jetting basics.... the way I've checked for a lean main jet is to find a nice flat area, go to WOT and back off a tad. If you pick up speed, the main is too lean.

I'll have to give that a try.

Crim

i've done my share of screwing around with my jets
if you have the stock CV carb, it comes with a 110 jet. like the guys said, if you are doing the uni-filter mod then you will want at least a 130 jet to start. i'm in canton and i run a 145 jet in my CV carb. the plug is a light brown color and it runs fine. if you need any jets, let me know. i have a 110,125,127.5,130,132 & 135 that i've used for tuning mine. i never had to mess around with the pilot jet. i can throw a jet in UPS for you and you should have it the next day. i got some "test" jets from mike at buggypartsnw so i'll pass on the favor.
let me know.