I have heard it said that with 48cc motors people need to lean up their carbs, I know for a fact that with my 66cc motor running a CNS carb was way too lean. I had to drill out my jet a bit and then she came to life....
Asking around I found the consensus is that the stock CNS main jet opening should be the size of a #70 drill bit, mine was smaller, I could only fit my #72 drill bit through it. Before I drilled it out she ran good up until around 75% throttle, above that she bogged bad. I was able to determine the condition above 75% throttle was lean by riding the bike into the throttle range where it bogged, while bogging I opened the "choke" enrichment jet and the bogging went away. Since the extra fuel from the enrichment jet fixed the bogging I knew it needed more fuel, so I drilled.
I first drilled from the original #72 opening size down to a #69 (by mistake, I was aiming for #70 but mixed up the drill bits) When I put the carb back on the bike the bogging was gone above 75% throttle and she ran great over all. I am very happy with my CNS carb now. Since then I have opened her up to #66 drill size with not much improvement, I needed to increase the air flow at this point I think to see any more improvement but winter was setting in and I didn't get a chance.
A couple of observations about the CNS carb...
First of all, lots of people think the white spacer/bushing is the seal, it is not. The seal is that black rubber grommet that is inside the carb neck, the white spacer just holds the carb in place. You MUST seat the intake on the BLACK GROMMET properly or you will get air leaks. I push the carb down the intake until it bottoms out, then kinda wiggle it a bit and push it on that last little bit so it seats into the black grommet and then tighten it down. I have never had to use any sealer on my carb to prevent air leaks when I install the carb in this manner.
Secondly, the "choke" on this carb is an extra enrichment fuel jet, not an air restricting choke. This jet is extremely small and clogs easily, if this jet clogs you will have poor idle and low throttle range performance. My bike rode rough at low speeds until I reamed out the crap from the enrichment jet, after that my low end smoothed out nicely.
Finally, I find the float on the CNS carb has a strong influence on how it functions. You may need to adjust the float setting a bit to get your carb performance where you need it, specially if your carb is not mounted level.
This all may seem like a lot of work, but the carb is really simple and not hard to work on at all. When you remove it from the bike take the two screws off the top and the cables pull right out, now the carb comes off easily without being attached to any cables. There are 4 screws that hold the bowl on, inside there aren't many parts, the only parts that come out are the float, its pivot pin and the needle that is underneath it. Thats it. I fiddled with the float adjustment a couple times before I got it right, and took a very thin wire and pushed it through the enrichment jet to clean it, which I do every time I open her up now. The hole is so tiny I had a hard time getting something that would fit through it, I ended up stripping the paper of a wire from a garbage bag tie and used that bare wire. The first garbage bag tie I tried was from a quality bag, the tie was plastic covered and the wire was too large to fit (believe it or not). The next one was from a generic garbage bag and it was a paper covered wire which was thinner and fit through the enrichment jet perfectly. I will state it again, cleaning out this enrichment jet is very important for good idle and low end smoothness on the CNS carb.
If I read the message properly, you purchased a CNS carb as an upgrade and quickly gave up on it and went back to the NT or Speed carb. Most people seem to be doing the opposite, they have a CNS and buy an NT or Speed to replace it. At any rate, I would like to encourage you to work with the CNS carb a bit when you have time, keeping in mind what I have written above. A little playing with the float and getting those jets properly sized and cleaned should get her running well, I think properly tuned you might be more satisfied with the performance of the CNS carb than your stock carb....
