Re: so i need your opinion please
I agree about teh NT and speed carbs, with the right jet and needle settings, there's not really a need to go bigger even after a few good mods.
Personally I don't like the Speed carb, but it's not the actual carb I hate, it's that fuel shutoff valve that was giving me all kinds of trouble shedding off rubber particles and blocking off the main jet or keeping the needle and seat valve from closing so the engine would either starve at the worst time or it would flood. I finally gutted out the fuel valve and sealed it off, problem solved.
The NT carb just has a fuel inlet so it don't have this problem and will perform just fine even after a few good engine mods.
Right now I'm running a CNS on mine and I'm really liking it, the CNS doesn't come without it's problems tho... First and foremost, the CNS won't just clamp to the stock manifold, the plastic shim leaks like crazy, and it comes jetted too rich.
When I installed the CNS on mine, I took out the plastic spacer shim and threw it in the nearest trash can and used a 3/4" long section of 3/4" ID silicone hose to take up the space, then used RTV on the outside of the clamp and stuffed it into every void and opening on the clamp, then installed it to the manifold and let it cure overnight.
After it cured I installed the manifold onto the engine with the carb preinstalled on the manifold, connected the fuel line, turned the fuel on and checked for leaks, no leaks, gave the starter a quick pull and it fired right up. It didn't stay runnning because the choke (or enrichner in this case) wasn't needed and the idle was set too low, but with choke off and idle adjusted up it fired and ran.
What I really like about this carb is that it has an idle mixture adjustment and once set, the engine idles smoother than ever and I can idle it under 1000 rpm, but that was just to see if it would, I set my idle at 1300 to 1400 rpm for regular use.
Throttle response is nice,even set too rich so when I swap out the jet later on today it should be right where I want it to be and I'm thinking it should do good with either a #65 or 67.5 main jet.
The other good plus about the CNS is it's a Mikuni clone and was used on the Yamaha PW80 and a few other 80cc 2 stroke yammys so extra parts are easy to get for it as well as you can upgrade it later on with a Mikuni rebuild kit. IT also has a power jet port already installed but this won't work just by connecting the overflow drain line to the port, you still need to regulate the fuel going in by making a restrictor (I'd start around .020 to .030" for restrictor size and adjust from there) and an air bleed to emulsify the fuel or it'll just flood the carb. I'm not using the power jet option yet but after break in I might tinker with it and get it working if I can't get the midrange to tune with the idle and top end tuned right. Power jets are really nice for compensating for altitude and temperature, and wher I live and ride, I really don't have those extremes so it most likely won't be worth messing with, but it's there if needed.
My final opinion on the CNS carb is that it's a really good carb when you have bigger mods like more advanced porting, higher compression, and a good pipe. With it's easy tunability one could dial in just about any setup.
For bikes that are close to stock or even with some more serious mods, the NT will do just fine and it's easy to set up, just get the jetting properly and the right needle setting and you're set... real easy and low maintainance as well as long term reliability.
The Speed carb, I wouldn't run out and buy one, but if the bike came with one it's a good carb, I'd recommend getting rid of the built in fuel valve and using a regular banjo fitting there, just shut of the fuel at the tank petcock. The Speed carb is another "bolt on and go" type carb that's very low maintainance and easy to tune by changing the main jet and setting the needle, once set there's nothing left to do but ride.
NT or Speed for stock to moderately modified engines CNS for those who like to fine tune everything, and no real need to go bigger than 15mm unless you're looking for serious power with reed valves or other major mods. For most people, the NT will be the best choice.