CNS performance carb?

Discussion in '2-Stroke Engines' started by nashy10, Mar 21, 2014.

  1. nashy10

    nashy10 Member

    Guess i shouldve done some more research on this carb before i bought it. i bought it because i thought it had a idle screw and an air mixture screw (separate) i guess i was wrong. anyways, i noticed it started on the first turn over literally every time and provided a bit better acceleration, but it wont even go past 30kmh? i took it apart to see if i could richen/lean the mixture but there seems to be no way to tune this carb for it to work right/fast... what the hell? did i make a big mistake buying this or is there a way to make this worth the purchase? put stock carb back on and huge difference in top speed and higher rpms run better... getting almost 65kmh easily with the stock carb.. whats going on here? could i at least use the jett from the cns to enhance the stock carb? only reason i bought this carb was to get a bit better torque and power.

  2. battery

    battery Member

    what is the intake diameter at WOT? also what size is the jet? how does the plug look? does it Bog when you twirk the throttle?
  3. HeadSmess

    HeadSmess Well-Known Member

    same old same old. throw on another part without doing any research.

    yes, you need to do more research. its called "jetting"

    the carbs ARE adjustable, its just via a drill-bit and often some solder, not by twiddling a screw ;)

    with some luck the jet from the stock carb might just fit the new carb and be the right size, too! iunno. if its M5 it will work. dont waste my time with these "performance carbs" seeing as a straight venturi with simple needle valve would suit my riding...WOT all the way. who needs smooth transition and throttle response on a pansy lil engine such as these things? its not like they will pop wheelies and flick you off into the weeds...
  4. mrfubs

    mrfubs New Member

    the cns is a awesome carb, mine is tuned almost perfect i had to go with a lot bigger main jet and a bigger idle jet. my engine is built to the tilt 3 transfer port added, ramped and windowed piston, balanced my crank, exhaust and intake port work and case to cylinder port match. i also fit a yz80 dirtbike expansion chamber on it so it needs a lot more fuel.

    When i first put the carb on with out doing anything it would basically only idle and would bog with any throttle input telling me it needed some jetting. learn how to read your spark plug, once you get the jetting close you can read the plug to fine tune it to perfection.

    Your cns carb does have a air/fuel screw and a idle screw, the air fuel screw is hidden under a brass cap that you need to pry off. at the end of the day the CNS is a way more tuneable carb than the stock and will perform better, there is one vent tube you need to block off aswell.

    Post a pic of the carb of both sides so i can tell what generation it is.
  5. nashy10

    nashy10 Member

    photo 2.jpg photo 1.jpg

    This is it with its air filter off. a little dirty, sorry for the absence just got enough money to get my bike some new parts.
  6. mrfubs

    mrfubs New Member

    Okay on the last pic u posted the picture of the side of the carb where the idle screw is. Too the left of that is your air fuel screw its under that brass cap so find a way to pry it off it is a thin brass cap. Also those 2 vents on the carb u have 2 hoses on, one needs to be blocked off i dont remember which one ill look when im home. If it bogs you need a bigger main jet
  7. badmoon

    badmoon Member

    I'm with a #68 main jet now it's too lean i've buy #70,#72,#74,#76 main jet from Sickbikepart i'll try those .

    If possible I want to know which vent hose need to be blocked mine is all open and it run well .

    With this carb i'm not limited in RPM because more flow come from so i've gain some rpm using it instead of NT carb less four stroking too .

    I'm using the NT needle because the CNS v3 as only one place to put the e-clip.

    Next week i'll received my new jet and I will comment on that here I think it will be better.

    THE CNS stock MAIN JET is just too small too use cause WOT to bog and stop the engine because it's too lean.

    I want to know where to find a good IDLE jet for the CNS too .
  8. Fabian

    Fabian Well-Known Member

    There is nothing wrong with the standard NT carburettor. It just needs to be jetted correctly for your environmental conditions.

    In my opinion the Walbro style (diaphragm) carburettor is the best option because it allows you to adjust the air/fuel mixture with external jetting screws. A secondary benefit is that a diaphragm carburettor does not suffer from air leaks of fuel leaks. They just work!

    The premium option is to combine a diaphragm carburettor with a reed valve intake system; getting more and less and none of all of the things you want: more low and midrange rpm performance and less intake noise, and without any fuel and/or air leaks.
  9. jaguar

    jaguar Well-Known Member

    yeah but doesn't the Walbro need a hose running to a nipple you have to install on the cylinder to access the crank pressure/vacuum?
    If so then I don't consider it a good option.
    Get a Mikuni or Dellorto carb
  10. Fabian

    Fabian Well-Known Member

    Jaguar, you know very well that a diaphragm carburettor needs a crankcase pulse signal, and that plumbing a signal line from the crankcase to the carburettor is a fairly simple task.

    Why would you waste your time with a float style carburettor when a much better option is available, especially if you are in the process of installing an aftermarket carburettor.

    Next to fuel injection, the diaphragm method is the next best option.
  11. jaguar

    jaguar Well-Known Member

    "simple" maybe to home fabricators but not to the common buyer.
    Unless all the extra hardware needed comes with the Walbro with a detailed instructions for installation then it isn't worth recommending to the common Joe in my opinion.
  12. Fabian

    Fabian Well-Known Member

    Jaguar, think about what you are saying.

    You are well aware of the troubles newbies are presented with when motorising their bikes; the forum being packed to the rafters with newbies asking the most basic questions which have solutions both more simple and more complex than plumbing a crankcase pulse return line.

    If a newbie is incapable of plumbing a crankcase pulse line, then they shouldn't even think about motorising their bike.
  13. johnny skidemarks

    johnny skidemarks New Member


    Id like a little more info on were the best place to drill and tap a hole for vacuum/pulse i think a walbro maybe the way togo. Can you get walbro that will bolt to gt5 jug
  14. Fabian

    Fabian Well-Known Member

    Rock Solid Engines sells all the parts you need to convert the intake system to both a reed valve and/or diaphragm setup.

    This is my setup tutorial:

    If you do install a diaphragm type carburettor you "MUST" order the thermal isolation spacer.
    Make absolutely sure that you order the thermal isolation spacer. I cannot emphasise this enough.

    As an alternative to drilling out a 4mm capscrew, you can use a rear derailleur cable adjuster screw, which happens to be 4mm and already has the centre drilled out.
  15. johnny skidemarks

    johnny skidemarks New Member

    Do you really need the reed valve ,cant you just find a walbro carb with the i believe is 31mm bolt spacing and bolt it direct to the jug and tap for vacuum. Iam look for best hp for best buck spent. I have a hp cns carb that i just bought i think with the right tune up it should work ok for the money.For walbro kit and its a nice kit its a bit pricey .thank you
  16. Fabian

    Fabian Well-Known Member

    It doesn't matter where you get the crankcase pulse signal from, so long as you get a crankcase signal to the diaphragm carburettor.

    From a technical perspective, you do not need to install a reed valve intake, but, from a practical perspective, the reed valve system noticeably boosts low and midrange torque, whilst at the same time it reduces intake noise.
    For me, the reduced intake noise is more desirable than the boost in low and midrange torque.

    I have tried just about all of the carburettors on the market, and have always reverted back to the standard NT (with regards to a float style) carburettor.
    In my opinion, there is nothing wrong with the NT carburettor when jetted correctly, it performs equally as well as any other carburettor that's correctly jetted, and that includes a diaphragm carburettor.

    If you want to save money, just make sure that the NT carburettor is correctly jetted and you will have equal power potential as to any other carburettor of comparable venturi size.
    My bike is currently running with an NT carburettor because i've been lazy to change it over to the diaphragm carburettor; after using the NT as a last ditch effort in trying to solve an electrical issue, of which the problem didn't turn out to be electrical.

    Considering the engine in my bike uses the older style caged needle roller big end connecting rod bearing, i was expecting it to fail before 3,000 kilometers, but with the Jaguar CDI, it just keeps on running perfectly, so i'm at the point where i'll be switching back to the diaphragm carburettor.
  17. johnny skidemarks

    johnny skidemarks New Member

    Thank you fabian for all your help. Iam going to play with this hp cns carb ,but how in the world do the these guys get almost 10 horse out of these little motors
  18. Fabian

    Fabian Well-Known Member

    Power is dependent on money and is also inversely proportional to engine longevity.

    If you want 10 horsepower, you'd be foolish to invest large sums of money into a Chinese bicycle engine when much better options are available straight off the shelf.
    My opinion is that 9.4 horsepower being pushed through a bicycle frame is simply inviting disaster and very likely to see the rider end up in the hospital emergency ward in a fairly short time; with life threatening injuries, but, if you really want to put your body on the line on an urban roadway, the info is below:

    Last edited by a moderator: Dec 15, 2015
  19. johnny skidemarks

    johnny skidemarks New Member

    Just want to thank fabian for your help and link thanks again
  20. HeadSmess

    HeadSmess Well-Known Member

    not all walbros NEED a crank pulse.

    hell, i just ripped one off a stihl lawn blower, drilled a hole in the top as i was going to attach a line there, decided i would just bolt it onto my lil huasheng 25cc fourstroke with no pulse at all as an experiment...FOUR STROKE, i repeat FOUR STROKE, and, well..its sucking fuel up 200 mm from a coke bottle, running quite happily with the "pulse" side of the diaphgram open to atmosphere. makes no difference if i cover it with my finger or not :)
    yes, the carb spacer block has no pulse orifice. its a 4 stroke. and i epoxied the orifice in the carb itself, because, as mentioned, i was going to attach an external line, and two, the spacer didnt happen to cover the orifice. if i had known it would make no difference i would have left it!

    oh, and theres the bing diaphgram carb from my old fichtel sachs km48 rotary... that has no pulse as the fuel pump is separate and locates elsewhere...

    on every other aspect i agree wholheartedly...the NT does the job, easily, simply , and properly. anything else is just having yourself on. better atomisation? the crank does that better than any carb can. better mixture control over the throttle range? erm... dont know about everyone else but my carb has basically two settings.... idling and WOT. its not like i have to nurse the throttle in case it rears up and spits me off at 200km/h+ down the freeway...