Carby Observations on the CNS Carburator

Discussion in '2-Stroke Engines' started by Sgt. Howard, May 18, 2012.

  1. Sgt. Howard

    Sgt. Howard Member

    ... also known as the 'High Speed' carb- this is the one found on Grubee 'China Trollop' kits, it has a hose breaking vacume between the filter and the throat plus the nylon insert mounting it onto the intake- I've heard mixed reviews on this device and have to admit the four that I have mounted have been a bit of a ... challange... until I've solved three main issues;
    1) that CHEEZY nylon insert/sleeve/bushing- pulled dimentions off the carb, off the intake and off the ... piece of rubbish... and cut one out of thick walled 1" aluminum tubing. If this thread generates interest, I'll post the dimentions for those with a lathe
    2) that stupid rubber tube from the air cleaner space breaking vacume to the venturi... remove the filter, insert a fitting screw into the tube where it inserts into the cleaner and put filter back.
    3) open up the throttle handle- notice the white nylon stop that prevents the handle from allowing a low idle. This is not an accident- with an ultra lean mix (remember the tube and the leaky nylon bushing?) the only way to keep the engine running AT ALL is to shove it up to near throttle wide open... shave off about 1/4" of the white nylon stop and re-assemble... then screw the turn channel under the handle as far into the handle body as it will go. This ought to allow you to drop the idle to something more reasonable...
    The idea behind what you encounter in this carb is simple- running lean produces less smog. It looks good on paper. Unfortunately it also cooks your engine and grinds your bearings. Typical California requirements. Fitting the screw as I have described leaves you with a tube that outwardly looks to be correctly installed but in fact is plugged. The nylon bushing will eventually turn into cottage cheese no matter what sealant you try to use- with the aluminum bushing, there is no leak or wobble. Lowering your idle to what will work with the other two issues solved gives you more throttle control and better engine life. With all three issues dealt with... THIS CARB ROCKS!!!! POWER UP THE KAZOO!!!
    How does this check with other's experiances? :army:

  2. BoltsMissing

    BoltsMissing Active Member

    Similar to what you have stated, went well on the "70"cc but another MB'r tried it on a "48" cc, had complications.

    At high end, Good.
  3. dougsr.874

    dougsr.874 Active Member

    How to repair a CNS carb

    Upon receipt of said item, immediately throw it as far across an open field as possible, then congratulate yourself for knowing how to rid yourself of such a dastardly piece of ----.
    Last edited: May 19, 2012
  4. Sgt. Howard

    Sgt. Howard Member

    Mine are all mounted on 48-49cc engines... since I have done the modifications I have had excellent, reliable performance. Yes, it's a bit of bother to do the modifications... the bushing is a bit of an issue... but the other mods are simple to do.
    A pictorial, perhaps? :army:


    I have been lucky enough to have never seen one of the CNS carbs but it's nice knowing what I need to do when I get one.
    Just one question. Do they have an idle adjustment screw like the old carb?
  6. Sgt. Howard

    Sgt. Howard Member

    They do, but until you shave the stop on the throttle handle you cannot set a reasonable idle... and untill you do the other two tricks, it will not hold a reasonable idle
  7. New Member

    Ok Sgt. i found your thread and i dont have the hose at the air filter to the venturi that you referred to. But i did connect one bowl vent tubes to the other and it made quit a difference. If i'm bothering you just say so but i'm desperatley trying to get this engine running its best. I've got the plug burning a nice light brown now and have good low end but have no power at all at the top higher speeds. It sounds like its fowled when i get in the upper RPM's. I live in salt lake city so the altitude is a bit high about 4800 feet. I've tried jetting both ways and have settled on a 68 and set the needle down one notch(had to make my own notch cause there aren't any on this carb) CNS V3. And broke into the air/fuel mixture screw and its turned 2.5 out from closed. Regapped the spark to .025 both the factory plug(3 prong) and NGK BH5ES. I'm out of ideas have any suggestions for me would be greatly appreciated. Thanks Zuide

  8. Sgt. Howard

    Sgt. Howard Member

    Are you still useing the nylon bushing between the carb and the intake?
  9. Sgt. Howard

    Sgt. Howard Member

    To all- there is apparently more than one CNS carb variation. The tricks I have developed work for the ones as such illustrated

    Attached Files:

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  10. sp454e

    sp454e Member

    h/p cns carb motor running rich elivation 500 ft no adjustments on the needle. will not run above 20 mph. plug gap at .6mm. stock plug.
  11. MaillotJaune

    MaillotJaune New Member

    CNS carb

    In the interest of interest. I am applying this info to my build which is nearing completion. Still carb install and tuning, chain adjustment / truing , cable adjustments - throttle cable too long - clutch , choke etc. I have the newer cns cone carb and plan to follow Sgt. H 's advice. Starting with the plastic insert replacement. I would like to try one of your aluminum replacements if you have the availability.
  12. Sgt. Howard

    Sgt. Howard Member

    Sadly out of stock and my lathe is incomplete- had to disassemble for the last move and have not yet found all the parts. What I HAVE learned is to cut the ring for a snug fit on the manifold, JB weld it there and the carb bolts onto that- works just dandy.