OMG I have to tune a CNS!!! Help

Todaleft said:
I tried 2 CNS carbs on a 66cc with no luck. Got a speed carb today and the bike runs frickin' great.

Not going to give up on CNS tho. Once the engine is broken in properly I'll be drilling the jet out and strapping that picky ***** back on there. I honestly think it can be a better carb once it's tuned properly.

One question, what exactly is 4-stroking? And what does it sound/feel like? Cuz one the way up to top speed the sound of the engine changes (of course), just wondering if it's normal-ish.

Thanks for all your guy's help!! This forum saved my ass!....and countless hours.

Wish I had pics cuz my bike looks dope.
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I had a hard time with my CNS carb at first, in the long run I got it tuned up to perform great, I think probably better than the NT or RT carb, though I don't have any of those to compare.
From my experience the float adjustment in the CNS is critical, if the float is going too high before it pinches off the needle valve (gas level in the bowl too high) you will get a lot of four stroking cause lots of gas will be flowing. I had gas coming out everywhere before I did the float adjustment, from the vent, from the muffler, too much in the motor as evidenced by four stroking... After properly adjusting the float all that went away.
Another thing to watch on the CNS carb is the idle or pilot jet. Unlike the NT or RT carb, the CNS has 3 jets. The main jet, the pilot jet and the enrichment jet (choke). The pilot jet is really small, easy to clog, if it is not clear your low end speed will be very rough, you will probably need to screw your idle screw in all the way to get idle (which isnt right). The main jet from my experience might not be sized correctly fromt he factory. Some say the factory size should the that of a #70 drill bit, mine would only fit a #72 bit through (which is smaller). I opened mine to #69 (drill bit) and the bike ran really well with that jet and the stock air filter. I recently bought a CNS rebuild kit, the main jet in this kit did indeed fit a #70 drill bit through it like others had reported. I have heard 48cc riders say the carbs are too rich for their motors, perhaps the jet I got with my kit was meant for a 48cc motor, I dont know....
 
nightcruiser said:
Might be a good theory, but not true in practice. Unlike some of the others, when I pull this tube from the air filter my bike runs like crap, I lose 7-10MPH from my speed. Put the tube back in the air filter and it runs good again, top speed is restored... I have repeated this test several times, same result each time....
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That's interesting. It shouldn't make any difference since it's only a float bowl vent.
 
biknut said:
That's interesting. It shouldn't make any difference since it's only a float bowl vent.
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I know, I had made the same initial assumption as well, until I experienced it for myself. I even repeated the test just to be sure, didn't seem it would make such a difference but it did...

My experience with the vent tube was opposite of what most others were reporting, mine runs better with it connected, others say disconnecting it makes theirs run better (I think those are 48cc motors, I have 66cc).

I have been developing a bit of a theory about that tube. A friend with an NT carb said he noticed his bike doesn't run as well when the fuel tank is low, guess maybe lower pressure from less fuel sitting above the carb. I never notice any difference in how my bike runs with full or empty tank with my CNS carb. Connecting a tube under the air filter would create a vacuum on that tube that increased with the speed of the engine, connected to the carb vent this would decrease the pressure in the bowl and make fuel flow faster as the motor increases speed, due to increased suction on the vent tube. (in my theory)

Working from that theory I am thinking perhaps the different effect of pulling that tube might be explained by different float adjustments/different fuel levels in the carb...

If the float is set a bit on high side, and the bowl always pretty full, the suction on this tube could cause the bowl to overfill and make the bike run rich, perhaps even overflow a bit. In this case disconnecting the tube eliminates the vac in the bowl and lets the bike run on the unassisted flow of fuel. Makes sense with the 48cc bikes, natural gravity flow probably gives them enough fuel to get along fine, specially if the float is set on the high side, so the bike runs better with the tube disconnected.....

If the float/fuel level on the carb is set on the lower side and not holding much fuel in the bowl, the fuel level in the bowl may run low during high demand. Having this tube connected would make fuel flow into the bowl faster when the throttle is opened more to keep the fuel in the bowl at the proper level. In this case having the tube connected would make the bike run better, particularly in the upper throttle regions of a 66cc motor when the float is set on the low side....

Just a theory I have been bouncing around in my head, cause the effects of this tube are really like some kinda carburetor voodoo..
 
A carburetor does not really "squirt" the fuel into the intake air stream. Any "pressure" created by gravitational forces acting upon the fuel as it flows down the fuel line and into the carburetor will not increase the volume of fuel delivered into the intake charge unless your float is malfunctioning and the carburetor is flooding.

A carburetor works on the venturi effect.
Venturi effect - Wikipedia, the free encyclopedia
This is based on Bernoulli's principal.
Bernoulli's principle - Wikipedia, the free encyclopedia

The center portion of the carburetor throat is smaller than the entrance (air cleaner side) and the exit (manifold side). This area is called the venturi.
As the air passes through this narrowed section of the carburetor throat the air speed through it increases and the pressure decreases.
The float bowl of any carburetor must somehow be vented. This allows atmospheric pressure to exist inside the float bowl chamber. Atmospheric pressure is 14.7 PSI at sea level.
As stated above, during operation the ambient pressure decreases in the throat of a carburetor.
When this happens the atmospheric pressure present in the float chamber is higher than the ambient pressure present in the venturi and the fuel is pushed through the fuel jet, jet holder, and into the throat of the carburetor.
If the float chamber vent were plugged up or non existent the pressure differential would be wrong or not existent and the carburetor would malfunction.

A vacuum, even very slight, placed on the vent hose will reduce the pressure inside the float chamber and thusly reduce the volume of fuel delivered into the venturi of the carburetor.
 
GearNut said:
A carburetor does not really "squirt" the fuel into the intake air stream. Any "pressure" created by gravitational forces acting upon the fuel as it flows down the fuel line and into the carburetor will not increase the volume of fuel delivered into the intake charge unless your float is malfunctioning and the carburetor is flooding.

A carburetor works on the venturi effect.
Venturi effect - Wikipedia, the free encyclopedia
This is based on Bernoulli's principal.
Bernoulli's principle - Wikipedia, the free encyclopedia

The center portion of the carburetor throat is smaller than the entrance (air cleaner side) and the exit (manifold side). This area is called the venturi.
As the air passes through this narrowed section of the carburetor throat the air speed through it increases and the pressure decreases.
The float bowl of any carburetor must somehow be vented. This allows atmospheric pressure to exist inside the float bowl chamber. Atmospheric pressure is 14.7 PSI at sea level.
As stated above, during operation the ambient pressure decreases in the throat of a carburetor.
When this happens the atmospheric pressure present in the float chamber is higher than the ambient pressure present in the venturi and the fuel is pushed through the fuel jet, jet holder, and into the throat of the carburetor.
If the float chamber vent were plugged up or non existent the pressure differential would be wrong or not existent and the carburetor would malfunction.

A vacuum, even very slight, placed on the vent hose will reduce the pressure inside the float chamber and thusly reduce the volume of fuel delivered into the venturi of the carburetor.
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I really never said fuel was "squirted" into the intake stream, actually, I never addressed the function of the carb beyond the fuel delivery to the bowl from the gas tank. I have no dispute with your explanation of carb operation...

If there is no vacuum on this line when plugged into the air filter, then why does it have such a dramatic effect, in opposite directions, for different motors??? You would think connected, disconnected, plugged, whatever... it would make no difference, but it makes a huge difference. It would seem if there was a vacuum on that line it would effect the atmospheric balance inside the carb and therefore effect the mix and the way the motor runs. (like it does in reality when it is disconnected on my bike and others) The effect would also vary, increasing with the rpm and air flow in the engine. At low speed unplugging the tube has no effect, at higher rpm is has a big effect, so that seems to track as well....
If you can explain why this tube has this effect on my motors and others I would really appreciate the lesson, cause I am eager to learn....
 
I can only surmise that cleaner element has enough restriction to air passing through it that a slight reduction of pressure is formed inside the air cleaner housing. When the vent hose is plugged into the back of the air cleaner the pressure inside the float chamber is reduced too. Not eliminated, not a vacuum inside the float chamber, just reduced pressure compared to atmospheric pressure. That would certainly affect how the carburetor operates.

Some engines like it, some don't.
 
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My 2 cents would be the bowl vent run to the air cleaner might have a significant effect if the float was improperly set way too high, or the needle valve not operating properly.
...Both ez to occur on these finicky little hi-quality carbs. lol
Routing the vent to the intake is most likely and epa requirement, or mandate.
Personally I'm very pleased to be consuming such a very small amount of fuel for my errands around town. These are very green machines, no matter what the regulators might say.
rc
 
GearNut said:
I can only surmise that cleaner element has enough restriction to air passing through it that a slight reduction of pressure is formed inside the air cleaner housing. When the vent hose is plugged into the back of the air cleaner the pressure inside the float chamber is reduced too. Not eliminated, not a vacuum inside the float chamber, just reduced pressure compared to atmospheric pressure. That would certainly affect how the carburetor operates.

Some engines like it, some don't.
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Ok, that tracks with what I am saying, except how do you reduce the pressure in the bowl??? Well, suction or vacuum on that hose caused by the restriction of the air filter. Perhaps this is not a typical "vacuum line", it still has some suction on it, that varies with the engine speed, which effects the pressure BALANCE inside the bowl. Like you said, some bikes like it, some don't. I still hypothesis that this depends a bit on how the float is set and how high the fuel level is in the bowl to start with....
 
rustycase said:
My 2 cents would be the bowl vent run to the air cleaner might have a significant effect if the float was improperly set way too high, or the needle valve not operating properly.
...Both ez to occur on these finicky little hi-quality carbs. lol
Routing the vent to the intake is most likely and epa requirement, or mandate.
Personally I'm very pleased to be consuming such a very small amount of fuel for my errands around town. These are very green machines, no matter what the regulators might say.
rc
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Could very well be EPA restrictions that wont allow the carb to vent to open air. Venting to the air filter would be an easy way to make sure any escaping gas fumes get comsumed by the engine. These other "vacuum" effects of that connection may be just a byproduct of an EPA mandate.
The effects on the way the motor runs are very real, however. Next spring when riding weather is back I am gonna tune the carb a couple different ways to experiment a bit further. I will pull the vent tube from the air filter and then retune the carb, starting with float adjustment (allowing a little more fuel in the bowl, since I seem to run a bit light on fuel at WOT when I disconnect that tube). I am curious to see if the bike runs as well when tuned up with tube disconnected as it does when tuned to having the tube connected....
I also plan to put some sort of aftermarket high flow air filter on the bike next season as well (if I can find one to fit this newest version of the CNS carb). Reducing the resistance of the air filter should reduce the suction on this line and therefore reduce the effect of this tube. Come to think of it, I had experimented with the stock air filter by opening up some extra area to draw in air, when I did this I lost my top end speed just like when I had this hose pulled... I just put that together as cause and effect, less restriction = less suction = running like the hose is disconnected... So it seems if I run an air filter with less restriction I am gonna have to tune the carb similarly to how it need be tuned with this hose disconnected... Of coarse I realize the carb needs to be retuned whenever you change the dampening in the system such as the air filter or exhaust...
 
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hello everyone im new to here (but not to cns carbs) in the picture that is in the first post. the hose that come out of the float pan is not supposed to hook up to the top it is and overflow and is meant to hang to the ground or collector. it is a "racing" (i use the term lightly) not a fuel efficient. doing this and lowing the floats my bike runs great don't have speedo but i keep up to cars.
 
Does anyone know how to guage the vacume through the air filter intake? The reason is I think the 48cc and 66cc draw diff. amounts of air, removing the vent tube like i posted on the first page seems to work mostly on 48cc engines, on all six 48cc kits with a cns that my friends and I have built removing the hose was a great improvement.
 
Greg58 said:
Does anyone know how to guage the vacume through the air filter intake? The reason is I think the 48cc and 66cc draw diff. amounts of air, removing the vent tube like i posted on the first page seems to work mostly on 48cc engines, on all six 48cc kits with a cns that my friends and I have built removing the hose was a great improvement.
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You could connect a vacuum tester the hole in the air filter and see what it reads, specially as the engine revs high. It seems apparent that there is some vacuum present since it has such an effect on how the motor runs.
I am thinking I might remove that hose and block off the hole in the air filter before I tune the carb next time and see how that works out. It will be interesting to test the carb when tuned properly without this hose connected.
I am also guessing that once I change over to an aftermarket high flow air filter (and retune the carb) the vacuum effect on this hose will be reduced quite a bit. Wish the summer wasn't over, I have so much experimentation I want to do...
 
If the float is too high, you could starve your engine for fuel, run too lean, and burn up the cylinder. Check your plug to see if it's too lean. If the float is too low, fuel will drain out of the drainage tube sticking out behind the bowl and possibly other places.

NightCruiser- I stumbled across a good combination for the CNS. My CNS would not run with the SBP HD air filter. I guessed it was too much air flow because it acted the same as it did with no air filter at all. I put the stock filter back on and was back to excessive 4-stroking. I wound up drilling the jet with a #66 bit. With the stock filter on it was clear I was getting too much fuel when the jet kicked in. I was ready to give up and put the NT on when I decided I might as well try it with the SBP HD air filter before tossing it and I got a great surprise. Now when the jet kicks in there is no 4-stroking and it runs great. After many miles I think that the jet is slightly too large. A #67 bit would probably be a little better. My vent tube is removed because even with this set-up a little bog comes in at about 3/4 throttle with it hooked-up. I'm in Atlanta, GA at @ 1000' above sea level. A jet drilled with a #67 bit(not the same as a #67 jet), the SBP HD air filter, and removing the vent tube should get you some good results. A small engine mechanic looked at my CNS before all of this and told me that when hooked up, it looks like the vent tube could actually lower air pressure in the bowl at high rpm's which could restrain fuel flow. Makes sense to me.
 
PerryP180 said:
A small engine mechanic looked at my CNS before all of this and told me that when hooked up, it looks like the vent tube could actually lower air pressure in the bowl at high rpm's which could restrain fuel flow. Makes sense to me.
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Thanks for that, I am not a mechanic, but after looking at that tube I came to the conclusion it has to lower the pressure in the bowl, though others said no way there is any vacuum on that line.
Tricky little beast but when you get her running its nice. I have a spare air filter, opened up some extra slots inside to let some more air flow, so depending on the jet I am using I can use a filter that lets more or less air flow. I had a similar experience as you, too much air or too much gas and you get the bog....
Thanks for posting
 
I've got a grubee skyhawk 66cc and have always had to feather the choke to get it to quit bogging down at 3/4 to WOT. I tried jet sizes 64 - 71 and c clip positions from bottom to top and back again. All it seems to change is the amount of choke I had to apply. A couple of days ago, I pulled the vent tube off the CNS Gen B carb, and now it works smooth from stop to WOT without choke applied. Now I'll go back and figure out which jet size works best. It could be the CNS Gen B carb is the only CNS carb with the problem. (I also tried several float settings before pulling vent tube.)
 
porch lizard said:
I've got a grubee skyhawk 66cc and have always had to feather the choke to get it to quit bogging down at 3/4 to WOT. I tried jet sizes 64 - 71 and c clip positions from bottom to top and back again. All it seems to change is the amount of choke I had to apply. A couple of days ago, I pulled the vent tube off the CNS Gen B carb, and now it works smooth from stop to WOT without choke applied. Now I'll go back and figure out which jet size works best. It could be the CNS Gen B carb is the only CNS carb with the problem. (I also tried several float settings before pulling vent tube.)
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I have the C-32CNS-EPA, I guess that would be version C?
 
Another quick and easy way to figure out what is going on is to pull on the choke while it is bogging(This is really a circuit to richen up the fuel delivery). If it comes to life and runs fine with the choke on, it is too lean. Rejet untill pulling on the choke actually takes away from the motor. I live on the east coast and changes have to be made when temps drop 10 or 15 degrees this time of the year. I am at a#78 jet when its about 50 degrees.
 
Can't understand it.....how would RAISING the float eliminate fuel from dripping out of the exhaust or overflow ? It seems the opposite would make more sense....just sayin'
 
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