OMG I have to tune a CNS!!! Help

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'

Nightcruiser...
The label on my CNS carb is: C-32CNS66-EPA
It has the red filter with thin circular foam inside, and only one black vent tube, and a bottom overflow outlet.

Looks just like mine, mine doesn"t have data tags. It still does'nt answer my question.

No data plate on carb., but " the manual " says 2010/2011 models have a "special " CNS YD EPA carb.

Also, (as long as I'm whining ) I had to remove my heat shield in order to clear the crank, does that affect the way it burns fuel in the combustion chamber ? (engine operating temp. )

There's the older CNS that has the smaller throat where the air filter connects (the one you can use a hose clamp to put a K&N style air filter on), and there is the newer CNS that has the wider air filter opening and red plastic air filter (like I have), and the newer grubee kits are going to have the older style CNS carb (with smaller throat and paper K&N style air filter) but those carbs will have the air mix screw eliminated or sealed as per EPA requirements. (the new grubee kits at gasbike are pictured with this carb) The middle batch of CNS carb (that I have) were made on a different production line and are reputed to have lower quality than the rest. Unfortunately the new ones have the mix screws sealed but they are being made in the better reputed production line. So the ideal CNS carb to have seems to be the older ones with the smaller throat at the air filter that still have the adjustment screws....

Bill Gaidos said:

Also, (as long as I'm whining ) I had to remove my heat shield in order to clear the crank, does that affect the way it burns fuel in the combustion chamber ? (engine operating temp. )

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Removing the heat shield from the muffler will not affect how the combustion occurs in the cylinder.

The advice concerning adjusting a carburetor's float level was backwards.
If the float is adjusted too high it can flood the carburetor, conversely if it is too low it can starve the carburetor of fuel at high RPM's.

Wow! Tuning this cnsv2 carb is a pain in the butt for lazy people like me.... Thanks for the very descriptive help.
-1969

Here I am again...I've lowered the float level and eliminated the wet plug and fuel dripping out of the muffler, have a great low,smooth idle,good mid-range performance, but notice a little hesitation and surging when going wide open. I attribute this to my needle clip being on the very top notch...easy enough to remedy, but what I can't seem to do is rid the carbon fouling of the plug over several days.......all I can think of is too weak a spark or wrong heat range of plug, bad CDI or coil wire. Can you suggest a different plug (BP6HS)...I've even been considering an Iridium plug -hotter spark,less fouling, but also $11.00 each (ouch)

Bill Gaidos said:

Here I am again...I've lowered the float level and eliminated the wet plug and fuel dripping out of the muffler, have a great low,smooth idle,good mid-range performance, but notice a little hesitation and surging when going wide open. I attribute this to my needle clip being on the very top notch...easy enough to remedy, but what I can't seem to do is rid the carbon fouling of the plug over several days.......all I can think of is too weak a spark or wrong heat range of plug, bad CDI or coil wire. Can you suggest a different plug (BP6HS)...I've even been considering an Iridium plug -hotter spark,less fouling, but also $11.00 each (ouch)

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I have this same problem when going up a hill. It only happens if I'm giving it too much throttle. My jet is drilled with a #66 bit which equates to @ #80 jet. These motors have only so much power and giving more throttle causes the things you described. Too much fuel will also foul the plug. I'm in Atlanta, GA at 1000' above sea level. If you are higher then your motor will be more sensitive to excess throttle. If you are using the stock jet then this is probably not your problem unless you are in the mountains above 2500'. Your acceleration above 3/4 throttle is gradual so ease up on the throttle twist and see what happens.

I'm in Tampa, about as close to sea level as you can be without waterwings, but thanx for your input.

A BP5HS plug would be hotter.
The closest NGK iridium spark plug I could find at NGK's web site was an IGR7A-G, NKG catalog #3106. (Used in a 2003 to 2005 Mitsubishi Lancer Evo)
It is a colder plug than a BP6HS, but since the fine wire Iridium plugs run differently, perhaps it would still work out?

Hey Bill,

It would HURT me to spend $11 on a plug for one of these lil' chinagirls!
I mean, gosh!, it would be painful!!!

IMO they are just not worth it.

Why take a cheap date to the opera???
Go to the pub, buy her a draft, and give her some quarters for the jukebox!

However, you might do well with a size smaller main jet if adjusting that needle doesn't bring you the burn you wish for.

OR, you could give the Opti2 a try.
...then mess with jets once you get switched over to it...

Best
rc

I just got one of these but a clone
looks the same
anyways mine only had a bowl drain line connected no other included lines
from the many pictures ive found it looks like all other nipples stay open to the air

Now to the tuning
ive tuned small carbs in the past but always stock ones
not a brand new one
and of course knew the jet sizes
I assumed since they claim its for 80cc then the jetting would be correct
I found with the air screw it would run better at 1/2 turn out
From little I know that really lean, and I think it was because when I wold go to idle it would idle very high for about 1 min, then drop very low

if I adjusted the air screw out ( im guessing rich) it would bog instantly and die
again ive used this screw on other bikes in the past and know it takes very little turns, and having to wait a few seconds between turns

I even tried the high idle trick and adjusted the air screw until it idled higher then lower

So acceleration felt very smooth, then a bump im guessing that the needle. I did try it richer but no help there.
Then full throttle I never got as fast as I did with the stock I even lost maybe 4-5 mph top speed

Now it never felt lean, because in the past if I would be lean and slowly drop the throttle a touch I would go faster. Just like my stock carb when I was jetting that for my larger filter.

But this new carb just didnt want to open up.
I could rev it on the stand and really get it screaming but on the road no luck

The micro drill bits I have arent the right sizes to drill out so I will have to order some, but in my case is this still a sign of being lean?





Anyone live at sea level and would like to tune it for me?
I dont want to give up on this carb yet but so far my stock one kicks butt

From what most of us have learned these carbs are very lean on the jets, I have not tuned a 66cc but had to drill the main jet on my 48cc with the high flow air filter. Biknut started this thread and had to do the same with his build of a 66cc, what size jet depends on what your plug looks like.

http://motorbicycling.com/showthread.php?goto=newpost&t=24873

yeah im trying to find all the bits from one seller
then I will drill them out like the first posts had said

never drilled out jets before usually I buy the correct types
so from what I understand there are no jets available for that carb?

its weird you would think it would be jetted for the air filter, unless it comes off a bike or scooter that uses a air box and they just slap the filter on it


anyways I have a used plug in it right now but its black with slight tan ceramic
but thats from my stock carb jetted to the filter im using

I will have to get a new plug once I get the drill bits