OMG I have to tune a CNS!!! Help

Greg58 said:
When you say it boggs does it spit back through the carb? if it does like mine did it is getting too much fuel for the air coming under the slide at partial open. turns out that the mixture was so rich that it was partial plug fouling on mine till i worked with the e-clip. As soon as it boggs hit the kill switch and do a plug check, I found a wet plug.
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I don't think it was spitting out of the carb. I don't think it ran good enough to spit.

Everything I've done so far, was to make it richer. I've drilled the main jet twice so far. Going for three soon. My needle is now in the richest position.

How is your carb set up?
 
mine is on a 48cc with the e-clip on the middle slot with a sbp air filter.Four strokes a little till it warms up. by the way all I have to do to crank mine is lift the back wheel and spin it by hand, it starts every time.
 
By the way I have read that the jets are the same no matter which cc engine kit the carb comes with.
 
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Greg58 said:
mine is on a 48cc with the e-clip on the middle slot with a sbp air filter.Four strokes a little till it warms up. by the way all I have to do to crank mine is lift the back wheel and spin it by hand, it starts every time.
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So your CNS 2 is running
stock jet
middle slot on the needle
?
 
biknut said:
.


I'm thinking more main jet. Seems like it's possibly going to need to be two sizes bigger from where I am now at a #69 drill.

The next step is a #68 drill bit.

.
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Major Success!!!!! but not without a little more tweeking.

I drilled the main jet with a #68, and it ran even better, especially on the top end. I thought I could feel surging under acceleration, so I decided on even more main jet. Now I drilled it with a #67, the biggest drill bit I bought today. A #67 seems to hit the frickin spot. From relitivly low speed, I now can give it full throttle and it accelerates to speeds higher than I want it to rev to yet. This is an amazing transformation.
 
It's not all love and roses though. I have yet to mess with the low speed much yet. Right now it starts Ok, Idles Ok, and accelerates OK, but it has a nasty little spot right off idle. If you give more throttle it goes away. I think it 4 strokes there too.

Only thing I've done to the mixture screw is set it at 1 1/2 turns out. The needle might need to be lowered a little now that I've richened the main jet up so much.
 
biknut said:
All in all it's running pretty darn now good for a brand new engine running heavy mix.
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After you are done running your break-in mixture, be prepared to do all your soldering/drilling/testing all over again.

Another thought: Maybe reducing the length of the needle so that it comes out of the jet sooner when you give it throttle?
 
rohmell said:
After you are done running your break-in mixture, be prepared to do all your soldering/drilling/testing all over again.
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That doesn't bother me now that I know it can run well with this carb. What was bothering me was everyone saying you can't get this carb to work, and buy a NT carb. I've proved to myself that's not true.

I think the problem most people have with this CNS carb is that from the factory it's so lean that it's a million miles from being right on the main circuit. Most people obviously give up trying before they get close to right. I don't blame them, because I was thinking it was a lost cause too.
 
This is the deal with the CNS carb. It's actually a better, more sophisticated carb than a NT or Speed carb, but possibly because of new EPA laws that went into effect this year, or maybe even total ignorance on the part of the factory, the carb comes out of the box jetted so lean a 80 won't even run. A 48 might run a little better but probably not much.

Then to really confuse the issue, the actual size of the main jet isn't that much different from the size jet found in a NT carb, but that doesn't mean much because the CNS is a different animal.

I'm pretty sure that the reason it runs so lean with the same size jet is because the CNS has a two fuel circuits, instead of the NT's one. On the CNS, one air bleed is feeding two circuits, instead of the NT's one circuit. The pilot and the main. The NT just has a main.

Usually motors that come with a NT carb have to be leaned it out from the factory setting, especially if it's a 48cc.

This is my opinion of what's going on. Because the pilot jet relieves some of the pressure on the main circuit, the main doesn't suck as hard as on a NT carb. That's the reason you have to have a much bigger main jet than a NT has in order to compensate for the reduced suck. This means the same things we do to get a NT carb to work better won't work on a CNS carb.

In the end you get the same amount of gas, but the CNS is more precise over a wider rpm range.

My question is, does it come from the factory jetted so lean it won't run, because EPA laws force the factory to do so, or is it because the factory is so stupid they though the same size jet would be Ok?
 
You did what I did, I would not give up on this carb because this looks like the way these kits are going.
 
LOOOking at your fuel tanks.
Did you bother to vent them?

You went to a lot of trouble to richen up the fuel air mix.
 
biknut said:
Major Success!!!!! but not without a little more tweeking.

I drilled the main jet with a #68, and it ran even better, especially on the top end. I thought I could feel surging under acceleration, so I decided on even more main jet. Now I drilled it with a #67, the biggest drill bit I bought today. A #67 seems to hit the frickin spot. From relitivly low speed, I now can give it full throttle and it accelerates to speeds higher than I want it to rev to yet. This is an amazing transformation.
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Well after a little more testing I decided I want more. I finely ended up drilling the main jet to a #64. Changing the spark plug to a NGK BP6HS made a huge difference too.

This Grubee SkyHawk 80 that would barely run out of the box, now runs like hot stink. Now after the motor warms up it runs smoother than before and there's very little 4 stroking.

If you're having trouble getting one of these to run right I'm not surprised. This is what I recommend.

Replace the stock Chinese Spark Plug, Z4JC, with a NGK BP6HS.

Move the needle to the richest position. Stock comes in the middle.

Buy some machinist drill bits size #67, #66, #65, #64. Stock is about a #71. Start with the smallest bit, the #67.

Check you spark plug for guidance. The best jet size will probably be the #65, or #64.

If it still doesn't run good try raising the float level. I raised mine twice but didn't notice much difference. I just left it as it is now.

A lot of people remove the rubber float bowl vent tube. I tried it with, and without several times. I couldn't tell much difference either way. My opinion is if it's not kinked it won't make any difference
.
 
Biknut: THIS is exactly the stuff we need on this forum: Doggedly persistent people who know how to document their experiments for the rest of us.

And yes, it seems like to get EPA certification, they needed to lean these poor little things out. The CNS carb used to be an 'upgrade' carb over the NT. Good to know they're fixable!
 
Tohri said:
Biknut: THIS is exactly the stuff we need on this forum: Doggedly persistent people who know how to document their experiments for the rest of us.

And yes, it seems like to get EPA certification, they needed to lean these poor little things out. The CNS carb used to be an 'upgrade' carb over the NT. Good to know they're fixable!
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Thank you,

I'm really just trying to get this one running well for a friend, and also I might decide I want one if it ends up being fast.

I can understand why people are having such a hard time getting these carbs to work. They're so far off out of the box.

I wish someone would start selling jets for CNS carbs.
 
Way to go biknut!!!
Reading all this really makes me want to get a CNS2 and fiddle with it even more so. You have now done what I want to do with one.
A question and a pondering idea for the CNS:
Does the CNS have a cross drilled emulsion tube for the main jet? If so, altering the size of the holes as well as the number of holes will effect when it "comes on" in relation to the throttle position/ air volume flowing through the carburetor.
I find it quite strange that this carb has one air bleed for 2 different fuel circuits.
All the carbs I am familiar with have separate air bleeds for each circuit.
What I am trying to work out in my mind is a way to somehow separate the event timing of each circuit better to allow finer fuel delivery tuning and a way to overcome any flat spots.
 
GearNut said:
Way to go biknut!!!
Reading all this really makes me want to get a CNS2 and fiddle with it even more so. You have now done what I want to do with one.
A question and a pondering idea for the CNS:
Does the CNS have a cross drilled emulsion tube for the main jet? If so, altering the size of the holes as well as the number of holes will effect when it "comes on" in relation to the throttle position/ air volume flowing through the carburetor.
I find it quite strange that this carb has one air bleed for 2 different fuel circuits.
All the carbs I am familiar with have separate air bleeds for each circuit.
What I am trying to work out in my mind is a way to somehow separate the event timing of each circuit better to allow finer fuel delivery tuning and a way to overcome any flat spots.
Click to expand...

Yeah it's fun to play with a carb, especially when it's not your bike.

If there is an emulsion tube I haven't seen it yet.
 
The thing about this carb is it performs different on a 48cc, all six of the bike we have built here in Ga. had to have the vent tube removed to run right and to get a good plug reading. The e-clip setting on all are on the 2nd or 3rd groove, this works well at 1000 ft a.s.l.
 
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