OMG I have to tune a CNS!!! Help

biknut

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Sep 28, 2010
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This is Duchess. A beautiful silver blue Micargi-Rover beach cruiser, with a silver Grubee SkyHawk 80. It's only a few hours old now, and has been around the block for about 3 miles so far. It belongs to my friend Corinne.

http://www.youtube.com/watch?v=dXysa1FNzEs


When I fired this beautiful bike up for the first time, a few hours ago, it started easily enough with full choke. After a little warm up, what I noticed was a total lack of power above about a 1/4 throttle. All it will do is bog if you give it any throttle. It can barely get up to 10 mph. After a little screw turning I was at least able to get it to idle pretty good, so I don't think it's leaking air.

That's the total extent of the tuning I've done so far. This carb has a aftermarket air cleaner, but the red plastic one is what came on it.

I've been reading about the typical problems people are having with the CNS carb for months, but this is the first time I've had one in hand. I won't toss it in the trash until it's running good lol.

Based on the comments I've read about this carb, I have to agree. Everyone pretty much nailed the description of the symptoms. What I want to know now is what to do next. Some people claim they've made it work, but the how seems a little vague.


Edit//After a few more weeks time, and tuning experience by myself, and fellow forum menbers tuning this carb, this is what we came up with.



How to tune the CNS V2 Carb.
_________________________________________________________________________________

Before even starting, it's highly recommended you toss your stock spark plug, and install a
NGK B6HS, or BP6HS. Both of these plugs are proven beyond a doubt to be dependable, and run well. Doesn't seem to matter whether you have a 50, or 80cc. About $2.50 each


1: Raise the float level.
Turn the Carb upside down. There's a seam in roughly the center of the plastic float. Adjust the float till the seam in the middle is parallel to the sealing surface of the float bowl.

2: Top end adjustment is as follows. Drill out the main jet. Check the color of the plug to help you decide tuning. Black is RICH, White is LEAN
Buy Machinist bits #64 #65 #66 #67 #68 #69 #70 (Stock jet size is #70)
Drill out the jet one dril size at a time starting with smallest, #69. Test and check for full throttle response and spark plug color. Below 1000ft altitude #65 and #64 seem pretty close. This can vary depending on the state of tune, but if you're reading this your engine probably isn't too exotic. If you drill too large of a hole, and want to go leaner, fill the jet with solder, and re-drill with a smaller drill bit.

3: Low and mid range, adjust the C-Clip on the carb needle.
(Top is lean and bottom is rich) Check your spark plug for direction.
For me, the correct position has always been obvious after a couple trys.

4: Idle adjustment.
The Idle mixture, and idle speed adjustment screws are on the right side of the CNS V2 Carb. The adjustment screw closest to the air filter is the idle mixture screw. The adjuster screw more toward the middle of the carb is the idle speed adjustment.

Basicly you just turn the idle speed adjuster screw in far enough to hold the slide up high enough to hold an idle. As you screw the adjuster in it's a good idea to open the throttle a little so the adjuster doesn't goudge the side of the slide. Now that it's idling, try turning the idle mixture screw both directions to find the fastest idle. Now go back to the idle speed adjuster, and adjust as needed.

Foot note: In some cases, but not all, removing the breather tube from the air filter may result in better performance.
 
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biknut

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One thing I noticed, that I don't remember mentioned, is when mounting this carb on the stock manifold you push it on what seems like all the way, then give it a harder push at the end, and it sort of snaps into place. Then you tighten the clamp. I think it's sealing, and I didn't use any sealer.
 

biknut

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I guess what I need to do first is check the float. Several people said they had to adjust the float level, but they didn't really explain how they measured the adjustment.

One guy seemed to imply the float was riding too low in the bowl, and the motor wasn't getting enough gas.
 

biknut

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Sep 28, 2010
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Most of the complaints I've seen revolved around the motor not having any power and a lot of bogging. That's exactly what I've got.

Alot of people complain about air leaks, but that not what I think I have. I wonder if people don't know about making the carb snap onto the manifold, or is that something new?
 

biknut

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Sep 28, 2010
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Is there any agreement with this. These instructions say to adjust the float so that when the tang on the float just makes contact with the needle, the bottom of the float and the float bowl mating surface the are parallel to each other, or about 16mm from the bottom of the float to the float bowl mating surface.

Of coruse the carb is a different one off a cowasucky.

Float Level Adjustment
 

Greg58

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May 1, 2011
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Try pulling the little black hose out of the back of the air filter base, that is the vent tube from the bowl. do not plug the hose leave it open, cover the hole with a plastic trim barb like used on auto trim. I have what looks like the same filter from sbp., I had to oil it first with k/n oil because it was getting too much air. Mine was drawing fuel from the bowl, I hope this helps you.

Cns mid range bogg solved!
 
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biknut

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Sep 28, 2010
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Try pulling the little black hose out of the back of the air filter base, that is the vent tube from the bowl. do not plug the hose leave it open, cover the hole with a plastic trim barb like used on auto trim. I have what looks like the same filter from sbp., I had to oil it first with k/n oil because it was getting too much air. Mine was drawing fuel from the bowl, I hope this helps you.

Cns mid range bogg solved!
Thanks for the suggestion. I was thinking about doing that. I don't see why the float bowl vent needs to go anywhere anyway.

So far I can't even get up to the mid range. I'm stuck riding around just off idle. Any gas at all and it just bogs.
 

Greg58

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May 1, 2011
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I have built four and been involved with two more kits with cns carbs. On all of these engines we found that you can tune the cns after you correct this vent problem.
 

biknut

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I have built four and been involved with two more kits with cns carbs. On all of these engines we found that you can tune the cns after you correct this vent problem.
That's encouraging. Besides the vent, what type of changes have you had to make? I'm guessing that since the EPA was involved the carb is probably set up lean out of the box.

Any suggestions about setting up the float?
 

biknut

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I hope I can get this carb to work because I like having the choke on the handle bars. I also like that it uses the same aftermarket air filters as the other carbs.

Will the jets I bought for my NT carb work in a CNS carb?
 

Greg58

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May 1, 2011
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I do not own a nt, all I have worked with is the cnsb. Mine is on a 48cc so main jet was o/k, the idle jet was part. cloged I had to run a torch tip cleaner through it. The air mixture screw is for idle only, so adjust the idle and air screws for idle then the e-clip for mid range and main jet size for wot.
 

biknut

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Sep 28, 2010
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This is what I tried so far today.

I've raised to float level 2 times This is how I measured it. Turn the carb upside down. Measuring from the top of the rubber float bowl gasket, to the highest point on the float, 49/64"
After the first adjustment changed to 47/64"
After the second adjustment, to 46/64"

This had a slight favorable effect.

Next I drilled out the main jet. I can't tell you exactly what size it is stock, but a # 71 drill bit was the biggest one that would fit through it. I drilled it out to the next size bigger, a #70.

When I tried to start the motor with the same amount of choke as before, this time it flooded. I turned off the choke and it started for the first time with no choke.

It seems to run a little better, but is still really crap. It starts and idles fine. Now I can give it a little more gas. I got it up to about 15 mph. A 5 mph improvement and new record. Trying to give it any more throttle after that it just bogs like a biache.

I'm going in the garage now and have another go at it. I don't know whether to raise the float more, or drill out the main jet bigger.

I ran some drills through the stock jet before drilling it and I seems to be about a #70. it seems to be about the same size as the stock jet that came in my NT carb.
 
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biknut

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Sep 28, 2010
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I just raised the float a little more. This was my third adjustment. I didn't touch anything else.

When I explain how it's running be advised, whenever I say choke, we know I mean enrichener.

Out of the box it was very cold blooded for starting. It needed a lot of choke to start and you couldn't turn off the choke for a long time till it warmed up. It seemed to always run a little better with the choke slightly on.
Now it starts about the same as always, but you can turn the choke almost all the way off right away. It's running better after this last float adjustment. Now it will run out to about 15 mph and has a little throttle response on the bottom end. If you grab a big hand full of throttle it still bogs out, but not as bad as before.

I thinking now the float has to be about high enough.

Should I drill the jet out more? Hummm
 

biknut

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Sep 28, 2010
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It's running better but, still weak on the bottom end, and no top end. Adding choke doesn't have nearly as much effect as it did before.

The needle is still in the richest position. Everything I've done so far made it richer, and it's definitely running better now.

The problem is that's not saying much.
 

Greg58

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May 1, 2011
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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.
 

biknut

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Sep 28, 2010
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You know the answer, NT.
Yeah, I can't deny that's the most rational thing to do.

Naturally that's not what I'm planning to do.


It's running better but, still weak on the bottom end, and no top end. Adding choke doesn't have nearly as much effect as it did before.

The needle is still in the richest position. Everything I've done so far made it richer, and it's definitely running better now.

The problem is that's not saying much.

I just got back from test riding after drilling the main jet. Starting out with a #71. going to #70. Now a #69. This last change has had the biggest positive effect so far, but it still sucks on the top end. I've now made enough of an improvment that it's starting to get interesting.

It'll cruise 15 or 16 mph, estimated. Now it takes full throttle for the first time, without immediately bogging down and dieing. Now it kind of holds it's own at 15 mph wfo. It's not accelerating, but it's not slowing down much either. I thought I might be hearing some piston pinging at full throttle for the first time. I think before it didn't make enough power to piston ping.


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.

.