CNS Carby tuning problems

RLorange

New Member
Jun 21, 2008
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Australia
Hello Everyone.

I have recently purchased one of the new style CNS carbys but I am unable to tune it properly. Very frustrating.

It starts easily and is tuned well enough for a trip but it has a problem that I can't identify yet.

It Idles fine and I have made adjustments to the idle circuit but when you open up the throttle it bogs down from 1/2 to 3/4 throttle then opens up again to full throttle. Also it seems to take a long time to rev down to idle again although I have reduced this with the idle mixture adjustment.

This carby has a slider and pin which I assume is the problem but I have tried all of the clip settings without success. This carb does have a proper air circuit which premixes air with the fuel leading to and surrounding the main pin/valve.

My questions are:

What does this bogging down at 1/2 to 3/4 rev mean? lean or rich condition?

Could there be a blockage in the internal circuit?

Could the pin have the wrong taper?

Also what about the main jet? I am assuming that because it runs well at full-open throttle the main jet is not the problem.

Any help would be much appreciated :)
 

RLorange

New Member
Jun 21, 2008
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Australia
I think I have solved my problem:

To rich in the mid-range throttle because the needle taper is too steep.

I checked the needle against the one from my stock NT carby, is has a much thinner cross section. A very fortunate discovery is that the two needles are exactly the same size in every other respect.

Unfortunately I cant test out the NT needle until this rain stops!!
 

misteright1_99

New Member
Mar 21, 2008
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Treasure Coast, Florida
I had the same problem with my Mikuni. I tried the HT stock needle because it was sooo much different than the Mikuni needle, and it didnt work for me. Finally I realized that the slide cutaway was too small (not enough air at midrange) and was unable to correct it with just needle position. My slide had a 1mm cutaway and I increased it to 1.5mm and helped and finally ended up with a 2mm cutway to flow the air I needed and midrange. The idle circuit on MOST carbs is misleading, they work thru the full range. Does the screw adjust air or fuel? The way I set mine (air screw) is to start engine, bring up to temp, set idle up to higher speed, turn the adjustment so that engine speed is at its highest (usually out) then turn in till engine starts to stumble, then 1/4 turn out. But like I said this only a start, this might give you too much fuel OR air at midrange and WOT, if the carb works like a Mikuni. To get some kind of idea of main jet size, you can do a plug chop, start the bike, bring up to temp, ride the MB at full throttle and while at full throttle, kill the engine, and pull in the clutch at the same time, let the engine cool, and pull the plug, this will give you a rough idea if the main jet is right. You can do the same at mid.
Good Luck......

Here is a good link on tuning a Mikuni carb, I am not familiar with CNS but maybe this will help

Mikuni carburetor operation and tuning
 
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RLorange

New Member
Jun 21, 2008
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Australia
Here is a link for the CNS.

PLEASE READ THESE INSTRUCTIONS BEFORE INSTALLATION

There is not a lot of info on it, my local dealer is trying to get the manual translated from chinese.

I have a Dellorto PHBG replica which I never got working (it is too large 19mm) which it resembles closely, maybe its a replica of the PHVA which I have not examined.

I tested my NT needle and it runs a lot better now. Will be doing some plug chops in the near future.

It has a really annoying Fuel overflow valve thingy which is regularly squirting out petrol!. I can imagine this is really needed so I think I will be bogging it up very soon.

Very interesting about the cutaway.

This carb. comes with a big cutaway compared with the one from my NT. Maybe it is worth trying the NT slide as it fits as well.
 

RLorange

New Member
Jun 21, 2008
127
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Australia
Well I can't use the NT slide even though it is exactly the same size and shape the cutaway is on the wrong side! The slide guide notch and idle speed notch are reversed.

My first plug chop shows a nice tan colour at WOT maybe a bit dark. The jet is definately bigger than the NT but this doesn't worry me because the NT has no pre-mix circuit to draw air in and around the needle valve so more fuel is drawn and hence a smaller jet is needed because it is not 'pre-leaned'... If this makes any sense. Also these carbys are kitted specifcally for these engines so I would assume the jetting is not wildly out.

My NT needle seems to be doing the job but I have yet to give it a decent test run.