Jet size for CNS carb?

B

Black_Moons

New Member
Hi. Today im wondering, Why does everyone change to a 0.68mm jet it seems? Is that just people using the stock or speed carb? Does nobody jet to a bigger jet?

On my other bike (Fat cat exhaust, CNS carb, 80cc ZB motor), I had to rejet to 0.73 and I still need to blip the choke sometimes up hills to keep it from sputtering (well, 4 stroken really) Thinking of getting 0.74/0.75mm jets to try out next. Can't understand 0.68, it would be wayyy to lean.

New bike, Stock jet, I had the E-clip 2nd to the bottom (Needle raised, Rich)
it was 4 stroken about half the time, a little less after it warmed up, and engaging the choke killed them motor right quick with no improvement. assumed it was running too rich due to choke killing motor. Raise the E-clip to lean it out and.. its 4 stroken a LOT and almost never going into 2 stroke. Gonna try my left over 0.72 in it.. and likey order some 74/75's tomarrow.
(Note, e-clip near bottom with stock jet was fine with stock exhaust, but its **** with new SBP exhaust)

Maybe this jet problem is the reason why people think CNS carbs suck?
 
I have CNS problems too. Rev fine with clutch squeezed but on the road bogs bad when I openup the throttle. Little choke helped. I too was able to help it by raising the needle and better when all the way warmed up. Like to get some different main jets but where? May have to drill to get a bigger jet but I hate to do that. One reason why I liked the old NT carb. jet to big, easy to solder and drill. to small, just drill it out a little.
 
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Allready checked for air leaks, And trust me, Running too lean CAN cause 4 stroken, just like running too rich will. No idea why, other then during 4 stroking, the air/fuel charge is not ignited every revolution, so there may be left over charge for the next cycle.

Moved to a 0.72 jet in my 2nd bike (biggest I had) and the 4 stroken is nearly gone at the exact same needle settings it was 4 stroken at with the stock 0.70.

I don't get what you mean by 'conditions not predictable'.
I make it more rich by moving needle up, It runs better.
I make it more lean by moving needle down, it runs like crap
I make it more rich by bigger jet, it runs better.

The only thing thats not predictable is the fact the choke on this carb seems to kill the motor outright when turned on, Where as my other CNS carb equiped bike that runs a little lean, it actualy makes it go from 4 stroken to 2 stroken, when used just right (Short blip on then off again)

Masterm222: I got my jets from the local motorbike parts place.. Took me awhile going through the catalog to find em however.

The package states this:
Importations Thabault ltee
Gicleur 72 BW's/Zuma
2344000 (for the 0.72)
2344070 (for the 0.70)
Theres lots of other sizes too.. I ordered in a 68, 70, 71, 72, 73, and just recently, 74, 75 and 78 (No 76/77 available?)

While the stock exhaust ran OK with the 0.70 in the CNS carb, Both my bikes after upgrading to expansion exhausts needed a 72 or 73 to even begin running well. I think I gotta up one of my bikes from 73 to 74 or even 75, since it still 4 strokes up hills now and then unless I blip the choke.

If anyone wants, I can check the catalog at the motorbike place when I go to pick up the jets I ordered in a week or two, to tell exactly what brand/model they are.
 
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The bike I am working on doesn't "4-stoke" it just bogs bad when I open up the throttle. This is one of the GASBIKE 2010 kits with the CNS. I am building this one for a guy on a Sting-ray XL. long header pipe with a glasspack. So not as much back preasure as stock I would think since it is straight through. They come with a three electrode plug. Little bit of brown on one side of the center electrode otherwise gray. Looks lean to me. Re sealed everything one more time to make sure it was not due to a leak but still the same. Going to try a totally different carb and intake from one of the kits I have in stock but I am going to drill the jet a size or two bigger. Got to figure this out soon because another bike for a build shows up Sunday! That and I have two more kits with CNS carbs to install. Like to figure these carbs out before I get busy this summer.

Black Moons: Thanks for the info, What you think? Would I have better luck at a Kawasaki, Yamaha or Suzuki dealer? Long bike ride to any of them so it would be nice to find them the first try!
 
Good point on the exhaust making it run lean. Pedal hit stock exhaust unless I mod it but i might be able to get a running start for one try just to see.Expansion chamber is out for now because customer doesn't want to buy one yet and it can't be a curved one on this prodject. Has to be straight to fit so i was going to build one just for that bike in my "spare time" LOL For now, bigger jet-have drill set.
 
a REALLY secure vise + two peices of wood on the jaws can bend pipes (And even expansion chambers) a degree or two without crimping them badly.

Sand+heat works much better however, especialy for anything past the most minor bending.
 
Another good point. The main jet was enlarged a size during my lunch hour today but no time to test tonight. Got dark by the time I was done with my side auto tech work so the bike has to wait until tomorrow. If it isn't ok with the jet mod I am going to ream the stock exhaust mounting holes and try it that way. Thanks, let youhow this works out.
 
Re-jetting the carb was the ticket. Now the thing runs. I was finally able to make some WOT runs and the plug still looks a little to lean. Working on another with a similar problem. This one has a SBP expansion chamber exhaust. That mini drill bit set I bought is coming in handy.
 
Are you using anything besides the kill button on your white leads, such as a bike light? My engine was bogging down and acting like its not getting enough power to the CDI, and then when I disconnected the tail-light, the engine suddenly runs smoother and it starts even faster. So, for now, my tail-light is hooked up to an old 6v rechargeable alarm battery. Nice and bright though!
 
I am having a similar problem. I got a sbp air filter and exhaust, so now it runs super lean. I was able to enlarge the jet a little with simple tools but I need to find a place to buy more jets. sbp has some but I don't think they fit CNS carbs. Or do they? I am waiting for them to email me back.
 
Someone please correct me if I am wrong but I believe the CNS has a larger intake than the NT. This is why some need to go to a bigger jet with the CNS than with the NT. We do now have stock of jets for the CNS carb and are just working on getting them set up on our website. They should be available Sunday or Monday 10/3/11.
 
Ghost good to hear you have jets, I had to drill mine out after installing the sbp air filter. The stock filter is so restrictive it makes these engines run with a smaller jet. The good news is after getting the right jet size my 48cc runs as good as I think is possible. I ended up at a #64 drill, sounds big I know but that what mine likes.
 
A #64 drill is approximately .9144 mm which is a 91 jet. That is pretty big, but if that is what works then good deal. I don't think we will carry anything larger than an 80 jet unless the market demands it.
 
Masterm222 said:
I have CNS problems too. Rev fine with clutch squeezed but on the road bogs bad when I openup the throttle. Little choke helped. I too was able to help it by raising the needle and better when all the way warmed up. Like to get some different main jets but where? May have to drill to get a bigger jet but I hate to do that. One reason why I liked the old NT carb. jet to big, easy to solder and drill. to small, just drill it out a little.
Click to expand...

I have a racing (lol) cns carb I have found some jets that are Dellorto PHVA PHBL 6mm Main Jets for $2.88 each plus cheap shipping will this work and what sizes should I order thanks http://www.1977mopeds.com/parts/carb...phbg-2730.html .shft.
 
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