Cns carburetor jets

Discussion in 'General Questions' started by Frankenstein, Oct 4, 2016.

  1. Frankenstein

    Frankenstein Active Member

    Don't give me crap about Cns carbs they run really nice and the new ones with 4 tubes are also very nice as long as you plug off the air inlet tube that is commonly hooked directly to the overflow tube (wtf are people thinking) and block off the second tube closest to the air filter (it's not a breather it's an oil inject tube used on some Yamaha dirt bikes, leaving it open as a breather allows excess air into the system.) the second one is the actual breather, either hook that one right to the over flow (not exactly recommended, it's called a breather for a reason, draining fuel via the overflow/drain tube requires you have both ends open anyway)

    Now to the meat, the jets on this carb are of some large over sized thread with a bit more head to the jet, anyone know what exactly this size is and how to get it? I'm about to give up searching and just drill the damn thing out and retap to the clone Dellorto jet size and use my existing stockpile to jet the carb...

    Third brass in to the right of the gasket is what I'm talking about. 20161003_124117.jpg
     

  2. jaguar

    jaguar Well-Known Member

  3. Frankenstein

    Frankenstein Active Member

    No unfortunately not, I've determined I have 6mm fine threads on the jet. I can't find any site that specifically notes it fine vs coarse thread.

    But this is OK! I took the jet and modded it to accept a standard 5mm x .8 thread which is the Dellorto thread size, now the original jet screws in and in that the standard jet gets screwed in. Here's what I did:
    Filled the gaps with silver solder on the top flathead slot, added some along the base of the head to reinforce the deeper threads. Also some size comparison.
    20161004_212254.jpg 20161004_212306.jpg 20161004_212547.jpg
    Tap is 5mm.

    I drilled out the center hole to 1.02mm, since it's my largest micro bit, had to do it old school since my dremel recently broke and I couldn't use my press for it.
    20161004_221421.jpg

    Quick size comparison again showing the clean cut hole.
    20161004_221655.jpg

    Did a bit of cleanup on top
    20161004_222751.jpg 20161004_222916.jpg

    Touched up the outside and inside a bit.
    20161004_234829.jpg

    Another quick size up
    20161004_235442.jpg

    Prepping the patient for further surgery.
    20161005_123313.jpg
     
  4. Frankenstein

    Frankenstein Active Member

    Drilled out for 5mm tap.
    20161005_123912.jpg


    Tapped
    20161005_125411.jpg


    20161005_125454.jpg

    Jet screwed in, not perfect but effective for sure.
    20161005_125652.jpg

    Size 70 jet not obstructed by slight off center, happy about that.
    20161005_125751.jpg
    Another look, pretty nice.
    20161005_125758.jpg

    And fitted to the carb, doesn't touch bowl bottom, but the brass insert that goes in the same hole (the needle sits inside that) from the same place as the jet, the jet doesn't seat as deep as it did before, so the insert might slip down a bit, which would change the richness of the low and mid range, I will fix that bridge when I come to cross it.
    20161005_130358.jpg

    Over all its very effective and I'm happy with the results, I won't have to buy new jets, just use the old ones I've been using and have plenty of.
     
  5. Frankenstein

    Frankenstein Active Member

    I'll also add WOW! The new carb is far better than the Dellorto clone, it's a bit choppy on the top end (too lean might be detonating, using mid grade and off my break in mix, can't find the jet collection) but I can fix that rather nicely by using the enrichment lever and then turning the idle up a bit to keep it running.

    I can feel the astroglide smoothness of the rpm acceleration, and my powerband is a bit more defined than it was before for some reason. I can also accelerate up hills better than before (could have to do with just comming off break in and the temp outside dropped considerably between tuning earlier and comming home from work at midnight.)

    The Dellorto clone of course has a single adjustment which is a main jet, very easy to use but lacks the stock nt carb quality of having a 2nd adjustment range on the needle for idle through mid range. The Cns has an idle jet with a mixture screw to accommodate it, then has the needle setting, and has the main jet for WOT, and has an enrichment setting, which is nothing like a choke since a choke is actually letting less air in per fuel unit while enrichment is letting even more fuel in with the same air and fuel as before. This actually gives me an idea however, the Cns has an extra inlet to the air intake side, it's a barbed tube that is right after the filter. If a person wanted to inject a very light stream of no2 into their motor this would be perfect if dosed correctly while using the enrichment slide.

    Anyways, the carb is definitely an improvement over the nt and Dellorto. If somebody wanted to buy one at a reasonable price then look for a pw80 carburetor. It's the exact same thing that places like bikeberry sell as a Cns carb, most places will be selling this type as a Cns for a motorized bike but at a very ridiculously overpriced amount (bikeberry at 80 dollars for carb, throttle and cables.) I can get the pw80 carb at only $15 on ebay, and I know a throttle with 2 cables and a thumb lever isn't worth $65...

    If you really needed jets to make the job easy then I suppose you can get them for this carb but I can tell you that solder is pennies and the skill is free, and for 30 bucks you can get a complete kit for tapping threads and such for a 3mm through 7mm. I was able to use them to do this project in about a day between family life. Since I have plenty of jets for 5mm Dellorto it seemed like a better idea to spend 30 dollars and some time to get a full range of adjustment than it would to probably spend twice as much on the same jet size ranges or just as much on new drill bits to solder and re drill a single jet over and over, at least this way I can quickly pull a jet and put the next size in. I can also give up quickly and put a Dellorto clone on the motor if I'm feeling far too impatient or haven't got time to tune, and I still have jets available to work with.

    Also the stock jets fit, and stock are cheap enough to I don't mind cutting and soldering if I need to while this specialty jet could yield issues if used too much in the testing phase from repeatedly cutting and soldering.

    Well that's about that. Can't say I'm disappointed with the carb, and the small problem was barely a problem since I also ready had the skill and time to finish it, the only tool I needed was the tap set and it even came with drill bits.

    Again this is not a grubee style Cns though I think they recently started using this kind, either way 2 of the ports (odd intake tube right after the filter, and the second one that's closest to the filter on the right side, which is an oil injection point for pw80 motors with the appropriate pump. They both need capping off to work nicely otherwise you get too much air at low rpm and high, one both aren't filtered either, so it's a problem.
     
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