Carby Re-jetting NT carb to prevent four stroking- what size jet to try first?

Discussion in '2-Stroke Engines' started by rowens, Jun 24, 2012.

  1. rowens

    rowens New Member

    Hi everyone, I am a new builder and have just successfully completed a Skyhawk installation. Thanks for this forum, I would be lost without it, having no experience with engines and my build would likely be sitting in the garage unused due to performance issues.

    Through reading posts here, I have determined this engine has four stroking- wet black plug, runs rough and jerky at all throttle positions except at WOT going up steep hills it will run smooth. I watched the YouTube four stroking video and is exactly what my engine is doing.

    I have moved the needle see clip to the top groove - still four stroking. I assume the next step is to get and install a smaller carb jet but I have several questions. Please forgive my ignorance in this.

    Q - what is the thread size of jet of NT carb, 5 mm or 6 mm?

    Q - what size is the stock jet of the NT carb? Can I go two sizes smaller and adjust the needle depth, or is it better to downsize one step at a time, I see there are about 11 sizes from 0.78 to 0.59 available at Sick Bikes and I would rather not buy them all to try.

    Any help would be appreciated - sorry if I sound like a cheapskate not wanting to buy lots of carb jets but I am a senior, and on a very tight budget. ---Rob in Port Hope, Ontario.
    Last edited by a moderator: Dec 15, 2015

  2. tooljunkie

    tooljunkie Member

    i should have taken pictures when i soldered my jet and drilled it two number drills smaller.
    i bought a set of number drills from princess auto for 4.99. ships without any issues,been doing it for years.
    i soldered the jet closed and drilled it two sizes smaller.instead of putting drill bit in drill,i put jet in drill and held bit to drill hole.
    i had a welding torch tip drill set,i used the bit holder (pin vise) to hold the bit.small vise grips would work too.
    my engine is new,only 40 km's on it and i didnt want to get too lean during break-in
    it still four strokes,less now and seems to improve as i drive it.
    after a hundred km's i will re-jet again.

    so far top end is 46 kph.
    hope this helps.
  3. Fabian

    Fabian Well-Known Member

    The best way is to purchase every jet size between Number 60 and Number 80.

    Start with the standard jetting (approx Number 65) and go richer and then leaner by 4 jet sizes each way. The engine will run either better or worse.
    This will give you a clear indication on which direction to take regarding fine tuning the jet size.
  4. rowens

    rowens New Member

    Thanks for your quick reply.... I am new to carbs... OK, the way you suggest I probably won't have to buy more than 3 jets - to get it right, then I can(hopefully) fine tine with circel clip on the needle.
    Does the NT car take a 5mm thread or 6mm, do you know? I don't have a caliper to measure it.
  5. rowens

    rowens New Member

    I didn't know there was number dill bits that small available to regular guys. I know how to solder (ham radio operator VA3OWE) so I could try this if I can get the bits. What size is "two number drills smaller"? thanks for the tip on jet in drill - makes sense, but I would havenever thought of it myself. What a great forum!
  6. Fabian

    Fabian Well-Known Member

    @ rowens:

    That's what i said: " I probably won't have to buy more than 3 jets - to get it right,"


    i now have a box with 20 jets
  7. Greg58

    Greg58 Member

    You can get a micro drill set from amazon for less than 10$, the one I have has a hand drill with it. Stock jet on my nt was #71 drill bit. Drill bits #60-#80,hand drill, plastic case.
    Last edited: Jun 25, 2012
  8. rowens

    rowens New Member

    Hi everyone-thanks for all your help - what great resources. If not for this Forum I would be less than satisfied with my running build.

    I bought the mini drill bits today for $5 at Princess Auto- trying the bits in my stock jetto determine it'se size of drill bit that fits the jet now is drill #71 = .026" or .66 mm.

    So if I go two drill numbers smaller = bit #73 - .024" or .61mm
    Wish me luck, these drill bits are so small they look fragile. I'll go slow, use cutting oil and wear my safety glasses! Rob in Port Hope, ON.
  9. Fabian

    Fabian Well-Known Member

    Realistically, and on DIY home style machinery you have no hope of drilling a perfect diameter hole. It's hard enough to maintain consistency using big dollar commercial equipment when drilling micro holes.

    If wishing for accuracy in the tuning process, you've got little alternative but to purchase a good (numerical) range of genuine Dellorto jets.

    Just fork out the cash and be done with messing around...
    Anthony 4k likes this.
  10. rowens

    rowens New Member

    Hi All,

    I soldered shut the jet hole and drilled the .61mm new jet, installed and test drive. Engine now smooths out nicely when given a hard acceleration, but still 4 strokes. Top end speed has improved, but is new engine withless than 50 km on it, so am not holding WOT or top speed longer than 10 seconds for now. I will stay with this for a while and see what develops.

    Note to Fabian- you are right, getting a centered hole with smooth edges is darn near impossible. I have ordred three Deloro jets from Sick Bikes so I can spend more time riding than repairing, but I am learning a lot with hands-on and discovering the `magic`that carbs do.

    Rob on Port Hope, ON.
  11. Greg58

    Greg58 Member

    Or buy one jet three or four sizes smaller, you already have the bits. Try the jet each time you drill it, one size bigger at a time.
  12. Fabian

    Fabian Well-Known Member

    I've been through this process and if i were to do it again (starting from scratch) i'd be ordering a twin needle Walbro carburettor from one of the Vendors.
    In Australia, Rock Solid Engines carries these carburettors on their website.

    No more jets to fuss about and you have independent low speed and high speed mixture screws that can be adjusted on the fly - reliable, dependable and just brilliant for a motorized bicycle application.
  13. Fabian

    Fabian Well-Known Member

    The best way to go is Electronic Fuel Injection.
    One of the Vendors on here is working on it but the time line for public release is unknown.

    One thing is for sure: i want to be the first person to have a laptop programmable Ignition system and EFI system and i don't care how much it costs...
  14. tooljunkie

    tooljunkie Member

    lol,Fabian i know what you mean.
    as a diagnostic tech for computer controlled cars,i had to deal with efi faults almost daily.
    i'm out of it now,and know i never have to go back.
    i like efi,as long as its durable.

    MegaSquirt has a laptop programmable efi computer.1 cylinder to 12 cylinder.
    runs up to 10 or 12,000 rpm's.
    been a while since i looked at the site,if it even still exists.
    the biggest issue would be powering it.
  15. Fabian

    Fabian Well-Known Member

    @ tooljunkie

    My bike is already equipped with a 12 volt system, so powering the EFI is no problem (depending on the current draw) but i'm very interested to hear about you method of getting such an system to work, i.e. the listed weight, the physical size, the method of mounting and the complexity of installation?

  16. tooljunkie

    tooljunkie Member

    i would think megasquirt would have something that would work for you,do a search for megasquirt and if you make some enquiries i think you would get an answer.
    the show efi for motorcycles.
    they used to have a build your own system,you buy the injector,and various other parts.
    happy hunting!
  17. Anthony 4k

    Anthony 4k New Member

    Plug chops
  18. Adam88

    Adam88 Guest

    Just to chime in, whoever said the stock nt carb jet is a 64, is way off. I'm a professional tech and build engines for a living, the 64 jet in my kit is twice the size of the stock nt or speed carb jets, FYI.

    With a reed valve, v stack and filter, expansion pipe, decked jug and high comp head I run the by carb with a 60 jet, still runs rich. I just ordered a kit for 30 through 60. Btw the nt carbs accept Holley style jets. Good luck with your build. Btw these engines come out of the box with vacuum leaks, tighten things up before you rejet. They tend to leak at the crankcase seal, manifold gaskets, the carb flange, the carb throttle bore where it accepts the cable, an oring under the cap helps allot, also an o ring between flange and carb helps. If you is3 the rce reed valve, it leaks air out of the box as well, just disassemble and coat gaskets with a fuel resistant sealer, and do not use the flange gasket it comes with it's trash. Good luck. Btw my top speed is just under 42 mph. Scary on a bicycle