Carby NT Carb Tuning Basics

Discussion in '2-Stroke Engines' started by AussieSteve, Dec 27, 2009.

  1. Ivan H

    Ivan H New Member

    Hi Steve, u say to do a plug chop, bring the motor up to running temp, then fit a new plug. I just want to point out that it is adviseable to NEVER remove a plug from ur motor when hot, there is a very real chance of the thread coming out of the head with the plug. Always remove plugs with the motor cold. To do a plug chop, put a new plug in, bring up to temp then do a good longish run at wotever throttle position & proceed as Steve says. Cheers

  2. Krisma

    Krisma New Member

    So, is the picture label for the choke right? Up is "off" no choke and down is "on", choke? Can't read the Chinese on the carb.
  3. Ivan H

    Ivan H New Member

    No mate, up is choke on, down is choke off.
  4. Krisma

    Krisma New Member

    Found choke problem. The 'C' washer was slightly bent and must have bound the spring and let the slider and needle float off the seat. Took a thick alum. washer, cut it out and she worked great.
    Been having another problem getting the idle down. With the idle screw all the way out, it still runs at high revs. Looking at the slider, it seems that the only adjustment the idle screw does is when you go in, CW, it lifts the slider off it seat. Without the screw, it should idle very slowly or not at all.
  5. AussieSteve

    AussieSteve Active Member

    First check that the slide is returning right to the bottom. (It might not be if you have the cable casing adjusted too tightly, you need a couple of mm play.)

    If it still idles high, it's an indication that you have an air leak and the mixture is lean. Check your plug colour to confirm. Good luck.

    *** I'm not returning to MBc, (don't ride any more), but just happened to be passing. No PMs etc, please, everyone.

    ... AussieSteve
  6. rabobanks

    rabobanks New Member

    Thank you

    Thank you aussie Steve for this very detailed post. Helped me out a ton!
  7. Krisma

    Krisma New Member

    Was the needle

    The needle was also slightly bent and had a tiny divot. straightened and blended out the needle, idle is down to where she should be.
  8. ptikiptiki

    ptikiptiki New Member

    Is that the same with the carb speed?
    Last edited by a moderator: Dec 15, 2015
  9. AussieSteve

    AussieSteve Active Member

    Is what the same? I don't understand - carbs don't have a speed. You need to be a bit clearer.
  10. DoIt AllPauls

    DoIt AllPauls Member

  11. Tressie

    Tressie Member

    Great thread AussieSteve.

    Told me everything I needed to know to sort out my 48cc HT's 4-stroking (and possibly detonation?) problem.

    Earlier, I figured out that I needed a leaner fuel/ air mixture, but dropping the sliding needle fully and re-jetting from 0.65mm to 0.5mm wasn't quite enough.

    Lowering the float's closed position by about 2mm to the correct 21mm hang immediately fixed the problem, and I now have a 'sweet' sounding and more powerful (+economical?) engine.


    T :grin5:
  12. gothicguy64

    gothicguy64 Member

    u got to love steves help . I use it and I like u am orig from good ole perth (floreat pk ),but now I say/live in dirty syd ....
    ohh I also have a sbp exhaust I brought from steve ..motor is a rse 48cc hp2 .with more billet than stock even the broken bike was a rse shaft bike ...

  13. 1max

    1max Member

    if I have a thick black coating on my spark plug does that mean its lean?
  14. 1max

    1max Member

    if I have a thick black coating on my spark plug does that mean its lean?
  15. piecepatrol99

    piecepatrol99 Member

    I've lost top end power after boostbottle install :-(

    Hi folks.
    My name is Ryan. I purchased a dual boost bottle kit for my 49cc 2 stroke.
    Before the purchase I had near no low end power but decent mid and top end.
    After installing the bottles, low end went from mouse to charging bull in comparing power. But top end is now barely a house cat with a bad attitude.
    I've adjusted the C clip on the NT carb for better idle (I had to lean it one notch). Still no top end power to speak of :-(
    My question is: does any one have a suggestion on how best to tune the carb (specifically jet change) to get my top end back and bring the idle down a little more?
    I ordered a set of jets and I'm trying to avoid the trial and error phase...

    On a side note: the nut that holds the muffler end cap/tail pipe came loose and bottom end power went through the roof (for such a small motor). I can't/won't run with no exhaust. I'm looking to quiet it more as it is...
    Any help in getting my idle down (with out stalling) and getting my top end power back is greatly appreciated :)
  16. mrbg

    mrbg Member

    I bought a couple of jets from sick bike parts sz #64 that solved the bogging at full throttle. I live in long beach, I'm probably around 75'ASL.I have a question about the float level if you bend the brass tangs down a little to lower the float level will that decrease the amount of fuel in the bowl or do you raise the float to decrease the amount of fuel in the bowl?
  17. mrbg

    mrbg Member

    Thanks for the info, well done. The float height at 21mm, works out to 13/16?does that sound right?
  18. crassius

    crassius Well-Known Member

    float closer to bottom of bowl will mean less fuel in bowl
  19. mrbg

    mrbg Member

    That's what I thought the 21mm or 13/16ths is the spec height? I went to a smaller jet and it ran great for awhile,then it was bogging at wot. I checked for airleak by spraying with water bottle and found a slight leak @ basegasket and I didn't have one handy so I goobered on some copper gasket adhesive it helped a bit. Still won't rev out cleanly so I thought if I lowered the gas height in the float bowl mite help. I know, change out the basegasket
  20. mrbg

    mrbg Member

    forgot to mention

    The cylinder head studs started backing out from centercases I took a shortcut and lifted cyl a bit and backed out the studs and put some loctite in the case's and put it back together with copper gasket adhesive. Frustrating