Carby NT Carb Tuning Basics

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

  1. crassius

    crassius Well-Known Member

    if you have acorn nuts on those studs, be sure they're not binding at the top - either break the tops off, replace with regular nuts, or add enough washers to give them more room inside

  2. mrbg

    mrbg Member

    Ya i got rid of the acorns
  3. mrbg

    mrbg Member

    I was riding home last night and my bike started actin funny, it would die when I stopped it would start with full choke and run while I fiddled with the choke between open and closed. I made it home, luckily I was only 2 mi.out. today I cleaned up the motor and started process of elimination fuel line, filter, sparkplug dry n tan. I pulled the carb slide out and the mainjet had rattled out so I pulled the carb cleaned the jet put loktite on,figured while it's open I'll check float height it was at 5/8" or16mm i set it 21mm,i took it for a ride it runs good @WOT,but it has flat spot in the middle, needle is set on topnotch. Anyone have a suggestion?
  4. butre

    butre Well-Known Member

    sounds lean
    mrbg likes this.
  5. mrbg

    mrbg Member

    Ya I was gonna raise the needle a notch and see that helps or raising the float a hair
  6. rcastro3185

    rcastro3185 New Member

    So do i need a new main jet?

    My bike starts to choke out? Bog down? After 75% trottle. Does that mean its to rich or lean? What size main jet would i need? Please help thanl you.
  7. crassius

    crassius Well-Known Member

    could be too rich, but 2-strokes will do this of revs aren't high enough to take extra throttle opening - if you are slowly building up speed & this happens, I'd try lowering float level to see what happens & if it helps, I'd try a smaller jet
  8. willwills90

    willwills90 New Member

    hey guys, what is the advatage of these 35 euro high performance racing carbs anyway?
  9. crassius

    crassius Well-Known Member

    they allow more air to flow when you have made performance mods to the motor
  10. bikejock

    bikejock Member

    Great tuning tips for the NT. This little carburetor is a good design. Simple & way less quarky than the 4 stroke carburetors the HS142 Honda clone engines use which I've had nothing but problems with. The NT carbs almost never seem to fail & are easily modifiable. I replaced a 4 stroke carb with the 2 stroke NT carb on my HS142 4 stroke engine & it starts up every time with no issues. Gas mileage seems to be a bit less with the NT carb on there but I think it was worth it.
  11. mrbg

    mrbg Member

    Float Bowl Gasket ^

    Also see srdavo's Carburetor Assembly Pics for a bit more info.

    Brief Description Of Operation
    The NT side-draught carburetor operates by drawing air into the air inlet past the slide needle jet. 'Venturi' action draws fuel from the float bowl upwards into the in-going airstream.
    The slide regulates the flow of air into the engine. A slide needle extends below the slide into the slide needle jet to regulate fuel flow during low to about ¾ throttle openings. When the throttle is opened further, from ¾ to full throttle, the 'main jet', screwed into the bottom of the slide needle jet, regulates fuel flow. The slide needle is adjustable, up/down, to increase, (enrich), or reduce, (lean out), the amount of fuel in the mid-range mixture. To adjust ¾ to full throttle mixture, the size of the main jet must be changed, by replacement usually, although drilling or soldering and drilling is an alternative. Larger = richer and vice versa.
    There is no idle circuit, pilot jet or idle mixture screw on these carburetors. The slide needle setting also controls the idle mixture.

    Specs vary with individual carbs and suppliers.
    For a 66cc engine, I've heard of stock main jets sized at 0.6mm,(MBB Imports), 0.7mm,(Many US kits) and 0.79mm, (ZBox 66cc).
    In the end, many seem to run well on about an 0.72 to 0.73mm main jet, with standard exhaust and porting.
    Bore diameter: 14mm.
    Float level: 21mm.

    The NT carb supplied with most typical HT engine kits is very simple and so should be easy to tune, but many have trouble getting it right, resulting in low power, low revs, smoky exhausts, fouled plugs and excessive buildup on heads/pistons if the mixture is over-rich or, in the other extreme, overheating, melted pistons, seized engines if the mixture is (way) too lean.

    The most common problem is a lean mixture due to an air leak where the carburetor attaches to the inlet tube or where the inlet tube attaches to the cylinder.

    The second most common problem is a rich mixture, (usually after sealing the inlet leaks), resulting in four-stroking and reduced power output, often in both the mid-throttle openings and at full throttle. See 'Four-Stroking', in 'Diagnostics' below.

    N.B. Before attempting any carby adjustments, ensure that the carb to inlet joint is well sealed, the inlet gasket is in good condition, the carb is clean, the fuel filter and air filter aren't blocked and that the inlet tube nuts are tight.

    There is only one direct method of mixture adjustment on these carburetors. This affects low to ¾ throttle openings. The slide needle has a series of notches near the top, (four on earlier carbs and five on the later ones), so that the attached 'C' clip can be removed and refitted to raise or lower the needle relative to the slide and slide needle valve.
    When the needle is set lower, relative to the slide, less fuel is allowed into the mixture for a given amount of air, making the mix leaner and vice versa.
    To adjust the mixture at ¾ to full throttle openings, the size of the main jet needs to be changed, smaller for a leaner mixture or larger to make it richer.

    One of the best ways to check the mixture is by doing a 'plug chop'. In effect, the engine is first warmed up to operating temperature, then a new plug fitted. Run the engine at the throttle opening for the area of concern for a bit.
    ie. Slide needle setting - about ½ throttle.
    Main jet - full throttle.

    Now, without changing throttle opening, pull in the clutch, hit the kill switch at the same time and roll to a stop.
    Check the plug color. If it's tan, the mixture in that range is OK. If black and/or wet, the mixture is too rich. If the plug is grey or whitish the mixture is too lean.
    I had thought about including pics of plugs, but don't have any that aren't tan, right now.

    One of the more common signs of a rich mixture is 'Four Stroking'. This occurs when the fuel/air mixture is so rich that the engine only gets enough air to fire every second revolution, reducing power and making the engine sound like it is only doing half as many RPM. An overly smoky exhaust is another indication that the mixture may be too rich.
    An example of 'Four Stroking', (thanks Al.Fisherman, for pointing it out):-
    Four Stroking (YouTube)

    Any corrections or other suggestions?

    (If this has been useful, please give it a rating.)

    ... Steve[/QUOTE]
    FurryOnTheInside likes this.
  12. mrbg

    mrbg Member

    I got a email about using the the NT on the Huashang. Where's the info for swapping out the stock carb with the NT ? I've got a NT with 66 thru 70 jets, I'm at 100' ASL.
  13. bikejock

    bikejock Member

    I use a manifold adapter I bought from bikeberry. Had to get shorter bolts & cut out a new gasket for it though but works like a charm.
  14. mrbg

    mrbg Member

    Thanks I'll take a look on bikeberry . what about the jet
  15. Ed kendall

    Ed kendall Guest

    Me and my son are building a motorized bike it's sort of a cheap 80cc black motor but we ordered us high performance carburetor the motor is bogging down when we give it gas and not running properly we can hear that I've tried changing the spark plugs do you have any ideas on how I might be able to make this motor run better it will barely pull you down the road without pedaling thanks for your time