NT Carb One Piece Main Jet Blues

Discussion in '2-Stroke Engines' started by Quanah, Jul 13, 2014.

  1. Quanah

    Quanah Member

    I would have thought this had been covered here, but I can't seem to find anything about it. I recently got a 66cc china motor kit and the main jet is one piece. Seems like I read that the old carbs had a slide needle with 4 grooves, and the newer ones have 5. Mine has 4. So I guess mines the older one? I have never seen a carb that didn't have the main jet that unscrewed from the slide needle jet tube. I haven't started this motor yet, but I am sure I will want to play with this main jets sizes when I do. I would also think that the type I have seen before would interchange, but can't seem to find the slide needle jet tube I need, jets aren't hard to find...checked all the major sellers of these china girls. Anyone with same problem?
     

  2. Quanah

    Quanah Member

    Maybe a picture is worth a 1000 words.
    [​IMG]
     
  3. Quanah

    Quanah Member

    Here it is. Looks like it comes apart, but it doesn't.
    [​IMG]
     
  4. crassius

    crassius Well-Known Member

    yes, that's the old style metering tube & jet

    that jet unscrews, but sometimes it can corrode so much that it is stuck pretty tight

    no need to take it off if you use the solder & drill method of changing jet size
     
  5. HeadSmess

    HeadSmess Well-Known Member

    agreed, thats the plain old jet and tube.

    and i hate it when the jet wont release!

    usually end up cranking the tube in hard then just tweaking the jet in with needle nose pliers, and then next time...the tube releases first again!!!!!


    AAAAAARRRRRGGGGHHHHH!!!!!!!!
     
  6. Quanah

    Quanah Member

    I'll look at it again, but I swear it is all one piece. Thanks for the input, I'll let you know what I find out.
     
  7. HeadSmess

    HeadSmess Well-Known Member

    think about it like this...

    why would they manufacture it with two hex's of two different diameters, that are slightly out of line with each other?

    simple. they dont ;) they use two separate pieces of hex bar in auto feed lathes that spit them out at a rate of ten a minute or so...

    (first hex tube ive seen now i think of it...mine are always round with two flats at the end... anyway. same thing.)


    that jet hole does like slightly large, so kudos for wanting to get a smaller one.

    ^^^crassius' suggestion^^^^ solder and drill it, the drills can be re-used, so a better purchase than new jets ;) other way is to use bits of fine copper wire but that can take some skill with a soldering iron...
     
  8. Quanah

    Quanah Member

    Well they did come apart. hard to believe how tight they were. I'll more than likely go for jets, because I don't have a set of numbered drills...although I have been wanting a set for some time now... we'll see.
    [[​IMG]/B]
     
  9. Quanah

    Quanah Member

    Oh yeah, this jet doesn't have a number on it, another reason to get a set of numbered drills, I could use them to find out what size the existing hole is.
     
  10. Fabian

    Fabian Well-Known Member

    The jet is designed to be a tight fit into the tube so that fuel can't leak down the jet threads and artificially richen the mixture over the set jetting size.

    After you've spent time running different sized jets in and out of the tube, it pays to buy a new tube so that the jets are once again a tight fit.

    Drilling out the jets to larger sizes is fraught with danger because a home drill press doesn't have the required accuracy of precision machinery.
    For the NT carburettor, i have a selection of 26 jet sizes from #68 to #90, though the widest extremes for accurate jetting on different 2-stroke Chinese engines has been between #70 and #86 with the average mostly hovering around #76 - #78, using ethanol free Australian lower south eastern 98 octane fuel. Someone on here said that Australian 98 octane works out to American 93 or 95 octane - can't remember exactly which one.
     
  11. Quanah

    Quanah Member

    I've heard it said to Loctite the jet...that just has "WRONG" written all over it. 10% ethanol is normal around here.
     
  12. Fabian

    Fabian Well-Known Member

    There is no need to use any kind of thread lock on the jet or the tube. They should both be a firm fit into their respective screw threads. In all of the hours of use my bike has had with the NT carburettor/s, i have never had a jet or tube come loose - NEVER.

    Whenever the jets no longer fit firmly into the tube, i simply purchase a new tube - problem solved and reliability is guaranteed.

    If you can't get ethanol free fuel, you will have to jet the carburettor accordingly; ensuring an optimal air/fuel ratio.
     
  13. HeadSmess

    HeadSmess Well-Known Member

    drill press? huh?

    anyways. fractional drills are different to number drills which are yet again different to metric incremental drills.

    the jet numbers are usually stated as so many tenths of a mm...ie, a 65 is 0.65mm. so do NOT order number drills unless you know what size that number actually refers to!

    http://www.smithbearing.com/pdf/ENG-FractionalChart.pdf


    note the numbers get LOWER as the drills get BIGGER!


    just buy a micro drill set, 20 dollars or so on ebay, and a pin vice chuck. metric micro drills. the fractionals are only really worth having on super high performance engines.

    usually, each manufacturer has their own standard, and let it be known now...mikuni jets (not all of them...i believe this system starts at 70 and over) have no relevance to diameter of hole, but to the amount they flow...two different sized holes, at the same pressure, can still flow the same amount of fuel, due to the surface finish, and the length of hole. and mikuni is the best! reamed and polished holes! but manufacturing tolerances decrees that no two jets are EVER exactly the same.


    anyways, i usually use a .55-.60

    ive had one that loved a .45!

    never gone larger than stock, which is usually around .75. they come rich as standard so they will always run... albeit rather badly.

    octane ratings are a joke, i just use plain old 91 ethanol...strange that BP has their ethanol as a 91, the low stuff, whilst others rate their ethanol mix as 95, or the standard stuff. in au, we get 91, 95 and 98 on tap... yes, our RON is not the same as the US RON.


    honda proved conclusively that they could reduce the octane to 70 or so, simply by letting their engines rev out to 16000rpm or so.... detonation is due to a combination of low speed, high loads, and high compressions.

    getting detonation? drop a gear, increase the rpm, reduce the load. simple.

    cant do any of these cus you already at redline? change fuel to higher octane.

    i run my engines fast, i dont need hi octanes. simple.

    hi octane fuel doesnt make more bang...it simply burns slower, and is LESS LIKELY to go bang! you will make more power with a low octane fuel.

    dont believe me? go do the research yourself and prove me wrong :)


    then also explain to me the difference between octane, iso octane, and the other variants of the octane molecule ;)
     
  14. Fabian

    Fabian Well-Known Member

    or you simply use the highest octane (non ethanol pump fuel) you can get if wishing to run at (engine preserving) reduced rpm; staying somewhere near the maximum torque band with the same load and the same compression ratio. To heck with any worry - just run the highest octane number and be done with it. Who gives a damn if it doesn't increase power for the low level of fuel consumption and low fuel cost of the extra octane number when paying at the cashier.


    Only if the compression ratio and port timing is altered to allow the bejesus to be revved out of the engine.
    Not everyone wants to rev the life out their engine - some people actually prefer reliability and prefer to ride sensibly than to just rip up and down the street; making a complete nuisance of themselves in the neighbourhood.
     
  15. HeadSmess

    HeadSmess Well-Known Member

    ahem.... please quote the next line next time :)

    and at 1L every 50km or so...even my old v8 sandman was better on the gas... mind you, that wasnt run WOT 99% of the time.


    remember fabian, youre the one that destroys con rod bearings, has spent countless frustrated hours and dollars searching for answers...

    not me ;) my role is simply as an advisor, always has been.
     
  16. Fabian

    Fabian Well-Known Member

    You only need to do a 3 second search and you'll find a city full of people who have had connecting rod bearing issues, though a reasonable number is caused by excessive rpm and the rest by the crappy stock CDI.

    My bike just keeps on ticking (sometimes audibly) but for a good period of time, it's been giving me perfect reliability.
    I don't carry tow ropes with me for my own needs but to haul other people's arses out of trouble with waves of low rpm torque.
     
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