Carby my carb issues

Discussion in '2-Stroke Engines' started by dotcom, Dec 10, 2014.

  1. dotcom

    dotcom Member

    My carb is the standard NT carb and I have the needle lowered some to increase performance but im wondering what to do when it bogs when I hit faster speeds and cannot stay full throttle. When I hit faster speeds, it pulls very well but not tuned properly so I can go full throttle and cruise. I have another bike that it seems I have adjusted it right where I can go full throttle and it won't bogg but this bike will bog if im at faster speeds and I can never stay on full throttle for more than a few seconds without bogging. Is it still getting too much gas? new jet? put the needle back? Very useful board

  2. Purple Haze

    Purple Haze Active Member

    Put a #66 jet in it. You are running too rich, they set them that way from the factory so they will start easily. Jets are available from SBP among others. After you do this, set the needle in the 2nd from top groove. This should make for a nice running bike with snappy throttle response.
  3. dotcom

    dotcom Member

    thanks I'll try it out :)
  4. Fabian

    Fabian Well-Known Member

    Fuel formulation varies around the world and even between states.

    In Australia, my engines seem to run optimally (on lower south eastern states 98 octane, which is similar to 93 octane American pump grade ethanol free fuel) with a jet size that's either dead on or somewhere close to a No 76.
    My current engine uses a No 77 for optimal carburation.

    Having said that, one of my engines needed a No 70 jet to run optimally, and another engine needed a No 86 jet to run optimally (of which both made the same power, by giving the same top speed on my favourite dyno hill), but for the most part there seems to be some level of consistency around a No 76 jet size.

    What is more interesting is that the fuel economy doesn't change (when the engine is tuned for optimal carburation), regardless of jet size.
    I consistently get 60 miles of trip distance out of my fuel tank, regardless if the carburettor is fitted with a No 70 or No 86 jet. This seems completely impossible, and to this day i am still scratching my head as to why fuel economy doesn't change when you can have two engines tuned to perfection, and both run different jet sizes for optimal carburation, and both return identical trip distance per tank of fuel.

    My experience has shown that there is no magic jet size number.
    I have found that the quickest way to optimise jet size is to start with a known that is on the rich side for the fuel composition in your area (in my case i start with a No 90) and then work backwards by 3 jet sizes at a time, till you get close to the butter zone, then start messing around with single number jet size increments to optimise the carburation to perfection.

    Having said that, a float style carburettor like the NT or the Delorto is no where near as user friendly for jetting adjustment or stays as clean as a Walbro style diaphragm carburettor.
    Last edited: Dec 16, 2014
  5. dotcom

    dotcom Member

    Its been some time since ive posted and having similar issues with the carb. The carb is the RT carb.
    As soon as I start the bike it runs perfectly like a champ better than the other stock carb but only for a few minutes, then no longer performs perfectly and then struggles pretty bad to get even close to top speed like when I first started it. This this same thing happens everyday. If I switch carbs back to stock, the stock runs normally so can't have anything to do with compression. I doubt it has anything to do with anything except the jetting because there is not much tuning that can be done to the carb besides rejetting. This one does not have a needle setting so I think the only option is to switch the jet but im wondering which jet. I have #60,#65,#70,#75,#80. I think the one in it is #70
    When I first start it, it takes off and rides wonderfully but after I come to a stop and try that again, I can't.
    Is this in fact caused by the jet by the way I explain the symptoms?
    Please and thanks again
  6. crassius

    crassius Well-Known Member

    first, I'd try raising & lowering float level to see if you're rich or lean, then re-jet with that info

    note that it takes about 8 miles to get those huge, steel crank wheels, which sit right in your intake path, up to a stable operating temp
  7. MotorBicycleRacing

    MotorBicycleRacing Well-Known Member

    65 jet because it's too rich.
    70 is stock jet.

    All those jets you bought are worthless except for the 65.

    The plastic float on an RT carb is not adjustable.
  8. jaguar

    jaguar Well-Known Member

    it likes the overly rich jetting while its still warming up but then once it gets up to normal temp it doesnt like it.
    thats the way two strokes are and why the carb has a choke on it to make it richer while warming up.
  9. dotcom

    dotcom Member

    Now im working with a slightly newer motor with the stock carb and #66 jet but it struggles just to take off and the idle screw is all the way in. I have to pedal alot just to start it and it still struggles even with alot of pedalling but eventually it will go fast . Then once I think its running and I have to stop, it idles but struggles to idle anymore and dies. Needle was on 2nd from top, then I tried 3rd from top and idle screw all the way in. Still basically same thing. Do I need a different jet size? Bigger? smaller?
  10. jaguar

    jaguar Well-Known Member

    The main jet just affects the mixture at full throttle.
    try all different needle clip positions. if none of those work then it could be that you have an intake leak, crank seals leaking, or loss of compression at the head. or an ignition system problem.
  11. crassius

    crassius Well-Known Member

    usually, if idle is all the way in, then one needs to drop the needle down to get a leaner mix (this should allow idle screw to be backed out several turns)
  12. KenM

    KenM Member

    Hi all, I am trying to get my carb setup right too. A stock 66cc setup.
    With the needle set in the top notch, it is to lean, runs like a champ but can not get a good idle, either to fast or to slow and stalls. And the spark plug is clean, no trace of carbon at all.
    In the second notch it is to rich, runs good, idle good, spark plug is a little black with carbon.
    Is there a way to fine tune it in the middle of the two notches? Or just run it on the rich side? Thanks in advance! Ken.
    Keep looking up!
  13. jaguar

    jaguar Well-Known Member

    these carbs are not truly tunable, period

    get a good one
    KenM likes this.
  14. crassius

    crassius Well-Known Member

    raising/lowering float level can get you 1/2 a notch
    KenM likes this.
  15. KenM

    KenM Member

    Thank you Mr. Crassius! I will have to try that out.
    I should of thought of that myself. I used to do set the float level on the Britsh motorcycles I had all the time, and then their were two of them. Then you had to syn. then. Ok , thanks again! Keep looking up! Ken.
  16. jaguar

    jaguar Well-Known Member

    yeah but it affects the jetting throughout the whole range equally
  17. HeadSmess

    HeadSmess Well-Known Member

    a few tricks.

    start at the beginning.

    basic carb circuits. (NT specifically)

    the idle mixture is controlled by the small wedge "cutaway" at the BOTTOM of the slide. not to be confused with the IDLE SLOT thats also at the bottom of the slide.
    a flat bottom slide is RICH. as the angle of this cutaway increases...they get leaner.

    so thats the IDLE circuit. normally, something you completely IGNORE unless you really know what youre doing...because the only way to go richer than the stock slide, is to make one. applying a file makes them leaner.

    (though you can file the base flatter, i guess. but that will also affect where the needle sits in the jet, which would then need raising blah blah )

    theres no pilot screw, so no need to worry about the idle side at all.

    the NEEDLE and the clip on it controls HALF throttle. mids. raising makes it richer, etc.

    it DOES NOT affect the top end.

    the only thing affecting WOT is the size of the jet. and the lil hole in the spraybar that the needle slides into.

    if it DIES at FULL throttle...its too lean. get a bigger jet.

    if it slows down but still makes power at full is too rich. get a smaller jet.

    i dont like this term "bogging" as some people think its when it dies at full throttle...other people think its when it just doesnt want to reach full rpm. and therefore, its hard to tell what someone means.

    now, just for this purpose, i keep an ngk 4 heat plug. they stay clean.

    throttle chops. start at the full throttle part. then half throttle, and finally idle if you want to bother. warning. get the idle bordering on too lean, and the smallest air leak will cause problems. why its best to overlook the idle part.

    after all this, performing good plug chops (warm the engine up properly or they will be false readings... as jag says, twostrokes (all engines really) start rich. and the rich condition gets progressively worse as they warm up. whereas a lean mix will richen slightly as the engine warms up. youre finding the line between being able to start easily, near maximum power when running, and longevity...)

    if the plugs coming out black...its rich.

    if its white, its lean. if its just starting to show some colour, perfect.

    then you can go back to a more standard plug, like a 6 or so... which should go tan or brown but never black...

    and of course, the plug chop is not perfect... its a guide. the heat range of the plug, and the oil (content) affect the reading. the real test is does it FEEL more powerful?
    Last edited: Jan 4, 2016
    KenM likes this.