Boring Out Throat of NT Carb

Discussion in 'Performance Mods' started by Fast Shag, Nov 3, 2014.

  1. Fast Shag

    Fast Shag Member

    Have any of you tried boring out stock NT carb with positive results? I know....Just go out and buy a better carb, but I want to play with what I have for now. Is the stock intake larger than stock NT carb? I know that any small adjustments go far with these motors....But if I could get just a little more air into this thing I think it would help. I'm not ready to pop the cylinder off yet and port match, but I have done some work matching up on the intake tube and jug as well as doing the same for the crappy SBP exhaust mount. (That was very disappointing to see that a US made part was so poorly made! The welds were horrible, and the flange was cut so bad and matched up it took me almost an hour to clean it up with a Dremel and match it to the gasket and jug!)

  2. Purple Haze

    Purple Haze Active Member

    Yes, I tried it way back when I first started building these things. I over-tightened the throttle housing, distorting the bore. Needless to say, it didn't work out well. Just get a "speed" carb which has a larger bore and be done with it. Actually, the larger bore seems to make these engines run and idle better.
  3. Fast Shag

    Fast Shag Member

    My flying Horse kit came with the Speed carb, but the housing around the fuel shut off valve broke! Bike Berry said it happened a lot, so they replaced it with the NT carb! I asked if it would hinder performance, they said no. Now I'm thinking they got away with replacing a $25 carb for a $12 one! Oh well. I was wondering what places you started boring out so it didn't interfere with the slide...
  4. butre

    butre Well-Known Member

    carb throats have a particular geometry, you can't just bore it out and expect it to work well
  5. Robot

    Robot New Member


    As I was about to mention as attempting to post a picture, the engine if it's a 66cc works off drafting and regardless of what people claim it can't be forced fuel. The picture here simply an exaggeration of why NOT ENOUGH VACUUM .

    Attached Files:

  6. Fast Shag

    Fast Shag Member

    lost me on that pic robot.....
  7. Fabian

    Fabian Well-Known Member


    There is nothing wrong with the power output of the operation of the standard NT carburettor once it has been jetted correctly.
  8. Fast Shag

    Fast Shag Member

    Sold. I'm not going to mess with it... Thanks for the input guys.
  9. darwin

    darwin Well-Known Member

    Reply to butres post above. You sure about that on these small engines, few carbs I've pulled apart seemed to just have smooth bore throats. No rifling or anything that I noticed. I was thinking a brake honer would work to expand it. Like the tool you use on drum brake cylinders with a drill...............could be wrong.
  10. Fabian

    Fabian Well-Known Member

    You need some level of atmospheric pressure drop where the fuel enters the air stream, which is why venturi is placed at the exact position of the needle and jetting tube.
    if you bore the throat out to be a smooth bore, you will find that the carburettor has terrible transition from idle to wide open throttle, if the throttle is opened quickly.

    There is good reason why manufacturers match the carburettor bore to the venturi diameter. Messing with those dimensions will degrade the transition between idle and wide open throttle.
    You can get around the problem by jetting the idle very, very rich but you will need to have quick rpm transition between idle and higher rpm.
    If you were to lug the engine in the lower midrange, the carburettor would be likely to give a lean air/fuel mixture because it lacks a venturi, which would manifest itself by the engine bogging down.

    If you are racing and have no need for good transition and only operate the engine at high rpm and wide open throttle, you can get away with boring the heck out the carburettor, but it's quite likely that it will need to be tow started.
    Fast Shag likes this.
  11. Purple Haze

    Purple Haze Active Member

    Oh well. I was wondering what places you started boring out so it didn't interfere with the slideI didn't bore it, per se; I have a very small hone which I used to get the slide to move freely in a distorted bore. After doing this, the engine ran exactly like Fabian described.
  12. Fabian

    Fabian Well-Known Member

    Completely lost me too!
  13. Fast Shag

    Fast Shag Member

    I didn't touch it. I ask questions first then thrash! :grin5:
  14. Fast Shag

    Fast Shag Member

    So here is the whole story.... My 66cc kit came with a Speed carb. The fuel valve broke off and cracked carb housing. They sent me a new free NT instead. Most of yall think the Speed works slightly better than the NT carb due to the fact that it has a larger bore and jet. So I decided to play around with it to see if I could repair it for sh#%ts and giggles. PB090032.jpg PB090033.jpg PB090035.jpg PB090036.jpg
  15. HeadSmess

    HeadSmess Well-Known Member

    NT carb is a parallel bore type. basic and cheap to make. they do the job.

    yes, they can be bored out, and yes it does help, and to further elucidate on previous posts... by boring a TAPER (the cut should just be finishing as you hit the slide, about 1 degree or so), you actually get the venturi to work properly, requiring a much smaller jet...

    and it weakens the bore. memory tells me that it was a right pain in the donkey to set the job up in the lathe, and then it broke after 3 months or so...

    wasnt really worth it :)

    neither is the mikuni vm18... hard to fit, and with a bore that about 3mm larger than the intake manifold, rather stupid actually...

    its been said before.... the NT does the job.these arent hi powered race machines, but docile, slow lumps of mutated beer cans... two strokes have a built in air/fuel blender/homogenizer... the carb is rather a moot point, other than supplying the the correct mixture....
  16. Fast Shag

    Fast Shag Member

    I was thinking about the concept of rounding out the throat up to the slide on both sides to let air role in smoother. Like blueprinting and rounding sharp edges on motor...Now that I think I've fixed the Speed carb, I'll just sit on the idea...At least I have 2 carbs now.
  17. Fabian

    Fabian Well-Known Member

    Couldn't agree more.
    I find it humorous to see people adapting lage bore carburettors to fit on the standard intake tube, then making claims that the engine has increased performance. Considering that the internal intake tube diameter hasn't increased in size, it is impossible to have a performance improvement with a larger carburettor, in such a situation.
  18. Fast Shag

    Fast Shag Member

    I slapped the Speed carb up on the bike today and there was large improvement in throttle response and got 2 mph better on top speed. The low end came outta the hole much better! Same jets, and needle placement from NT carb. Oh, and I fixed the leak! Is this saying that a slightly larger throat and slide will improve performance? I'm sold on that idea. FABIAN, Now you should all know that the intake tube on my bike is made of MUCH skinnier metal that the stock one that came with it, same outside diameter. There is more air going into the motor now. I guess there is no need to bore out, or mess with anything on the NT carb, given the fact that it would most likely ruin the carb. I do believe that the NT carb was made for the 49cc motor and Speed carb for 66cc motor. Correct me if I'm wrong.
  19. Fast Shag

    Fast Shag Member

    The stock intake tube that came with motor was 14mm inside diameter..

    The intake I replaced it with is 17mm inside diameter...

    I agree completely on what you guys are saying. I think the smaller tube was cheating the 66cc motor of air/fuel. This is why I believe there are performance gains in switching to the Speed carb, or having a larger bore.
    Last edited: Nov 11, 2014
  20. jaguar

    jaguar Well-Known Member

    this chart shows the minimum need bore diameters depending on the engine size and maximum RPM: