Boring Out Throat of NT Carb

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.
 
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.jpgPB090033.jpgPB090035.jpgPB090036.jpg
 
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....
 
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.
 
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....

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.
 
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.
 
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.

The stock intake tube that came with motor was 14mm inside diameter..
PB100053.jpg

The intake I replaced it with is 17mm inside diameter...
PB100052.jpg

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