Boring Out Throat of NT Carb

Fast Shag

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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!)
 
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.
 
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...
 
carb throats have a particular geometry, you can't just bore it out and expect it to work well
 
Re:

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 .
 

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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.
 
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.
 
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