I bought a Speed Carb (marked NTTC) from EBay and an intake manifold to test and make recommendations. Here's its specs: 15mm venturi diameter, .52mm main jet, 1.07 degree needle, 20mm float height, .8mm slide cutaway, 1.85mm needle diameter, and this one unfortunately didn't have the desirable spring loaded float needle that some speed carbs have. A jet kit is available for it from bicycle-engines.com that has 4 main jets, size .65mm, .7mm, .75mm, and .8mm. Treatland has a set of 5 jets from .6mm to .68mm for $12. I had to enlarge the main jet hole to .6mm for best running on my 48cc piston port engine with 175/120/120 port durations, and then to .75mm for my 48cc reed valved engine.
I noticed during assembly that the carb was hitting the ridge on the intake manifold which meant the manifold was not fully entering into the carb to seal well. This might allow an air leak. So I cut off 1.5mm from each of the 4 carb petals where it is marked with a black line in this photo. This may not be necessary with all intake manifolds but should be checked.
This pic shows the cut carb which allowed the manifold to enter fully without butting up against the carb petals.
Here you can see how rough the end of the intake manifold was. Both of its mating surfaces needed to be planed flat using sandpaper on a flat surface.
One other strange problem is that if the needle clip was put on the lower 2 clip positions then the needle would pull up out of the needle jet at full throttle which would make the carb mixture suddenly rich. So only the top 3 clip positions should be used.
Piston Port Intake
My 48cc piston port engine has a 175 exhaust duration and 120 transfer duration, and 120 intake duration. It has a balanced crank and it revs to 8000 RPM. With the needle clip on the top position the stock .52mm main jet had to be increased to .6mm which I did by using micro drill bits. It wouldn't hold idle at a normal idle speed and so I had to turn in the slide stop screw till the slide was 2.75mm open so it would have a high idle speed. Possibly at sea level the idle problem wouldn't of existed because this test was done at 3000 ft elevation. The higher up you go the leaner the jetting should be, including the idle mixture. Someone with a ported 66cc chinese engine (piston port with stock pipe) said his Speed Carb needed a .72mm (#72) main jet and his needle clip at middle position for it to run right at 1000 ft.
Reed Valve Intake
My 48cc reed valve engine with nearly the same durations had to have a .72mm main jet and the needle clip at the middle position. Idle slide height was 2.25mm. It idled so smooth.
This illustrates why all carbs should have a separate idle circuit, to adjust for altitude, temperature, humidity, and engine size.
According to my carb size calculator this carb on a ported engine should be good for street use of the 48cc engine. A 66cc needs a 18mm Mikuni carb for street use. Neither the Speed or NT carb should be used on a racing engine. They are only for street use due to small size.
These are my final recommendations up to 3000 ft. Notice the weak check mark that indicates it is do-able but not highly recommended. The piston port 66cc really needs an 18 or 19mm carb that has 60% more flow area than the 15mm Speed Carb.
A year ago in 2019 I measured the needles for a NT carb and a Speed Carb and the NT needle was fatter than the Speed needle which means its idle mixture is leaner due to decreased gas flow space between the needle shaft and the needle jet. So it would be better for piston port engines since they require leaner jet sizes due to the intake port opening late and there already being a good vacuum in the crankcase due to the rising piston. That sudden opening makes a strong vacuum wave front which draws up more gasoline thru the needle and main jet. This has to be countered with a smaller flow area and a smaller jet.
For my tests I took off the useless stock filter and made my own out of a foam scrub pad for kitchen use. Here is its photo. I hollowed out a shallow area for the carb using scissors, silicone sealed it onto the carb, and dabbled engine oil onto the foam with my finger since I didn't want to do that before it was glued in place.