Speed Carb 15mm $25 - some have a spring loaded float valve which is better than the NT's rigid valve. It's lacking a low speed circuit (idle jet + air screw) and different needles are not available (set the clip to the 3rd/middle position. Main jet size needed to be #72 Dellorto jet for my 48cc reed engine.) I only recommend this carb for 48cc reed valve engines. The stock NT is better for piston port intake engines. Click here for the $55 CDHPower G2 Reed Valve with 19mm OD for the NT or Speed carb.
Dellorto PHBG AS15 $110 - clamp on style (24mm, 23mm (like VM18), or 20mm with additional shim), fully adjustable, get a W19 needle for it because the W7 it comes with makes it too rich mid throttle.
Dellorto SHA 16mm $66 - other than changing the main jet this carb is non-adjustable. I had one and had to do lots of mods to it for it to be jetted right but at 7000 ft it was the best! When I moved down to sea level I couldn't get it to work right. [more] Clamp-on size is 19mm (which is right for the CDHPower reed valve 19mm size).
Keihin 16mm Carburetor Clone - clone of Keihin PTF 16mm. Pilot/Main jets are available but no needles. Intake bolts onto intake. Sold mostly for 4 strokes so maybe the needle jet is of the 4 stroke design.
Mikuni VM16 $85 - Clamp-on size is 20mm. Main jet 70, idle jet 25, Needle Jet D-8. slide cutaway 2.5, Needle 3DJ3. Get a 4DH11 needle for it if you find the carb to be too rich mid throttle. It probably needs a #60 main jet in it for a mildly ported 48cc.
Jamon Howell used to sell this manifold for the VM18 for use with piston port intake. I can't get him to respond to me so maybe he's another vax victim. But this shows you what one should look like if you have a machine shop make one for you.
OKO Flat Slide Carb (19, 21, 24, 26, 28mm) $110 - 34mm OD Spigot mount. Don't get any of the cheaper imitation carbs ($28) because they often have problems due to sloppy manufacturing tolerances and sometimes they won't even accept the normal jets for it. Oko can select the jets they think will work with your bike. Needle kit source
Dellorto PHBG 21mm CS clone $65 - Clamp-on size is 24mm. Click here for an original PHBG21 since I don't really recommend buying a clone carburetor since they usually have bad manufacturing tolerances. The genuine one comes with #50 idle jet, #92 main jet, W7 needle, 4mm slide cutaway. My jetting calculator shows it (on a reed valved engine) to provide a good full range air/fuel mixture for 69cc up to 10,000 RPM with a W12 needle (which is much less rich mid range than the over-rich W7), #25 idle jet, #70 main jet, and the slide cutaway increased to 6mm (done manually or buy one).
Mikuni VM24-512 $113 - Spigot mount, Pilot Jet 40, Main Jet 210, Needle Jet N-8, slide cutaway 1.5mm, Needle5L14. The inner diameter of the connecting hose needs to be 24mm for soft rubber or 26mm for hard rubber.
Dellorto PHBL 25 BS $100 - 31mm spigot mount (use $10 rubber connector) comes with #70 idle jet, #102 main jet, D22 needle (I recommend replacing this with D21 which is leaner).
Mikuni VM26 (#8074, the 2 stroke version) $77 - Spigot mount (use $10 rubber connector), Pilot Jet 60, Main Jet 190, Needle Jet N-8. slide cutaway 2, Needle 5E75. The inner diameter of the connecting hose needs to be 26mm for soft rubber or 28mm for hard rubber.
Click here to find out how to adjust a carburetor for perfect running thru all the throttle positions.
Here are my recommended carb sizes which comes from my carb sizing calculator which takes into account engine compression, cylinder bore, intake type, engine size, RPM. If your engine compression is more than 130psi then you can use a slightly larger carb.