This page is based on a bunch of research I did a few years ago, and is not based on running any of these gas-passers. I’ve settled on the Tillotson X for my Teenie Two installation. E-mail me in a few years to see if it worked out.
I haven't yet found a rule of thumb I like. Carbs in the range of 28 to 45 mm seem to work, depending on the size of the engine, the manifold arrangement, and the type of carb. When looking for alternatives, look in this general range. The stock Solex downdrafts ranged for 28 to 34 mm. (Solex is apparently now part of Mikuni by the way...)
Dual carb installations use slightly smaller carbs, or the same size with smaller jets. The "injector" style carbs (Ellison, POSA, Revflow) seem to be able to use smaller throats than the Constant Depression types. Measuring different things - butterfly valves, venturis, the initial inlet, or just the hole in the slide valve causes some of the confusion.
Certificated aircraft carbs are always updraft, and are mounted below the engine. Any fuel leaks go out the cowl, not onto the exhaust manifolds. It also makes for a low cowling top, and better visibility over the engine. Aircraft engines have their intake ports on the bottom of the head to suit the intake arrangement just described.
VW's were designed for use with a downdraft carb and have their intake port on top. Do you see the problem?
Of the carbs suitable for VW use, only the POSA/Supercarb, the Tillotson X, and the Revflow can be configured for updraft flow. Sidedraft carbs can still be mounted below the engine, with the proper manifold. Both these arrangements mean long, convoluted, heavy intake manifolds. Yes, long manifolds can increase volumetric efficiency, but only if there are individual, relatively straight runners for each cylinder, preferably with a carb throat for each runner. The reality of the common VW aeroengine manifold is poor mixture uniformity and throttle response. One thing that happens, particularly with carbs with poor atomization (I won't name names, but it starts with a P...), is that the air and fuel separate, un-mix, downstream of the carb. The gas droplets coalesce and form a film on the inside of the manifold, where engine compartment heat revaporizes them. The whole length of the manifold is effectively part of the carb. Now open the throttle quickly. If you do not have an accelerator pump, and few of the following carbs do, the incoming rush of airflow overwhelms the film vaporization mechanism, and the engine leans out, stumbles, and stalls.
This is not to say this type of manifold is un-airworthy. Thousands of installations prove the converse. It's just important to understand its limitation. And with the carb under the engine, gravity feed works pretty well, even in low wing applications.
The "KR-2" style is the best of the commonly available carb-below-the-engine manifolds. After initial branching, it has separate runners for each cylinder (Note: this assumes you're using dual port heads). Everything is a tradeoff though; it's both heavier and harder to cowl tightly than the "Sonerai" style Y-tube setup.
What other choices are there? Well, thousands of VP-1 fly with the stock Solex downdraft mounted above the engine. If you have a big cowl, like the WW1 replicas, this should work fine. If you feel you need more carb than that, the common dune buggy dual Kadron setup might fit by swapping the manifolds right for left to put the carbs closer to the centerline (note: you’ll have to modify, or make your own linkage if you do this), or by making your own manifolds.
I have Irv Furman's notes on mounting a Mikuni HS42 side draft on top of the engine; his general scheme could be adapted to work with any side draft that follows. Ellison recommends this location, and has a sketch on their web site of a very nice home built separate runner manifold. And Steve Bennet has just such a manifold for his rear drive system. Irv's mounting interferes with the normal top mount for the oil cooler. Remote coolers work, but they add hoses to the system I personally prefer to do without. Gravity feed is a non-starter for low wing designs with this and the following arrangement. And these arrangements need about 8 inches from the top of the case, way too much for Sonerai, probably marginal for KR-2, but easy for many other designs.
Another possibility worth exploring is dual side drafts mounted behind and slightly above the engine. This may only work if you have lots of space between the firewall and the back of the case. Look at how the carbs are arranged on a Rotax 912 for an example. You can keep the oil cooler on top, the cowl line is probably lower than the previous example, and performance should be better when properly tuned. Disadvantages include more weight, two carbs to heat, and linkage complications.
You need it, period. Figure out a way to provide it. Butterfly carbs on long intake manifolds in particular need lots of it.
HS-6 should work for any engine 1600cc or larger. Pick your needle using the Haynes Carburetor book (also covers Weber and Stromberg), look for an engine of similar displacement. HS4 for 1600 cc or smaller, or possibly for dual carb installations. Gower & Lee, UK carb experts, can get you parts.
Rugged, cheap (used) and reliable. But the needle does it all - no separate idle or mid range circuits. Need a drop of automatic transmission fluid in the dashpot every 3 months or so, or they will lean out on acceleration (good idea on Stromberg CD carbs too). Somewhat altitude compensating, but not much. Has a bad reputation for leaking fuel if not properly maintained.
Copies(?) are available new, called the Eliminator, for Harley's, in the same general price range as the Mikuni's, which are probably superior.
Irv Furman loves his HS-42 float bowl slide valve for single carb applications up to 2200 cc. HS-45 for bigger. May need seat and needle change if running a fuel pump instead of the motorcycle gravity feed normal for this carb. Needs to be choked to start. Very adjustable, three circuits separately control idle, mid-range, and full throttle operation. Sudco sells the bare HS-42 carbs, the cheapest Harley retrofit kit, which includes the carb, cables, air cleaner adapter, and instructions is about $370 discounted. Kits are widely available from Harley dealers or specialists. J& P Cycles has them, along with most of the other Harley carbs listed above and below.
There is also the RS series of Slide valve carbs in various sizes. The 32 mm slide valve carbs used on various two strokes were about $140 in 1997. Jets and needles for all Mikunis are widely available in a broad range, and cheap. California Power Systems or Sudco are your best bets. Many, many Japanese motorcycles run Mikunis, so they should be available used.
Motorcycle racing carbs, slide valve. I know little about them; they appear higher end than Mikuni. Sudco does Keihin too.
There are also Keihin Constant Velocity and butterfly carbs used on newer stock Harleys, although they do not seem to be very well liked by the hot Harley guys and gals. Presumably they are cheaper than the Bendix/Zeniths, Mikuni, Quicksilver and S&S carbs that commonly replace them. Should be a glut on the used market, and there are tuning kits available for them, check with the many Harley discount supply houses.
Side draft constant depression type, CD150 (1600cc and below?) or 175 (1800cc and above), although Limbach uses single CD-150 on some engines, presumably with larger (.100, .090 is standard) main jets, and there are several known dual CD-150 installations (including Limbach's larger engines).
Roughly the same advantages and disadvantages as SU, although they tend to be more expensive, and at least in the US, rare. Gower & Lee have the bits you need for tuning. Stromberg seemed to survive tightening emissions standards better and longer than SU, and this is a mixed blessing. The later carbs, the CDSE, are designed to run very lean off full throttle, and are often completely non-adjustable. They were set once at the factory, and then left alone for the life of the vehicle.
Many types. Type 64/32/392 is used on the 912 dual carb setup. These carbs are semi-expensive, $550 in 1997. They are a constant depression butterfly. Similar Bings (several different models) are standard equipment on BMW motorcycles, so check motorcycle salvage yards. The smaller slide valve Bings (36 mm), as used on Rotax 2 strokes are much cheaper, $180 in '97, and may work in a dual carb setup, or even single carb on smaller engines. Mixture controls are available for the smaller Bing carbs ($120 per carb in '97), as well as altitude compensating versions ($450 for one, or $678 for two in '97). California Power Systems has all the aircraft specific stuff, this is the US distributor, and they have even more, like parts for motorcycle stuff.
Float less(?) slide valve carb with mixture control, reasonably priced ( $235), usually mounted side draft. Available in a variety of sizes, call and discuss your application with Joe Horvath.
"A poorly metered leak", "Simple, cheap, reliable, durable". Your mileage may vary... Float less slide valve design, side or updraft. Supercarb adds mixture control. The needle does it all, you can somewhat adjust mid and low mixture by altering the taper of the needles. If they idle too rich, but otherwise run good, drill a little hole in the slide to bleed a little air at idle. Some sort of extension (or filter! Most Sonerai have neither...) on the intake side improves performance a lot, by keeping the fuel spray in the throat of the carb, and not being blown away by the airflow out of the cowl. A velocity stack or just a piece of hose stuck over the intake and clamped securely will do the job. Not currently being manufactured, but available at Fly Markets everywhere aviation junk is sold.
Expensive, better atomizing than a POSA, higher quality in general. Seems to prefer a higher fuel pressure than a POSA, although gravity feed, even in a Sonerai, seems to work, although you can experience un-commanded leaning with low fuel with this little head. $600.
Updraft (side intake, vertical exit) float bowl butterfly valve tractor and industrial engine (forklift?) carb, dating from the 1930's. Steve Bennet of Great Plains likes them, so they must have considerable virtue. Features include cable controlled choke, low speed idle bleed screw, mid to high range fully cockpit adjustable mixture control, and float bowl vent to help prevent vapor lock. I have read some posts in various lists and groups indicating difficulty in tuning, particularly at part throttle. I know of no source for tuning parts, the GPASC instructions have you drill the main jet to enrich the mixture. Mounts neatly in under engine installations. Model 1617 is for use on 1600 - 1700cc engines only (updraft) $193.41, Model 1821 is for use on 1835 - 2180cc engines only (updraft) $255.05, Model 1821S is for use on 1835 - 2180cc engines only (side draft). No mixture control $229.95. Not to be confused with....
Side draft Harley carbs. Float bowl, butterfly valve. 38 and 40 mm throats. About $230 and up. Sort of an old design, the 38mm was stock from '57 to the mid '70s. The 38 mm in particular must be widely available used, although most of them had non-adjustable main jets. Great Plains and HAPI once used this carb, but they don't anymore. Perhaps there is a reason...
Harley High performance carb. Slide valve float bowl side draft. Shiny! Part throttle mixture adjustment. Single circuit. Allegedly altitude compensating. Available in a variety of mounting styles and sizes. $280 and up. Interesting sizing note: Edelbrock recommends 2mm smaller throat for dual carb applications, and notes that stock cams cannot be used. Bigger carbs like "bigger" cams, more lift and overlap, and also respond well to porting and polishing. Gene Berg's tech notes basically say the same thing RE VW carburetion. Quicksilver needles look a lot like POSA needles!
Another Harley carb, all side draft, float bowl, butterfly valve, has enricher and accelerator pump. The Super B has 1-7/8" (47.6mm) throat at the butterfly, 1-9/16" (39.6mm) venturi, runs $260 or so. Super E is same throat size, and is about $200. Super G is the same thing only bigger, 2-1/16 and a 1 3/4" (44.5mm) venturi, and is probably too big for all but the biggest stroker engines. As near as I can tell, the B model is the same as the E, but for later engines, emissions certified, and setup for dual throttle cables, not an advantage for us. The Super E looks like a good deal for a side draft carb.
Yet another side draft Harley carb! Float less, thus suitable for aerobatics. $370 back in 1997. Also formerly used by HAPI and GPASC, but abandoned for other choices. No idea what size it is. Not to be confused with the...
Updraft carb, originally designed as a replacement for the all-cast iron Bendix carbs fitted to Model A Fords. All parts are still available! Works well on 1600 cc, perhaps up to 1800cc engines. I've got two cores (don’t pay more than $15 ea), ready for rebuild when the time comes. Available complete from dealers in Model A stuff, or at Model A swap meets.
The stock Solex downdraft butterfly carb is used on lots of Volksplanes and their ilk. 32PICT is about the most common, and is a good size for 1600cc engines. The early 1500 cc Type III fastbacks used dual Solex 32PDST-3 's. Clearly, this size will work well, but these carbs are rare in the US, as this model was never imported or supported by VWoA. A very common, relatively mild dual carb setup these days uses dual Kadron (Solex copies?) downdraft carbs. This is a wide and high installation, but may suit some conversions. Stock venturis are 28mm (as opposed to the air intake itself, which is 40mm), larger venturis are available, Low Bugget has them. Kadrons are common as dirt, used sets of Kadrons at swap meets, car shows, etc. with everything to run can be had for under $50 bucks!