There is nothing quite so simple, and at the same time complicated, as a carburetor. Your dictionary will tell you, correctly, that it is merely a device for mixing fuel (in this context, gasoline) with air, and although early examples employed several different means to this end it has long since been proven that the job is handled best by directing the air through a venturi in which a fuel-entry nozzle is incorporated. Pressure within the venturi is reduced below atmospheric in direct proportion to air velocity, which applies a suction to the fuel nozzle in the same proportion. Thus, fuel is drawn from the nozzle at a rate closely tied to air velocity, and the basic proportions of the combustible air / fuel mixture produced by the carburetor may be established by varying the cross-sectional area of the fuel entry as compared to that of the venturi. And, in point of fact, early carburetors were little more complicated than the device just described, with only a controlled-level fuel reservoir (the float chamber) and some form of throttle valve added.
Modern carburetors are still built around the basic venturi, fuel nozzle, reservoir, and throttle, but various details have been added. The most important of these is the “correction-air” feature, which is needed to compensate for the venturi / fuel-nozzle's great inherent defect: Uncompensated, the basic carburetor will deliver an air / fuel mixture in which even higher proportions of fuel appear as air velocity through the venturi increases. The reason for this is that the pressure-drop within the venturi is accompanied by a reduction in air density, and while flow from the fuel nozzle is in direct proportion with air velocity, the actual mass of air passing the nozzle does not remain in proportion. In consequence, mixture strength rises with increases in velocity unless measures are taken to prevent that from happening.
All carburetors except those with diaphragm-controlled metering make the necessary adjustment for decreasing air-density by means of a correction-air system, in which air at atmospheric pressure is delivered to the fuel nozzle via drilled or cast passages. In its more primitive forms, the air is simply led to an annular opening around the spray nozzle - as may be observed in Amal carburetors. As pressure within the carburetor throat drops, the differences in viscosity cause air-flow from the correction-air system to rise more rapidly than fuel-flow from the spray nozzle, and this does much to stabilize mixture strength. But much better mixture stability is obtained by directing the correction-air into a well below the spray-nozzle, from which it is pulled through a series of holes in an “emulsion” tube. This emulsion tube, which may be incorporated as part of the spray nozzle, takes in fuel at its lower end (usually) and admits the correction-air through a pattern of holes drilled in its sides. At low rates of flow, the well is almost entirely filled with fuel, and air passes only through the topmost holes in the emulsion tube. But as overall flow increases, the fuel level in the well drops to uncover more holes and the correction-air component of the froth delivered to the spray nozzle becomes larger.
By altering the hole-pattern in the emulsion tube, and with adjustments to the total amount of air being admitted to the correction-air system, the carburetor's mixture-delivery characteristics are tailored to suit a particular engine. It must be noted here that an absolutely-even mixture strength may not be desirable: Some high-output engines require a richer mixture at their torque and / or power peaks than at other speeds, which means that the correction-air system should be adjusted accordingly. Generally, a large main jet and small correction-air jet deliver a mixture increasing in richness with engine speed, (as you might expect), with the opposite being true of small main jets and large correction-air jets. But while the overall slope of mixture delivery is determined by the main jet / correction-air jet proportions, slight periods of richness and leanness may be created by alterations in the emulsion tube's hole-pattern. For example, an emulsion tube with large holes at its upper extremity and smaller ones farther down will tend to deliver a richer mixture at higher revs; the opposite condition is found when the upper holes are smaller than those in the rest of the hole-pattern. And, when the emulsion tube is a close fit inside the fuel well, it is possible to make a further adjustment with differences in diameter down the length of the tube: a collar, midway down the emulsion tube, can constitute a restriction within the fuel well large enough to become a kind of secondary correction-air jet. With maximum application of these mixture compensating techniques, it becomes possible to employ very large carburetor throat sizes relative to cylinder displacement, which is why the sophisticated Mikuni is a better choice than the Amal GP-pattern carburetor despite the latter's unquestioned advantage in air-flow capacity, size for size. A 35mm Amal GP will flow more air than a 35mm Mikuni, but you can fit a 40mm Mikuni on an engine that would develop the blind-staggers with an Amal GP larger than 35mm in throat size.
Other complications in carburetor construction have been introduced to cope with part-throttle conditions. At very low engine speeds, for instance, air velocity through the carburetor will not be high enough to pick up fuel from the main spray nozzle and the mixture must be provided by other means. Typically, idle-speed mixtures will be produced by a kind of carburetor within the carburetor: Under the floor of the carburetor throat, where the throttle valve seats, you will find a small chamber supplied with fuel and air, and with entry to the throat through a single hole downstream from the throttle valve, or through one hole downstream and one or more upstream from the throttle. The least complicated arrangement is one in which there is only a single hole behind the throttle valve, and the chamber is fed air past an adjustable needle valve while fuel is supplied through an orifice of fixed size. Here, the idle-speed mixture (which is a froth emerging from the entry hole in the carburetor throat) is controlled by varying the amount of air admitted to the chamber, and the idle mixture will be full-rich when the needle valve is closed. Other systems have a fixed pilot-air jet and an adjustable fuel jet, which reverses the rich / lean position of the needle - while still others have fixed air andfuel jets, and bulk flow from the idle-mixture chamber is controlled by the adjustable needle jet. These details are relatively unimportant to the tuner; what is important is that the idle-mixture system not only keeps the engine running at low speeds but also handles the transition between closed-throttle running and the point at which enough air is flowing through the throat to initiate the movement of fuel up from the main spray nozzle.
The transitional period is managed best by having feed holes both in front of and behind the throttle valve. With the throttle against its stop, some air passes under the valve and picks up the mixed fuel and bubbles coming from the idle feed hole, while air is diverted down through the hole in front of the throttle valve to mix with the fuel in the idle-mixture chamber. But as the throttle opens, the depression existing behind the valve moves forward to cover the upstream feed hole, which means that instead of air entering the hole, fuel is pulled from it, and the net result is that the carburetor is delivering enough additional fuel to compensate for the increase in air moving past the opening throttle. That is, of course, only the case when all the fuel and air passages are the right size - and when they are,the mixture will remain at the proper proportions until enough velocity is established past the main fuel nozzle to relieve the idle system of further duties.
Carburetors with butterfly-type throttles often have a pattern of holes ahead of the throttle valve, and these are called "progression" holes. As the valve disc pivots, and its lower edge swings forward, it moves ahead of the progression holes in order of placement, and each hole then switches from being an air-bleed to become a fuel jet. With the right pattern of progression holes, even a very large carburetor (large in terms of throat size relative to cylinder displacement) can be made to keep the engine running without stumbling while the transition is made to fuel flow from the main nozzle. Rarely are progression holes, in the multiple, found in slide-type carburetors. In these, the transitional period is handled by the slide cutaway - and the higher the cutaway, the leaner the transitional period's mixture.
Almost certainly, the carburetor you will be using will have a circular-slide throttle, because this is the type most commonly employed, and most successful, in the field of high-output motorcycle engines. Almost certainly, too, the carburetor you buy for your racing engine will have been jetted and given a slide cutaway suitable for a somewhat larger stock engine, which should warn you that a more-or-less complete retuning of the instrument will be necessary. Many tuners begin the retuning process by finding the correct main jet, and that is a good beginning unless there is an undiscovered problem with the mid-range metering system - the long, tapering needle clipped to the throttle slide, and the needle jet itself. These, in combination, constitute a variable fuel-metering valve, and if the flow is restricted between the needle and needle jet to an extent greater than the restriction provided by the main jet required to feed the engine at full throttle, then no amount of switching main jets will get the engine running properly. So the retuning process must always begin by determining if there is sufficient fuel flow past the needle jet to feed the engine. I have found that this matter can be settled very simply by lowering the needle to its last notch, which maximizes the flow restriction at the needle jet, and then removing the main jet entirely from the carburetor. The engine should then run, if none-too-well, on part-throttle, but flood as the throttle is opened fully. Should the engine be willing to run on full throttle, you may be sure that a larger needle jet is required.
After the selection of a needle / needle jet combination that will pass more fuel than the engine can digest, you then proceed to the problem of finding the correct main jet. Until you become really expert in the art of "reading" spark plugs, the right approach is to start with a huge main jet and then reduce the size until the engine will just barely run, on full throttle, without "four-stroking". Because of the benefits in cooling that are obtained with very rich mixtures, you will get very near the maximum power to be had from a high-output two-stroke engine with a mixture that verges on being so rich that misfiring occurs. The optimum usually will be found with a slightly leaner mixture than that bordering on four-stroking, but as the potential gain is rather small, and the risk of melting a piston is very large, leaner mixtures should be tried very cautiously.
While you are experimenting with main jets, the metering needle –which controls the mixture strength when the throttle is from about a quarter to three-quarters open - should be installed with its clip in the middle groove, or halfway through its adjustment range. You ultimately will probably change this setting, and perhaps switch to a different needle, but you'll need a starting point for the adjustments required to the throttle cutaway and the idle system. Start these adjustments by backing out the throttle-stop screw until the throttle is completely closed, and then turn it back in until the throttle is just barely cracked open. Having done that, you also close the idle mixture screw completely, and then open it two or three turns before starting the engine. The object, in juggling the two adjustments, is to keep working with the idle-mixture adjustment to increase the idle speed while dropping the idle by backing off on the throttle stop. Eventually you'll arrive at the lowest throttle setting at which the engine will idle satisfactorily, and the correct mixture at that throttle opening, unless, of course, the idle system is wrongly jetted. On carburetors with an "air" adjustment, and a fixed jet feeding fuel, you'll know that the fixed jet is too small if engine speed continues to rise (at a fixed throttle-stop setting) until the adjustment screw is turned all the way in to the closed position; at some point in this process the idle mixture should become over-rich, and if it does not the fuel jet is too small.
The opposite is, of course, true when engine speed continues to climb as the idle mixture adjustment screw is opened, without an optimum ever appearing. A similar, but opposite, rule applies for idle systems with fixed air jets and an adjustment for fuel flow. I cannot give you a listing of which manufacturers use what type of idle system, as this varies even between carburetors of a single make. To be certain, you'll have to take your carburetor apart (or check the maker's literature, if available) to see whether the mixture adjustment is for fuel or air.
Throttle cutaway will be the next matter for your attention, and this factor almost exclusively concerns what happens during the first eighth of throttle opening. It is possible to have atoo-low cutaway on the front edge of the throttle slide, but a carburetor intended for some big engine will almost always have too much cutaway for one with a smaller cylinder displacement. The too-high cutaway problem will be manifested in a tendency for the engine to cough and die when the throttle is opened, and the cure simply is a new throttle slide with less cut-away. When dealing with a single-cylinder engine, you can buy a replacement slide with the lowest cutaway offered, and file the bevel higher until off-idle running is clean. The same approach may also be taken with multiple cylinders, but it is very difficult to get the cutaways modified exactly the same and if finances permit you should just buy a selection of slides. Incidentally, a too-low cutaway will make the engine surge and burble at one-eighth throttle or less -and if you are forced to make a very large change in cutaway, you'll have to start the idle-system tuning process over from the beginning. Probably the best sequence in overall carburetor tuning is to begin with the main jet, then rough-adjust the needle and needle jet, after which the cutaway and idle jetting are managed more or less simultaneously as it is almost impossible to separate them completely.
Factories devote months to finding precisely the right metering needle, because a touring motorcycle spends most of its life being run somewhere between one- and three-quarter throttle. Fortunately, considerations of fuel economy are strictly secondary in racing, so you need not spend months switching needles and needle jets, but a racing motorcycle is much easier to ride when it at least runs cleanly on part throttle and you will have to make some effort in this direction. If there is surging and stuttering at steady throttle within the range controlled by the needle, then the mixture is too rich and the needle should be lowered. An engine that runs fairly cleanly at steady throttle but stumbles and hesitates as the throttle is opened farther is suffering from mid-range leanness and the carburetor's needle should be raised. Sometimes you will get both symptoms with the same needle, with an over-lean condition at one-quarter throttle changing to become over-rich as you approach three-quarters throttle. That should tell you the needle's taper is wrong, being too steep, which means that n needle with a more-shallow taper will be required. Obviously, the opposite may also be encountered. Unhappily, these needles are expensive, but there is no satisfactory alternative to buying a selection and trying them until the right one is found.
At one time a remote-mounted float chamber was the sine qua non of racing carburetors. Commonly, carburetors were bolted rigidly to the engines they fed and float chambers were mounted in rubber to isolate them from the worst effects of engine vibration - which can cause such extreme frothing of the fuel that accurate metering is impossible. But while remote mounting may have solved one problem, it introduced another: Acceleration and braking caused a surging in the long fuel lines linking the carburetor and float chamber which also upset metering. The answer, all along, was to incorporate everything in one casting and mount the whole carburetor/float chamber assembly in rubber. The difficulty in that was finding a rubber capable of withstanding a lot of heat, and constant exposure to gasoline, without disintegrating - and a rubber that could be bonded to a steel manifold flange to avoid the inconvenient space requirements of the old hose-and-clamps assembly. Advances in synthetic rubbers eventually brought us the right material, with the result that nearly all Japanese-made motorcycles now have their carburetors held in rubber sleeves that double as stub-manifolds, and the rest of the world's motorcycle makers follow Japan's lead when possible. With these manifold sleeves now available in a variety of sizes, there is little reason for anyone using a rigid carburetor mounting, and the equally wide availability of the Mikuni carburetor allows even less reason for employing superannuated instruments like the Amal GP. Diaphragm-type carburetors, like those universally found on kart engines, are relatively indifferent to vibration and need not be rubber-mounted, but all the rest benefit from being isolated. Even when no obvious symptoms of mixture instability appear, you may be sure that the tendency is there and of course vibration will cause a remarkably rapid wearing of throttle slides, needle and needle jet, and even the float valve. I would also unhesitatingly recommend the Mikuni carburetor as, for the moment at least, it does the best, most-delicate metering of any readily-available motorcycle carburetor. Another attraction of the Mikuni is that it is supplied in many different sizes, and backed by an excellent selection of jets, slides, needles, etc. Finally, the Mikuni is much better streamlined, internally, than it appears, and has a greater air-flow capacity, size for size, than almost any other carburetor. Consequently, there is little reason for choosing anything but the Mikuni unless you are very short of money and forced to take what you can get.
It is possible that one of the diaphragm-type "kart" carburetors may offer advantages in some specialized applications, and I know people who claim that good results are obtainable with things like the CV-series Keihin (the "constant-vacuum" carburetor found on Honda's CB350 and CB450). However, one should remember that the Keihin CV is designed to improve the broad-range performance of four-stroke engines and inherently poorly-suited to the two-stroke engine's quick-gulp intake characteristics - which means that it is something less than a perfect choice for the latter, in terms of sheer power, even though it might show some advantage on a trials engine. Similarly, the diaphragm-type carburetor was invented, and still is most widely used, to overcome the ordinary float-chamber's inability to feed fuel at steeply angled or inverted positions. There is nothing in its makeup to recommend it when you are looking for pure horsepower, and I would consider most diaphragm-type carburetors a good choice only in applications where there is so much jolting and jouncing involved that a conventional float-type instrument cannot function normally. The single exception here is the pressure-pulse carburetor developed by McCulloch, which employs the crankcase's pumping action to meter fuel-flow instead of a venturi. The pressure-pulse carburetor is thus capable of metering satisfactorily even at extremely low air-flow rates, and provides good throttle response and a broad range of power when used in relatively enormous sizes. In the original application, McCulloch's 100cc and 125cc kart engines were fitted with a pressure-pulse carburetor having a 1.375-inch throttle bore, which is much larger than would have been possible with a conventional carburetor. Subsequent development work showed that a slight constriction was necessary to create a venturi effect that would compensate for this carburetor's tendency toward high-speed lean-out, but it still is capable of combining remarkable peak power with a broad effective range. The single difficulty with pressure-pulse carburetors is that they are extremely sensitive to both cylinder displacement and crankcase compression ratios, working well only on engines for which they are specifically designed. Their air passages, which bleed crankcase pressure into and away from a chamber behind the metering diaphragm, have calibrated orifices and any variation in the conditions anticipated by the maker require a complete recalibration. It is not a job for amateurs.
Selecting carburetor throat size is enormously difficult: The four-stroke engine's carburetor may be chosen through relatively uncomplicated consideration of cylinder displacement and operating speed, but in the two-stroke engine's case there is an added difficulty introduced by the quick-gulp intake characteristic and by the overriding importance of pulsations in the intake tract. Engines with smallish intake ports and relatively long intake periods respond best to small carburetors; those having very wide, low intake ports will have shorter intake periods to provide the same specific time-area value and need a bigger carburetor-throat size if throttling is to be avoided. Frankly, unless you intend raising your engine's peaking speed very considerably, it is wise to retain the stock carburetor size even though you may want to exchange the original carburetor for a Mikuni. Switching to a large carburetor will alter tremendously the intake tract's resonant frequency and usually will require that the intake-tract length be changed to bring the sonic wave motions back into phase with the intake period. Otherwise, the loss of intake ram effect will more than offset any gains obtained through the larger carburetor's added flow capacity.
Despite all the difficulties introduced with a major change in carburetor size, the change becomes a necessity if other engine alterations work to raise peaking speeds by more than, say, 15-percent: A carburetor chosen with a power peak at 6000 rpm in mind is certain to cause some throttling when asked to deliver mixture at 8000 rpm. The problem here is, how do we determine the most suitable throat size? Everyone knows that while a bigger carburetor is sometimes indicated, "bigger" does not automatically become "better". Again, Yamaha's Naitoh and Nomura provide an answer, of sorts, with a handy formula for determining throttle bore:
Where D is throttle bore, in millimeters
K is a constant
C is cylinder displacement, in liters
N is rpm at peak power
For racing engines, the constant, K, is given as 0.8 to 0.9, and if you know how to work square-root problems it is easy enough to establish throat diameters for all engines. The catch here is that Yamaha's formula seems applicable only to road racing engines. For example, the Yamaha TR3 has a unit cylinder displacement of 173cc and develops maximum power at about 10,000 rpm, and if we assume that K = 0.85, then
D = 35.4 mm
Thus, the calculated throttle bore diameter is only slightly larger than that actually used on the TR3 engine, which indicates that the formula probably is valid for most road racing power units. But when we apply the same formula to Yamaha's DT-1, a 250cc single developing maximum power at about 6000 rpm, the calculated throttle-bore diameter is 31mm even if we use K = 0.8. In reality, the DT-1 is fitted with a 26mm carburetor and dynamometer tests have shown that this engine responds badly to a 30mm carburetor, which means that for the Yamaha "enduro" model, the best results are obtained at K = 0.64. On the other hand, the similar DT-1MX does perform well with a 30mm carburetor, and that works out to be K = 0.69. And the Sachs 125's displacement, power peak and carburetor size gives us K = 0.91, which demonstrates that there are factors here not given proper recognition in the Naitoh-Nomura formula. I am convinced that there is a "gulp-factor", consisting of all the things that influence the shape of the intake pulse, (timing, port shape, connecting rod length, etc.), complicating this matter. Someday, I may have time to solve the problem. For the moment, I can only tell you that K = 0.8 is a safe value for road racing engines, while motocross and enduro engines may need anything from K = 0.65 to K = 0.9, with relatively low, wide intake ports (which give a brief, strong intake gulp) favoring the higher values.
A final note on carburetion: In all two-stroke engines intake pulses are very strong and the sonic wave activity considerable, which has effects both good and unpleasant. On the credit side is that the high-amplitude pulsations do make it possible to obtain very high specific power from the mechanically-simple piston-port engine by blocking blowback during the second half of the intake period. But these same pulsations also have a terrible effect on the carburetor's ability to accurately meter fuel, by leading a large part of the air drawn into the engine past the spray nozzle three times: Air passes the nozzle moving into the intake tract, then reverses direction as a result of the pulse generated when the intake port chops shut, and passes the spray nozzle a third time as the next intake period begins. This may sound slightly improbable -but there is absolute evidence it is happening in the fog of fuel one sees dancing in front of the two-stroke engine's carburetor. Now if this triple-passage occurred at all engine speeds, no problem would arise; unhappily, intake tract resonances - the fundamental vibration and its harmonics – slide in and out of phase with changes in engine speed. Relatively small carburetors tend to damp these resonances, and therefore are less subject to vibrations in mixture strength, which makes them particularly suitable in any application where a broad power range is more important than maximum power. This problem with mixture delivery can easily become so severe that it will be impossible to obtain clean running, especially if the carburetor is inserted at the wrong point in the overall intake tract (which consists of the port, manifold -if any - and the pipe connecting the carburetor with the air cleaner as well as the carburetor itself). Maximum air flow will be obtained with the carburetor crowded close to the port window, and an extension on the carburetor's inlet to provide the correct tract length, but that arrangement also gives the worst conditions for mixture delivery. Positioning the carburetor at the intake tract's outer end reduces volumetric efficiency somewhat, but provides the best mixture-strength stability. Connections to the air cleaner should be as short as possible, but if it is necessary to separate the carburetor and air cleaner by more than a couple of inches, the passage linking them should be either a cone (diverging at least 15-degrees) or a parallel-wall tube having about 400-percent of the throttle bore's cross-sectional area. Both of these will provide essentially the same condition as a pure, atmospheric inlet, and prevent secondary resonances that also can upset fuel metering.