The carburetor can be intimidating to some for a variety of reasons. It has over a hundred parts, can be difficult to get running if it's not tuned correctly, and how it works remains a mystery to most people. In this section I will attempt to demonstrate the simplicity of the carburetor and "demystify" the way it works. With the understanding of how it does what it's supposed to do, you'll better be able to diagnose why it's doing what it's not supposed to do. In the process I will debunk some of the common myths regarding carburetors.

It is my hope that after reading these pages you will have more confidence in tuning and even rebuilding your carburetor, as well as a better ability to choose a carburetor that meets your engines needs, driving application, and driving style.

It's just a fuel sprayer; a really intricate fuel sprayer...

For those of you who are not intimidated by mechanical gadgets with a dozen moving parts, you're already beyond a rather large hurdle that some still need to pass. For the others, some consoling thoughts to help set you at ease are that as long as you don't destroy or bend any of the components on the carburetor, you're not going to break it; most anything you could ever do to it can be "undone" with the help of the Internet and a digital camera, and blowing up your engine is unlikely unless it's under boost. It's just a fuel sprayer; a really intricate fuel sprayer, with adjustments for the timing, mixture and amount of fuel coming out, that's all. Most of these are already preset as stock jet sizes.

...acts of desperation...

Troubleshooting a carburetor doesn't have to be such a hardship. The biggest mistake I see is that people unfamiliar with how a carburetor works tend to troubleshoot the carb very randomly. Then as acts of desperation, they begin adjusting things that have no bearing on the problem they're experiencing, thus compounding their troubles.

In the "Troubleshooting" guide, in the "Stock Nikki Section, I will describe a fool-proof checklist that follows the fuel, using the Nikki carburetor as an example. At the end of the guide will be  a "Nikki Carburetor Troubleshooting" chart.

...herding cats.

Tuning a carburetor can be a bit like herding cats. Often while you make changes to one variable, others change subsequently, and overall performance is no better for your latest effort. Whether you're tuning for ideal mixture and atomization at a particular RPM band, for best fuel economy, or you're just trying to get the idle correct, the "Tuning" page will show you easy steps to systematically tune your Nikki carburetor. It's highly recommended that you read the page entitled "Driving Technique" before attempting to tune for performance. This page helps to understand your tuning needs based upon your own driving style, and demonstrates why this is one of the most important, yet overlooked variables in carburetor tuning.

...Raising the dead.

A misbehaving carburetor can be as frustrating as anything. When all else has failed, and you know it's time to call 'time of death', that means you're faced with the challenge of a rebuild. While for some this comes easy, for others it appears as impossible as raising the dead. With help from the Rebuilding page, and a handful of tips and tricks, you'll wonder why you ever doubted yourself in the first place. For those who have no fear of the rebuild gremlins, you'll still find valuable information that will help you avoid some of the most common pitfalls of Nikki carb rebuilding.

 How a Carburetor Works:

Air Delivery

Air in motion is what powers the internal mechanism that meters the fuel inside the carburetor. At the heart of that mechanism is the venturi. The venturi is very simply a small section of pipe with an "hour-glass" profile inside it. Intuitively, it appears that the narrowing profile of the venturi would serve to impede the flow of air or fluid through it, when in fact if it's designed correctly, it is not an impediment at all. A properly designed venturi will flow air at the same rate as a plain piece of straight pipe with the same diameter and length. But the amazing counterintuitive efficiency of the venturi pales in comparison to what goes on deep within it...

  The nature of the venturi is such that as air flows through it, there is a pressure drop created at the narrowest diameter inside it. On the Nikki, like most carburetors, there is also a smaller "booster" venturi set down into the primary venturi. It amplifies the pressure drop, helping to draw fuel in at low RPMs, when the air flow is low.

Inside the booster venturi, located at the narrowest point where the pressure drop is the strongest, is the fuel outlet pipe. The venturi assembly is situated so that the fuel outlet pipe is above the fuel level in the float bowl, otherwise fuel would simply spill out into the venturi. The fuel pipe comes up from a circuit at the bottom of the float bowl, where the fuel jet limits the rate of fuel that can flow up the circuit and out the pipe.

When air flows through the venturi, the pressure at the end of the fuel pipe becomes less than the atmospheric pressure acting on the fuel in the float bowl, causing fuel to spill out of the fuel pipe and be carried into the air stream into the engine, ideally in a fine mist. The pressure differential, or vacuum, acting on the main circuit is often referred to as the "fuel signal". As the flow of air through the venturi increases, the pressure at the fuel pipe decreases, creating a more powerful signal. The pressure drop is not linearly proportional to the flow through the venturi. The pressure decreases at a higher rate than the air flow increases, which means that the ratio of fuel to air increases, also. But the carburetor has a way to "bleed off" some of that powerful signal to the fuel inlet pipe in order to maintain a usable air fuel ratio; the "Emulsion System".  Even a venturi of mediocre efficiency will be able to draw enough fuel at high velocity to perform well in the high RPMs as long as the venturi can flow enough air. Because the emulsion system can divert excessive vacuum present at the fuel pipe, the more efficient and powerful the venturi is, the better the carburetor will be at low air velocity, and thus in the low end of the power band.

I've often said jokingly that a cut up beer can with a straw stuffed in it's side would draw out enough fuel at the top end for the 12a. The fact is, most any carburetor that can flow more than the maximum theoretical needs of the 12a is capable of delivering near perfect mixture at WOT. But the larger the carburetor is, the more the low end will suffer. This holds true for an appropriately sized carburetor with inefficiently designed venturis, as well.

  The venturi utilizes something called the Bernoulli principal, named for Dutch mathematician Daniel Bernoulli (1700 - 1782). It states that as the speed of a moving fluid increases, it's pressure decreases. Though this is a bit of physics that has been applied to sewage removal and community plumbing in city dwellings for over 5000 years, Mr. Bernoulli is credited with coming up with the mathematical equations currently used in modern science.

The venturi is named for Italian physicist Giovanni Venturi (1746 - 1822), who dedicated the bulk of his work to fluid dynamics, so much so that the venturi with which he is accredited pales in comparison to the sum of his other discoveries.

  All venturis are not created equal!

There is actually an "ideal" venturi. It's not a theoretical myth, either. The ideal venturi would, of course, have a flow efficiency of 100%, and as large a pressure differential as possible at peak air flow through it. The venturi would have an inlet edge angle of 16 - 20 degrees and an outlet edge angle of 7 - 11 degrees. The equal inlet and outlet diameter would have a cross - sectional area 4 times that of the smallest inside diameter. (In other words, the smallest inside diameter would be ½ the largest diameter.) At 100% efficiency, the flow rate would be 137.7 cfm per square inch. The greatest pressure differential inside the venturi, measured at a few hundredths of an inch below the constriction, would be 25 inches of water, or about 0.9 psi.

From this ideal venturi, we can calculate the theoretical venturi configurations to meet the demands of any given engine, as well as estimate the overall flow efficiency of any given carburetor. And that's just what we're about to do...

Again, I'll use the 12a rotary stock port as the engine of choice for this exercise.

We've already dremonstrated in the Rotary Engine section the theoretical maximum needs of this engine to be 306 cfm @ 8400 RPM at a VE of 90%. For argument's sake, we could also take the theoretical needs of the 12a @ 10,000 RPM at a VE of 100%, which comes out to 405 cfm, to appease those still unconvinced of the relatively scant needs of the 12a.

Let's do both.

Keep in mind that this exercise is going to give us a theoretical venturi configuration necessary to meet the demands of the engine, and as such it's to be taken for granted that the venturis are indeed "ideal" in efficiency, taking into consideration no air flow impediments what so ever. (... in other words, those pesky extraneous components such as booster venturis, shafts and throttle vlaves.)

The equation looks like this:

 

Maximum Engine Demand cfm	= total area of venturi inlet/outlet
137.7 cfm/sq. Inch

How we divide up the combined area of the venturis is purely subjective. We can choose to apply the information to a two barrel or four barrel carburetor, though we would have to make an educated guess on the spread of the primary and secondary bores on the latter.

Let's rewrite the equation with the known variable plugged in:

 

306 cfm	= 2.22 inches sq
137.7 cfm/sq. Inch

For a 2 barrel carb, that's only 1.11 square inches of area for each venturi, and remember, since this is for an "ideal" venturi of 100% efficiency, that measurement is taken at the inlet diameter, not at the most constrictive point.

The math for determining the area of circle is simply this:

area = pi ( r^2)

The value for Pi is 22 divided by 7, and the radius is one half the diameter...

 

22	(r^2)	= 1.11 in^2
7

...which becomes:

(r^2) =	1.11 in^2
	3.1429

...and then:

(r^2) = .3532

r = .5943

The diameter of each barrel of a two barrel carburetor with perfectly efficient "ideal" venturis to meet the needs of a 12a with a maximum RPM of 8400 and a VE of 90% is 1.19 inches, or about 30 mm.

The same equation applied for the theoretical demands of a 12a with a maximum RPM of 10,000, and a VE of 100% results in a two barrel carburetor with venturis 1.37 inches in diameter, or about 35 mm.

Now  everyone knows from personal experience that this theoretical measurement is very far from reality. Most venturis are far from "ideal" simply due to manufacturing constraints, and then of course, there are the many necessary "extraneous" components I joked about earlier that impede air flow. These variables may seem to make the above equation seem rather useless for picking out a carburetor by it's cfm rating, but that's not what it's intended to be used for. Instead, we can apply this same "ideal venturi" theory math to carburetors with known flow rates and venturi cross sectional areas, and calculate their efficiency. By comparing carburetors with the same maximum flow rating, we can mathmatically determine which choice will yield the higher signal strength to the main circuit, which translates into better low end performance. -Remember, high end performance is mostly determined by the maximum cfm the carburetor can flow because as velocity increases, the internal pressure drop decreases multiplicatively. Even if the carburetor has inefficient venturis, as long as it can slightly surpass the flow needs of the engine, it's nearly gauranteed to be capable of delivering decent high end performance. In nearly every performance application aside of drag racing, of two identical set-ups, the one that also has good low end performance is the one that's going to out perform the other overall.

I'll do this for the stock Nikki, the Sterling Nikki, and the Racing beat Holley 465.

First we measure the venturis to find the combined cross-sectional area. Then we'll plug this number into the following equation derived from our exercise above to find the flow efficiency of the carburetor:

 

rated flow of carburetor in cfm	= flow efficiency (%)
venturi area  in sq. inches x 137.7 cfm per sq. inch

The combined venturi area is multiplied by137.7 cfm per sq. inch to give the maximum flow efficiency through the carburetor. The venturi area is measured at the inlet/outlet diameter because the venturi which we are comparing the real world carburetor venturis to flows at 100% efficiency even though the smallest internal diameter is half the inlet/outlet (largest) diameter.

 

Carburetor	Venturi Diameters (inches)				Combined Venturi area (inches sq.)	Flow at 100% efficiency (cfm)	Rated or Tested Flow (cfm)	Flow Efficiency
	Pri #1	Pri #2	Sec #1	Sec #2
Stock Nikki	1.09	1.09	1.35	1.35	4.73	651	313 cfm	48%
Sterling Nikki	1.09	1.09	1.35	1.35	4.73	651	465 cfm	71%
RB Holley 465	1.46	1.46	1.46	1.46	6.70	923	465 cfm	50%

The inside diameter of the primary venturis in the stock Nikki measure 20mm, or .79 inches. I increase these to 22mm, or .87 inches, and I also turn the venturis on a lathe to give better inlet and outlet angles. But you can see from doing the math that they are still far from the "ideal" venturi in that the inside diameter is much larger than the half the outside diameter ( ...ideally the inside diameter would be 13.8mm or .55 inches ). As this is all theoretical math, real world experience shows us that even with less than ideal venturi design, at fairly low air velocity the pressure differential created inside the mass produced carburetor venturi is still plenty strong enough to draw adequate fuel into the carburetor for a useable air / fuel mixture. Even so, when the venturi is optimized as much as possible, it does improve performance as well as fuel efficiency. Further flow testing is yet to be done to measure the pressure differential on all three carburetors for comparison.

Also, take note of the fact that the Racing Beat prepped Holley 465 has the same sized inlet and outlet diameters for both the primary and the secondary venturis. The inside diameter, however, is smaller on the Holley primaries. Though this can be considered an illustration of the above paragraph in that even "less than ideal" venturis can pull adequate fuel at a given air velocity threshold, it stands to reason that at least one of the pairs of the Holley venturis is much farther from ideal than the other. Since the venturis in the Holley are cast into place, I cannot remove them to easily measure the inlet and outlet angles. I suppose it wouldn't be too difficult to triangulate by measuring the constriction inside and also how far down the venturi it is. I'll update the chart when I do.

We've discussed the efficiency of venturi design, but we haven't taken into account the rest of the components with regards to how they disrupt and impede air flow. It only stands to reason that anything in the path of the incoming or outgoing air flow through the venturi is going to threaten efficiency, and there are certainly plenty of such components. Probably the worst offenders are the throttle plates at anything other than wide open throttle. But at WOT, the throttle shafts disrupt flow the greatest, followed, at least in the stock Nikki, by the booster venturi support arms. Also present in the direct path of incoming air are the OMP fittings, accelerator pump nozzles, and of course the choke, all of which impede the flow only through the primary venturis. Everything that you can see in the air path will effect air flow, and not in a positive way, either. Even changing the screws that hold the secondary throttle plates on from the stock machine screws to more delicate button-head cap screws had a measurable impact on flow efficiency in our testing.

Carl Perez did all of the flow testing on the development of the Sterling Nikki. I mention on other pages the great lengths that Carl and I went to in our efforts to increase flow through the Sterling Nikki. We found out rather early on that optimizing venturi profile was not even half the battle. We started out with a fairly optimized venturi that was conservatively cut as little as possible so as not to compromise fuel signal strength at low velocity. We then slimmed down everything that was impeding flow by clipping the extra booster supports and putting an airfoil profile on the remaining ones, cutting back the OMP lines that hung over the edges of the primary venturis, and milling both throttle shafts as thin as possible while still retaining a reasonable amount of strength. The result was a carburetor that flowed approximately 425 cfm with great low end power. As I'm convinced that it's plenty enough for even a heavily ported 12a, I could indulge myself here by plugging what I consider to be my best carburetor achievement into the above equation, and I get a flow efficiency of only 65%.  But alas, Carl and I agreed that impressive numbers is what sells carburetors, and if we couldn't catch people's attention with a cfm rating that was competitive with the least expensive after market performance carburetor choice for the 12a, the Racing Beat prepped Holley 465, our little cottage industry would falter, and all our efforts would be for nothing.

So I compromised the venturi design to make the Sterling flow the same as the Holley 465. Luckily, after many different cuts and tests, I was able to maintain the great low end of the Sterling 425. Through those testing stages, some early carburetors were shipped out with one of three different cuts, all yielding the same decent low end pull, but with different maximum cfm ratings. There was the Sterling 412, 425, 450... until we hit that magic marketable number, 465. That's the same cut I currently use, and have used for the past several years.

A carburetor can only flow as much as the rest of the system will allow. The most restrictive part of a complete engine system is referred to as the "bottleneck", and for stock and modified Nikki owners, the bottleneck is most certainly the stock manifold. The flow rate of the stock manifold is downright terrible compared to the flow capabilities of the carburetor. Some later models, with the anti-after burn valve installed flow as little as 288 cfm. There is great room for improvement in the stock manifold, even with mild porting just to remove the obvious flow impediments. The Racing Beat cast Holley manifold, on the other hand, is an absolutely beautiful free flowing design, and there's no doubt just by looking at it that it will deliver unimpeded high end performance. But the Holley carburetor is unfortunately not very optimally designed for low end performance, where the rotary engine really needs help, while the Sterling Nikki, flowing the same maximum CFM, is very well designed for low end performance but the manifold keeps it from reaching it's high end performance potential. Both carburetors are capable delivering the same high end performance, as they both exceed the maximum Mazda rotary induction needs. With a better manifold, the Sterling Nikki would plant the Racing beat Holley, hands down. Until the time when a manifold is designed and produced, it is only a "contender". - I am working on that, by the way. This is a stage I plan on focusing my efforts on this coming season (2009).

Fuel Delivery

On the stock Rx-7, fuel is pumped from the tank to the carburetor by means of small piston pump that rates at roughly 5.5 psi. The capability of the stock fuel pump to actually produce that pressure varies with the age of the vehicle, as the pumps do wear out. Generally, even a worn stock fuel pump can generate the necessary volume of fuel into an open bucket, but as soon as the slightest resistance is encountered, the volume diminishes as the pressure increases. Therefore, the fuel pump output "bucket test" cannot accurately distinguish between a stock pump capable of pushing 5.5 psi and one that can only push 4.75 psi when flow is restricted. If a flow restrictor in the form of a smaller diameter hose is applied to the end, then the test may be more accurate. However, since there is no standard by which to guage, the test would have to be done first without the restriction, and then with the restrictor in place... -an awful lot of work to test the pressure of the pump when you can simply hook a guage up to the carburetor.

The Nikki carburetor is meant to have a constant feed of fuel about 1 - 2 psi over what the actual useable internal fuel pressure should be. The Nikki incorporates an internal fuel regulator, as simple in design as they come. It's nothing more than a bypass hole drilled in the internal plumbing through which all of the fuel going back to the tank via the return line must pass. I have found three diameters of return bypass orifice used throughout the Mazda Nikkis, according to dates manufactured. The larger the hole is, the lower the pressure of the fuel inside the carb, and visa versa. Indeed, the most trouble free manor in which to guaranty that the pressure inside the carburetor falls with a relatively narrow range according to specifications is to utilize a fuel pump with only a slightly higher pressure rating, and then bleed off the excess pressure using this simple bypass design for the fuel return line. While for a mass manufacturing application this "cookie-cut" design helps to eliminate the need for more precise fuel system metering, for performance applications where fuel demand goes beyond the specifications, the system as is simply cannot provide adequate flow or consistent pressure.

The problems with using the stock fuel delivery system for a performance carbureted application are three-fold; First, the stock fuel pump, even when new, simply cannot move the volume necessary to maintain the needed pressure for wide open throttle (WOT) fuel demand. The second problem is that even if it could produce enough volume and pressure, -or if it is replaced with an aftermarket pump that produces only slightly more volume than the maximum fuel needed at WOT, the internal regulator is not designed for the higher pressure. The orifice would have to be recalibrated to tolerate the higher pressure and volume flowing through the system. The third problem, most particularly important for track racers, is that even if the internal fuel bypass orifice were carefully recalibrated to tolerate and regulate the new higher volume & pressure, the variable range of pressure inside the carburetor would be much wider than the stock system, and with a modified Nikki carburetor, it would be enough to severely interfere with tuning on the track. The owner of a street performance application can tolerate a little bit of "slop" in the internal carburetor fuel pressure without much notice, but the track racer jet tuning in an effort to shave a few seconds of off lap times will be chasing his tail.

The stock fuel delivery system needn't be completely revamped in order to work with a performance application. All that is needed is an upgraded fuel pump, a fuel pressure regulator (FPR) and a reliable fuel gauge. Since the internal regulator of the Nikki carburetor is calibrated to maintain between 2.75 and 4.25 psi with a pump output of 5.5 psi, the internal bypass will allow for no pressure build-up while the performance Nikki is fed between 1.75 and 2.5 psi. This is the "theraputic range" of fuel pressure that should be maintained inside the performance Nikki. The pump must be up to the task of supplying the fuel necessary to maintain this pressure at WOT fuel demands, and the regulator is set between the pump and the carburetor fuel inlet. At this point, there are two ways to finish the plumbing; The return line can be left intact, running from the carburetor to the tank, or the carburetor can be "dead-headed". If the return line is kept hooked to the carburetor, then the pressure gauge can be ported off the third port of the FPR. This is very convenient, and the simplest (and therefore most common) set-up. However, the guage will then only read what the pressure is going to the carburetor, not what the actual pressure within the carburetor is. Chances are that if the FPR reads 3.5 psi, the internal carburetor fuel pressure is lower at WOT due to the return line being such a ready escape for fuel.

The option to dead-head the carburetor can be done two ways, each with the FPR running in-line from the tank to the carburetor inlet. The carburetor return line should be capped with the fuel pressure guage, and the third port on the FPR (depending on the model) can either be blocked off completely, or it can feed back into the return line. Capping the return line of the carburetor with the fuel pressure guage is a convenient way of getting a very accurate reading of the pressure going to the carburetor. I personally have had little luck completely dead-heading the fuel delivery system, and I choose to use the tank return line. If a pump such as the Carter 7lb is used, the return line to the tank may not be necessary. I say "may" because finding a fluctuation of greater than .75 psi throughout the power band while testing can be a concern for some and not for others. Any higher capacity pump used in the fuel delivery system should definitely make use of the tank return.

Fuel pressure in the performance Nikki carburetor should be kept as low as possible while still meeting the demands of maximum engine RPM & load. I'll state that again; Fuel pressure in the performance Nikki carburetor should be kept as low as possible while still meeting the demands of maximum engine RPM & load. Anything beyond that pressure only serves to overwhelm the emulsion system inside the carburetor and make it less effective than it's full potential. In the stock Nikki, a marginally performing emulsion system may present an indiscernible loss in performance, but the fuel consumption may be noticeably higher. This is due to the fact that the fuel is not being emulsified before the carburetor atomizes it, and the mist is not optimal. Consequently, the engine runs rich when it needs power, but doesn't always run leaner as it should for low load and cruising. There may be power in the mid to high range, but the low end will suffer. Correcting the fuel pressure will require a full jet tune up. This is why in the beginning stages of performance tuning larger jets should be used in order to find the correct fuel pressure. The pressure should be reduced incrementally between runs with no jet changes until the optimal pressure is found, which is the point at which if the fuel pressure is reduced by .25 psi or less, high end performance will suffer. Once the pressure is set, the emulsion system will be working optimally, and the carburetor is then ready for jet tuning. The pressure inside the carburetor will probably fall somewhere between 1.75 and 2.75 psi, but only if adequate fuel volume is supplied to maintain that pressure under the heaviest fuel demands.

Fuel gauges and regulators should never be skimped on when it comes time for purchase. Poorly calibrated gauges are not uncommon, and some can be off by 5% or more. But an un-calibrated gauge would still tell you a measurement for comparison from one adjustment to the next.The biggest problem is that gauges are also very inconsistent. Fuel itself is very inconsistent, so it stands to reason that measuring fuel pressure needs a bit of attention. Be sure that the temperature of the fuel and the gauge are the same. or as close as possible, to the last time it was read. Pretty difficult, I know, but necessary for serious tuning.

The Main Circuit

  Although fuel readily evaporates, air cannot be saturated with enough fuel vapor needed for engine combustion. This means that liquid fuel must be mixed with the air, or atomized. Proper atomization of fuel is the key to successful tuning. Introducing fuel into the air as a fine mist in specific ratios is what we need the carburetor to do. It's difficult to atomize fuel into air using only a variable stream of air as the pressure source. There is fairly wide leeway for fuel atomization quality to be "good enough" for engine combustion, but fuel atomization needs to be nearly perfect to deliver optimum power. Liquid fuel does not stay suspended in air for very long, and even worse, turbulence within the manifold causes the fuel droplets to crash against the insides of the manifold, increasing the difficulty of keeping fuel mixed with the air consistently. Though these are problems inherent to any engine, unless the carburetor is jetted completely wrong, these occur at low enough RPMs where it's not too much of  concern.

  Air flows through the venturi, or "barrel" of the carburetor. The nature of the venturi is such that as air flows through it, there is a pressure drop created at a point inside it. There is also a mini, "booster", venturi set down into the primary venturi. It amplifies the pressure drop, helping to draw fuel in at low RPMs, when the air flow is lower. Inside the booster venturi is the fuel pipe. It is located at the point where the pressure drop is the strongest. It is also located above the fuel level in the float bowl, otherwise fuel would simply spill out into the venturi. The fuel pipe comes up from a circuit at the bottom of the float bowl, where the fuel jet limits the amount of fuel that can flow up the fuel pipe.

When air flows through the venturi, the pressure at the end of the fuel pipe becomes less than the atmospheric pressure acting on the fuel in the float bowl, causing fuel to spill out of the fuel pipe and be carried into air flowing into the engine in a mist. This is sometimes referred to as the "fuel signal". As the flow of air through the venturi increases, the pressure at the fuel pipe decreases, creating a more powerful signal. The pressure drop is not linearly proportional to the flow through the venturi; the pressure decreases at a higher rate than the air flow increases. This means that the ratio of fuel to air increases as air flow through the venturi increases. But the carburetor has a way to "bleed off" some of that powerful signal; the "Emulsion System".

For each barrel, the emulsion system consists of a thin brass tube that runs from the top of the main circuit down through the same path that the fuel tube  uses. It has an air jet on the top, which sits far above the level of fuel in the float bowls, and the bottom half stays submerged in fuel. At the bottom of the emulsion tube is a series of holes drilled cross-ways through it. As air flows inside the booster venturi, the pressure differential created vacuums out fuel from the pipe. When the vacuum becomes great enough, it begins to also suck air down into the main fuel circuit from the air jet located at the top of the emulsion tube. This results in the outgoing fuel being laden with air bubbles which helps the quality and consistency of fuel atomization. The size of the air jet on the top of the emulsions tube dictates the threshold of the vacuum needed to pull air into the main fuel circuit.

Arguably this is the system's primary job in the carburetor (hence it's name), as fuel atomization is of the utmost importance in carburetion. But an equally important job that the emulsions system does is bleed off some of that overbearing signal to the main circuit when air velocity through the venturi becomes high enough to draw too much fuel into the air stream. In fact, if the emulsion system were blocked (air jets plugged), as the air velocity was slowly increased, the pressure differential would eventually become great enough to siphon raw fuel right into the carburetor. The signal to the main circuit is bled off, or corrected by jets located in the tops of the emulsion tubes. These jets are commonly referred to as "air bleeds", "air jets", or "correction jets".

The Idle Circuit

  The main circuit utilizes the venturi to create a low pressure system, -essentually a vaucuum, to draw fuel into the engine. It works very well at high velocity due to the nature of the venturi. In fact, it works so well that at really high velocity (relatively speaking) the signal to the fuel circuit is actually too high, and it needs to be "bled off" by the air jets in the tops of the emulsion tubes.

That defines the idle fuel rate. The idle mixture is regulated by adjusting the amount of air entering the carb via the idle speed screw. Idle speed is controlled by a screw located on the lower right hand side of the cast iron throttle body. The bottom end of the idle speed screw serves as a physical stop for the primary throttle shaft. As it's moved in, the "closed" position of the primary valves is opened up, and more air flows in. Consequently, the idle speed is increased, but the mixture changes dramatically with even a minute adjustment of the idle speed screw, which is why it is necessary to adjust them alternately until the desired result is achieved.

There is a throttle stop screw located on the secondary shaft as well, but this serves only to keep the valves from being wedged closed inside the throttle body. It is not meant to be user-adjusted, and if it's been tampered with, or if the secondary valves have been removed and replaced without being properly seated, the result will be a permanently raised lowest idle speed setting. Whether the raised lowest setting actually compromises the idle depends on how much air is getting through the secondaries. It is highly suggested that the secondary throttle shaft and these associated components never be adjusted.

There are eight very important jets associated with the idle circuit on the Nikki carburetor. Four are all referred to as "slow air jet #2"; two for the primary main, and two for the secondary main. These jets serve as air bleeds for the idle mixture, and are located on the outer edges of the top of the main body. Set closer to the center of the carburetor are four "dual purpose" jets. These are all referred to as "slow air bleed # 1", and are specific to the primary and secondary main circuits just as the other four are. These jets play a much smaller role in the idle circuit, and play more of a role in circuit transition. They serve to ease the transition from idle operation to primary main circuit operation, and then to secondary main circuit operation.

Ideal idle air / fuel mixture for the 12a with the complex stock emissions control system in place was specified to be a rather rich 10:1, as it required a rich mixture to help fully burn up the idle exhaust. With a header exhaust (or even just the rat's nest removed), this is slightly too rich and can result in fouled plugs. I recommend an idle mixture of just under 11:1.

Circuit Transition

Circuit transition in the Nikki is typical of most four barrel carburetors. As the primary throttle is opened, a pump called the accelerator pump shoots a steady, small stream of fuel directly into each primary venturi. If it were not for the accelerator pump, opening the primary throttle would cause a lean condition that would result in a stumble, even if it were opened relatively slowly. But with the aid of the accelerator pump, the mixture is made very rich, which is ideal for the load associated with acceleration.

On the stock Nikki, the secondary main circuit is vacuum operated, and only opens under significant engine load. As such, there is no bog associated with the opening of a vacuum operated secondary throttle shaft.

But on a modified Nikki with mechanized secondaries, the same transition problem that occurs from running on the idle circuit to running on the primary main circuit also happens while transitioning from the primary main circuit to both the primary main and the secondary main circuits. The accelerator pump can be modified to accommodate this transition the same way it does the prior one. This is done by both modifying the linkage to keep it pumping throughout the opening of the secondaries, as well as modifying the inside of the pump to increase the volume of fuel that it can hold.

Circuit transition is tunable on the nikki via the accelerator pump. The accelerator pump can be adjusted to give a longer or shorter shot of fuel, and if it's too much either way, the result will be difficult to discern. In both cases, it will cause a bog. Optimised accelerator pump tuning is subjective to an individual's driving style; -[to a point. Remember, nothing will make the rotary accelerate as fast as you can slam the pedal.

For information on tuning the accelerator pump, visit the Tuning section.

There are four jets associated with the transition from the circuit idle to the main circuit. These are basically dual purpose jets as they serve to bleed signal as well as emulsify the fuel coming into the idle circuit. Fuel emulsified in a carburetor is fuel that is mixed physically (in stead of molecularly) with air. The small air bubbles trapped within the fuel make it atomize easier when it hits the incoming flow of air that carries it into the manifold.

These four jets are removeable, but not replaceable in the Nikki. In other words, there is no variety from which to choose. But they should be inspected upon a rebuild to ensure that they are not plugged with debris or solidified fuel residue.

Finally, there is one last adjustment that can be made with regards to circuit transition on the stock Nikki with vacuum secondaries, and that is the vacuum diaphragm box spring rate. Changing this spring rate can make the secondaries come on with either less load associated engine vacuum prompting with a lighter rate spring, or more load prompting with a higher rate spring. generally speaking, for performance gains, it would be desireable to lessen the rate of the spring. Often people will simply cut the spring, but in order for it to push fully against the diaphragm, the spring must then be stretched slightly to make it long enough. This can actually make the spring have a higher tension than it originally did.

I have found that the best way to lessen spring tension rate is to apply localized heat to a section of it and let it air cool.