Carburetor tuning can be very frustrating, even for an experienced tuner. It's often difficult to pin point a power problem,  sometimes when you make jet changes to correct it, another one presents itself. This is because most jet changes, particularly with the primary main circuit, have an effect throughout the whole range of the carburetor. Such a task akin to "cat herding"  can be avoided by following a handful of simple, sensible tips...

Have reasonable expectations regarding the power and response of both your engine and your carburetor.

Learn how the carburetor works. Take the time to understand what happens when you press the throttle.

Learn how to drive the car by manipulating the carburetor, taking advantage of it's strengths and avoiding it's weaknesses.

Keep a jet tuning log. Write down the changes you make and the resulting behavior of the car.

Be systematic in your jet changes. Keeping a log helps with this, and prevents a lot of wated time and frustration.

And perhaps most importantly, be patient. I often say, anyone expecting immediate "bolt-on" optimal power from an aftermarket performance carburetor without expecting some work is really kidding themselves.

Have reasonable expectations.

The stock Nikki carburetor is a two stage four barrel carb and lends itself very well to modification. The modified Nikki can be tuned for a variety of applications, including fuel conservation, street performance, track performance, and even drag racing. The more modifications done to the carburetor, the more diverse it's application. But it's as unreasonable to expect great fuel mileage from a carburetor that's optimally tuned for performance as it is to expect a carburetor that's set up for drag racing to be "streetable". Just because the carburetor is capable of doing either doesn't mean it can do both at the same time. The "best of both worlds" with regards to carburetor performance and fuel economy is completely subjective, as one must be sacrificed for the other.

  If you do not know already, take the time to learn where the power band starts in a rotary. Learn not to expect any significant power until around 3200 RPMs. Doing things like stomping the pedal from idle or shifting at 4000 RPMs is not going to yield power from a naturally aspirated rotary, no matter what carb you put on. Stomping the pedal from idle and expecting to bake the tires off the rims won't ever happen, either; -That's the stuff of V8s, boost or nitrous.

Also, if you're running a stock or modified Nikki carburetor, chances are you're also running a stock manifold. Even if the manifold is ported, it probably isn't going to match the flow capacity of the carburetor.  Expecting super high performance from the stock manifold is like expecting a miracle. That's not to say that good performance can't be had with one, but there's no denying that it remains the modified stock intake's bottleneck.

(Plans are in effect for a cost effective, free flowing alternative from SMW.)

  Learn a little carb theory.

Even if you've never changed a jet before, you can become familiar with what to expect from specific changes with a basic understanding of the relationship between the primary and secondary main circuits and the emulsion system. The small investment of time to learn about carburetor theory will save unnecessary test runs in your efforts to optimize performance. It will save a lot of frustration, time, and even expense if you're renting track time. Nothing seems to slow carb performance tuning progress quite like the frustration of knowing your paying for track time, and the clock is ticking.

On the Sterling Street Nikki, in addition to the two sets of fuel jets and the accelerator pump adjustment, there are also two set of air bleeds which, although used to subtly change the fuel curve throughout the power band, can completely screw up performance if they are too far out of whack. Learning about the emulsions system, for example, will help you understand what air bleed changing actually does to the mixture, and at what throttle position those changes become apparent, when they play their most vital role, and why it is so important to make changes in small increments.

Learn to drive.

The reader might be thinking, "Oh, now that's a bit presumptuous!", but the fact is, -and I'll bet any racer will agree, that experienced track drivers wouldn't just hop in a car with a modified Nikki carburetor and expect that they can drive in exactly the same manner as their old stocker. -And who should know better how to drive a sports car than them? Anyone who does try to drive a modified Nikki the same as a stock Nikki will quickly find out the limitations of things like mechanized secondaries; -"limitations" being relative to driving style, not performance.

The biggest operational difference of the Sterling Street Nikki is it's mechanized secondaries. Even someone with years of IT7 track racing experience will need to learn how to get the most power out of the carburetor. It's a whole new exercise for the right foot of anyone used to the automatic vacuum operated secondary throttle. It doesn't take much time to learn, but requires an understanding of the limitations of the engine, the mechanical limitations of the carburetor and some reasonable expectations to get the most performance out of it.

I recall on two occasions, spending long troubleshooting dialogs with people trying to help them get their Sterling Nikkis to perform better. In both instances, the only other carb they had for comparison was the stock Nikki. The Sterling wasn't "wowing" either of them, and they both complained of a "terrible bog" when they accelerated.

Eventually it was discovered that they were simply driving their rotaries like old ladies. I had taken for granted that they already knew the nature of the rotary engine, as well as the fact that they needed to operate the throttle completely differently now since the secondaries were no longer automated. 

Though the Sterling Street Nikki has an accelerator pump modification to improve the ease of transition from partial throttle to wide open throttle, it still can't replace fuel injection. And expecting yummy performance goodness by mashing the pedal on a rotary at anything under 4000 RPM is simply out of the question. Driving the carburetor instead of the car goes hand in hand with Tip # 2.

Keep a jet change log.

Carburetor performance tuning from scratch usually requires an arduous series of test runs and jet changes. Not keeping track of those changes and the results can make all your efforts add up to little more than trial and error. If you approach jet tuning as if it were trial and error, you're basically resigning yourself to a craps shoot, hoping you'll hit it somewhere near "optimum". There is a better way to approach jet tuning than just tossing a few different combinations at the carb and seeing what you get.

Write down the jet changes you make and the subsequent changes in performance. You'll have a history of what you've already tried, and you'll begin to recognize patterns. At first, for some folks, starting the "jet logbook" may just seem like a pain in the ass. But as patterns start to become apparent it will become an invaluable tool for the new track racer. Eventually experience will render the logbook obsolete, but that does not change it's importance in attaining that experience.

Write down little notes next to the test run data, such as engine backfire, popping at partial throttle, sputtering, etc. These all can be attributed to carburetor problems or adjustments, but they can also be caused by ignition issues. While these details may appear completely innocuous at first, just as recording jet changes begins to reveal patterns, jotting them down may also help uncover a correctable problem.

My jet change log sheet in PDF format can be copied from here .

Be systematic.

I'm self taught about carburetors and tuning just like millions of other enthusiasts. Of the many mistakes I have made in the past I can confidently say that 95% were due to not being methodical in my approach. Treat the carburetor that is not optimized for performance as though it has several individual issues that need to be addressed. Just as it is for troubleshooting any other problem, pinpointing where changes need to be made can only be successfully done by two ways; eliminating variables, and guessing. -And the mathematical odds of guessing correctly are not in one's favor when so many variables are involved.

I break tuning down into three basic parts, which is how I see most other people approach it, as well:

-the low end, in which the throttle range is from idle to the primary throttle opened as far as it will go before the secondary throttle shaft opens;

-The mid range, which starts from the initial opening of the secondary throttle shaft and ends at a midway point between the highest power-bearing RPM from the low end and redline;

-The high end, where the throttle range is from the pre-determined secondary throttle shaft midway point and redline.

If starting from scratch, for example if I'm testing a new venturi cut or modification, I set up the low end first without the aid of the accelerator pump. It's tricky, but it eliminates the biggest, sloppiest, most inconsistent variable on the whole carb. When I jet tuned IT7 configurations, I disconnected the vacuum secondaries so they wouldn't come on and interfere with my primary main tuning. After reconnecting the secondary main, I had to "tweak" the primary air bleeds, but I was confident that I had it as optimized as I could get it.

Breaking down the whole carburetor into it's individual segments allows you to have much more control in what I consider to be a bit of a scientific experiment.

Idle Tuning

Most people experiencing persistent idle tuning dfficulties with the Nikki actually have other problems, such as a vacuum leak, clogged air jets or a timing issue. But on a nice tight set-up with no leaks and no other engine problems, tuning the idle on a Nikki should be as simple as following a few easy steps, especially if you understand what the components and adjustments on the carburetor do. Attempting to tune the idle without any understanding of what the adjustments do is every bit the craps shoot as trying to performance jet the carb with no understanding of how the emulsion system works, though the chances for success are a bit better only due to the fact that there are fewer adjustments to make. If you read this in it's entirety and follow the instructions provided and still cannot get the carburetor to idle the engine at 750 RPM, then you have another problem. Some of the probable causes are listed at the bottom of this page.

Like most carburetors, the Nikki idle circuit is milled into the throttle body. With regards to idle tuning, basically there are two types of Nikki carburetor throttle body configurations that most of us need to concern ourselves with; 79 - 80 and 81- 85.

Before 1981, the Mazda designed Nikki incorporated three basic adjustments. One that was set at the factory and never meant to be reset by a mechanic or an owner, is the Primary Throttle Stop (PTS) screw. It is threaded through a tab on the cast steel throttle body, and a tab on a link attached to the primary throttle shaft rests against the bottom of this screw when the throttle is in the closed position. IIt is located just under the accelerator pump linlkage. The screw is set so that the primary throttle valves close fully, but do not get stuck closed, and then the screw is locked tightly into place with a nut.

There is a similar, smaller set-up for the secondary throttle shaft, located on the diagonal corner of the throttle body, underneath the the vacuum secondary link.

In the earlier Nikkis, both the air and the fuel in the idle mixture are adjusted using needle valves, located on the front-center of the throttle body. The larger screw on top regulates the air entering the idle mixture, and the lower, smaller brass screw regulates the fuel.

1978 - 1981 Nikkis:

* The idle air adjustment (IAA) regulates idle speed.              *  The idle fuel adjustment (IFA) regulates A/F mixture.

In 1981/82, Mazda (or Hitachi-Nikki) did away with the idle air adjustment, and inserted a fixed jet in it's place inside the throttle body. The fuel regulating needle valve was changed from brass to steel, but remained located in in the same spot, dead-center in the front. They were capped in the US with a 'tamper-proof' metal bullet cap that pops off easily with a flatblade screwdriver. The secondary throttle stop screw remained locked with a nut, but the locked Primary throttle stop (PTS) was changed to a simple adjustable screw with a spring between the head and the throttle body to keep it from vibrating out of adjustment. Now the idle engine speed could be more easily regulated. The idle fuel screw still remained the manor of regulating idle mixture.

As I understand it, there are 1982 Nikkis that still have the old style throttle body, using two idle needle valve screws.

1981/82 - 1985 Nikkis:

* The primary throttle stop adjustment (PTS) regulates idle speed.              *  The idle fuel adjustment (IFA) regulates A/F mixture.

Just as opening the throttle valves partially introduces more air to the engine, thus raising the RPM, adjusting the idle air adjustment or primary throttle stop sets the speed of the idle. Regulating the idle fuel adjustment changes the idle mixture, and as a result the idle speed will change slightly also, but not nearly as dramatically as when the idle air is adjusted. Adjusting the idle fuel can be thought of as a fine adjustment of the idle air, -in terms of engine RPM; the object being to set the engine idle speed using the idle air adjust screw and then set the idle mixture using the idle fuel adjust screw. Ultimately, one wants a relatively rich idle mixture, at an RPM of about 750 - 800.

However, the idle fuel adjustment will only introduce a small amount of fuel appropriate for a good idle mixture over a relatively narrow range of incoming idle air. Too much air, and no further amount of adjusting the idle fuel screw will make the idle mixture rich enough. It is also often necessary to alternate several times between adjusting the two components of the idle mixture to give the idle the correct mixture and engine speed. Sometimes setting the initial engine speed is the most difficult part of the process, usually with a refurbished carburetor where the adjustment screws have been completely removed and replaced. Particularly difficult can be a throttle body that has been rebuilt or modified, where the throttle valves have not fully seated themselves. Often customers with Sterling Nikkis find the need to reset idle after about a week, where the throttle valves have finally seated themselves from use.

The following instructions should pertain to any Nikki carburetor on a Mazda 12a rotary engine, and the measurements of idle speed, mixture and air  screws are approximations. The instructions below are meant to serve as a starting reference point. The settings needed to get achieve the desired idle can vary greatly from one carburetor to the next.

Ten Easy Steps to Idle Tuning:

• Step 1.    Adjust the Deceleration Dashpot and the AC Idle Compensation Valve so that they are not interfering with the primary throttle operation.
• Step 2,    Open the primary valves to get the engine running on the main circuit by turning the PTS in. (81½-85 & SMW modified Nikkis only.)
• Step 3.    Set the IFA to 1½ turns out from closed. (Never bear down on this screw to tighten!)
• Step 4.    Set the Idle Air screw to 1¼ turns out from closed on a stock Nikki, or ¼ turns from closed on an SMW modified Nikki. (...if you have one on your carburetor.)
• Step 5,    Start the engine, reduce the engine RPM using the IAA / PTS screw, and let it warm up.
• Step 6,    Turn the IAA screw in on pre - 81½ Nikkis, or the PTS out on post - 81½ Nikkis, until the engine almost shuts off. (This will lower the engine RPM.)
• Step 7.    Turn the IFA in, in ¼ turn increments, waiting 2 seconds after each change, until the engine starts to skip. (This will raise the engine RPM.)
• Step 8.    Repeat steps 6 & 7 until the desired idle speed is achieved. (Usually this is between 750 & 850 RPM.)
• Step 9.    Back the IFA out less than ¼ turn to ensure that the idle is not too lean. (This may require readjusting the Idle Speed screw first.)
• Step 10.  Readjust the Deceleration Dashpot and the AC Idle Compensation Valve.

  Step 1,    Adjust the Deceleration Dashpot and the AC Idle Compensation Valve so that they are not interfering with the primary throttle operation.

Commonly overlooked idle tuning pitfalls include not paying attention to extraneous components attached to the carburetor. Things like the Deceleration Dashpot and Air Conditioning Idle Compensation Valve have linkage that can interfere with the primary throttle shaft, and keep it from closing fully. Other pitfalls include not having enough slack in either the throttle cable or the fast idle cable. Still another possible trouble spot is the complex automatic choke assembly.

Since all of these components will need to be re-adjusted once the proper idle is established anyway, it is best to first adjust them to ensure they are not keeping the primary throttle shaft from closing fully.

Step 2,    Open the primary valves to get the engine running on the main circuit by turning the PTS in. (81½-85 & SMW modified Nikkis only.)

  The "PTS"(primary throttle stop) is located on the lower right side of the throttle body, looking at the primary venturi side of the carburetor. It's simply a screw that goes through the casting and serves as a stop for the throttle shaft in the closed position. Screwing it in opens up the primary valves just a bit, letting in more air, which raises the idle. Backing it out has the opposite effect.

  On the earlier Nikkis, this screw is at about a 45* angle to the top of the throttle body, and it has a locking nut. It's has been set at the factory, and it is not necessary, nor recommended that this screw be readjusted with the exception of the SMW modifed Nikkis. On these earlier throttle bodies, the large air control screw located above the idle mixture screw will have a similar effect of controlling idle speed by regulating the air in the mixture. On SMW modified Nikkis, certain performance models have been completely rebuilt, and the locking nut has been eliminated and a locking spring added to the idle speed screw to allow for it to be adjusted by the user. These can be tricky, as there are now three idle adjustments to make.

In the cases where the pre 81½  idle speed screw has been unlocked as described, the Air Screw should be set to ¼ turn from closed and left alone until a preliminary idle close to what is desired is established. Then it can be regulated slightly as an additional means of adjsutment.

In 1981/82, Nikki replaced the throttle body design with a similar one where the screw is parallel to the throttle body top and has a spring to keep it immobile from vibration, instead of a locking nut. It also has about a one inch long head to make it more accessible. The air regulation screw was also eliminated form the center of the front of the throttle body, and both idle speed and over all air allowed into the idle mixture are regulated by the idle speed screw.

Step 3,    Set the IFA to 1½ turns out from closed. (Never bear down on this screw to tighten!)

The Idle Fuel control is a small screw located front and center on the throttle body. The initial setting for this should be about 1½ turns out from closed. Never over tighten this screw! NEVER!!! This is a needle valve with a delicate, sharp point. It is made of soft brass on earlier Nikkis, and soft "junk steel" on the later ones, and the tip meets a hole drilled into the cast iron throttle body that has a very sharp, hard edge. Scoring the tip of this delicate needle valve is a sure-fire way to screw up your idle forever! (no pun intended.)

The idle Mixture screw regulates the amount of fuel in the idle mixture. The overall idle mixture air / fuel ratio should be about 8:1 on an Rx with a working emissions system, and no richer than 10:1 on an Rx without emissions system.

Step 4,    Set the IAA screw to 1¼ turns out from closed on a stock Nikki, or ¼ turns from closed on an SMW modified Nikki. (...if you have one on your carburetor.)

On later Nikkis, there is just one small screw that controls the amount of fuel allowed into the idle circuit. The fuel is introduced to the engine from a slit just beneath the edge of each partially closed primary throttle valve. Air flowing past the small gap in the closed primary bores creates a strong vacuum that draws the fuel out. However, on the early Nikki carbs, there is an additional, larger screw above the Idle Fuel adjustment. This screw is used to dampen that vacuum. It does nothing more than limit the amount of fuel that can be regulated by the Idle Mixture screw. In 1981½, the large air regulating screw was replaced by a pressed-in internal jet inside the throttle body due to a slightly redesigned emissions control setup.

Without a limiter of some sort, fuel would be siphoned into the intake via the idle circuit throughout the range of the throttle body.

Step 5,    Start the engine, reduce the engine RPM using the IAA / PTS screw, and let it warm up.

Obviously you can't tune a carburetor without the engine running, and to tune the idle properly, it needs to be warmed up, too. If the idle is tuned cold, when the engine warms up, the temperature will effect the fuel quite a bit, and the idle will lean out and raise the engine RPM.

If you guess at the settings, either by trial and error, or by using someone else's settings, you chance not getting the engine to start. If the engine cranks but doesn't start, it's likely to get flooded. Now you're in to wasting at least 10 minutes.

The carburetor does not need the idle circuit for the engine to run. It needs the idle circuit for the engine to idle. It's only there because the main circuit can't deliver any consistent mixture with so little air running through the venturis. But the main circuit will run at 1800 RPM.

Take a look at what that Idle Speed screw does again. It opens the primary throttle, so if you crank the screw in a bit, the carburetor will basically run on the main circuit. That's why I send all my carbs out with the idle speed screw tightened in so that the engine will initially start.

Set the IFA to an initial adjustment of 2½ turns out from closed. If you have a Nikki carburetor with an IAA as well, set that to about 2 turns out from closed. If you cannot get the idle to drop down to a reasonable RPM, the air adjustment may need to be closed a bit. Use ¼ turn increments, and repeat the tuning steps.

Before starting the engine, it's a good idea to practice locating the speed screw with a long screw driver so you can turn it down quickly after starting the engine. After turning theIAA / PTS in, when you initially start the engine it may rev right on up to 3500 RPM, and it's never a good idea to rev a cold rotary up high for very long.

  Fire up the engine and get out and turn down the IAA / PTS so that the engine is running at a comfortable warming-up RPM (around 1800 RPM). Let the engine get up to temperature because idle mixture changes while it's cold won't stay that way. Remember, the IAA turns in to lower the air, and the PTS turns out to lower air.

Step 6,    Turn the IAA screw in on pre - 81½ Nikkis, or the PTS out on post - 81½ Nikkis, until the engine almost shuts off. (This will lower the engine RPM.)

Once it's warmed up, turn the IAA in on pre - 81½ Nikkis, or back the PTS out on post - 81½ Nikkis, lowering the RPM. Keep going until the engine "hunts", or sounds like it wants to shut off. Turn the screw back in opposite direction about an eighth turn, to ensure the engine won't stall out.

Step 7,    Turn the IFA in, in ¼ turn increments, waiting 2 seconds after each change, until the engine starts to skip. (This will raise the engine RPM.)

Now direct your attention to the fuel mixture screw. Turning it in is going to let less fuel in, which is going to lean the mixture, which is going to raise the idle. Too lean a mixture, and the engine will die. To rich a mixture, and your plugs will get fouled.

Turn the screw clockwise until the engine begins to stumble. It's important to keep in mind that this adjustment tends to take a second to have a full effect, so make your turns in small increments, and wait 2 seconds for the result before continuing.

When the idle begins to stumble, back the mixture screw out just a bit (about an eighth turn), and back the speed screw out again until the engine begins to hunt.

Step 8,    Repeat steps 6 & 7 until the desired idle speed is achieved. (Usually this is between 750 & 850 RPM.)

Simply repeat this process until the engine is at the lowest RPM it can go and give a consistently purring idle. The RPM goal is 700 - 750 RPM, but I consistently get 650 on my stockport. (I do have an aluminum flywheel and direct fire ignition, however.)

Step 9,    Back the IFA out less than ¼ turn to ensure that the idle is not too lean. (This may require readjusting the Idle Speed screw first.)

When you've gotten the idle to where you like it, back out the Idle Mixture screw so that the idle is a little richer. This will cause the engine to bog and stall if you've already gotten the idle down to around 700 or 750 because the stockport rotary really doesn't want to idle at anything lower than 750 without an extremely lean mixture, so you should probably adjust the Idle Speed screw in a bit first. The reason we want a slightly rich mixture is because giving the engine a good workout is going to heat the fuel in the carburetor up. Because the fuel is expanded, the mixture becomes leaner, and coming to a stop can cause the engine to stall.

If you can't get below 1000 RPM, there is a problem. Either there is a vacuum leak somewhere, or there is an idle jetting issue, such as the incorrect jets, or a fuel blockage inside the idle circuit.

Step 10,  Readjust the Deceleration Dashpot and the AC Idle Compensation Valve.

Once the idle is all set, adjust the other components on the carburetor, like the Deceleration Dashpot and the AC Idle Compensation Valve, so that they do what they are supposed to do without effecting the initial idle.

The deceleration dashpot simply "cushions" the return of the primary throttle shaft, and should never interfere with the idle setting. The AC Idle Compensation valve, however, is designed to increase the idle via a vacuum signal to compensate for the drop in idle RPM due to the engine load from engaging the AC Compressor. Be sure that when the AC is switched off, the idle returns down to where you set it.

Keeping these components on the carburetor lubricated with white lithium grease is the best way to ensure they do not bind the linkage.

Persistent idle problems with stock carburetors include vacuum leaks, clogged air jets, and fuel delivery / pressure problems, including venting issues.

If you have trouble with a Sterling Nikki, it could be a matter of the throttle valves not seating fully closed. Exercising and lubricating the throttle shafts can usually cure this problem. Try this, as well as troubleshooting fuel delivery, pressure & venting scenerios.

Never be shy about emailing me or hitting my forum for help.

Accelerator Pump Tuning

The purpose of the accelerator pump (AP) is to introduce a rich mixture into the engine at very low engine RPM, where the air flow through the carburetor is too low to effectively meter out the necessary mixture for acceleration. It mainly serves as a transitional component from idle to part throttle.

The accelerator pump on the Nikki has an adjustment that is simply a nut at the end of the piece of linkage that runs through the pump lever. The lever is hinged via the pin in the pump cover and actuated by the primary throttle shaft. There is a spring on the shaft between the lever and the throttle shaft link. When the primary throttle shaft is opened, the spring pushes the lever that actuates the diaphragm within the housing. The fuel is pumped up through a circuit in the center of the main housing, and a small stream of fuel is forced into each primary bore.

The accelerator pump lever finishes it's travel at about the midway of the primary throttle shaft being fully opened, and the stream stops. The accelerator pump linkage shaft is spring loaded in order for the primary throttle shaft to be able to continue to wide open throttle even though the pump lever has gone as far as it can go. The secondary throttle shaft is locked shut by linkage that's under high spring tension. The action of the accelerator pump stops just short of the point where the primary throttle linkage begins to unlock the secondary throttle shaft.

The stock Nikki has vacuum operated secondaries that only open under significant engine load. If the accelerator pump were to continue to squirt fuel into the carb while they were trying to open, there would be a pronounced stumble, loss of power, and most likely some backfire, all due to the rich condition.

However, when the Nikki is modified to have mechanical secondaries, the lack of a steady stream from the accelerator pump while opening the secondaries causes a stumble due to a lean condition. this can be corrected by modifying the accelerator pump so that the housing holds more fuel, and the accelerator pump lever has a longer span of travel. This is a modification I make to the Sterling Nikki.

Adjusting the accelerator pump is purely subjective to driving style. The basic setting is with the diaphragm partially pushed in. At this point, the portion of the accelerator pump lever that is pinned in to the slot of the accelerator pump cover is parallel with the edges of the slot. It's a good starting point from which you can increase or decrease the pump shot.

The adjusting nut is located on the threaded rod that goes through the bottom of the AP lever. Turning the nut clockwise pushes the lever out from the diaphragm, increasing the volume of fuel that goes into the AP circuit. At the end of the circuit, where the nozzles are located, is a brass banjo bolt holding the nozzle head in place. Inside that banjo bolt is a pressed-in jet that controls the flow rate of the fuel stream. On the stock Nikki, this is rather small, and adjusting the nut for higher volume within the pump translates into the AP stream lasting for a longer duration at the same rate because the action is ultimately spring loaded via the external linkage spring (the diaphragm spring simply opens the diaphragm to draw in more fuel to the pump when the linkage is returned).

Turning the adjusting nut counter-clockwise pushes the diaphragm further in at the throttle starting point (idle), limiting the amount of fuel the diaphragm is allowed to draw in. As the throttle is engaged, the pump still pushes fuel through the nozzles at the same rate of flow that the jet in the banjo bolt dictates, but the stream will run out sooner.

On the Sterling Nikki, the jet has been drilled out to increase the fuel stream slightly, the pump has been hollowed out a bit to increase the maximum amount of fuel it can hold, and the linkage has been modified to continue the stream through the mechanical secondary throttle operation to aid in acceleration even at high engine load and RPM.

In either case, the AP should be adjusted by alternating ¼ turn increments of the adjustment screw with test runs that focus on acceleration to see what happens.

 

The accelerator pump lever with an extension on a Sterling Nikki. Adjustments are made the same way as stock. The nut on the rod going through the bottom of the AP shaft controls the volume of fuel the AP can take in (and thus pump out). Turning it clockwise increases the volume of fuel.

Noteworthy:

The accelerator pump begins introducing the same rate of fuel as soon as the primary throttle is opened, no matter how it's set.

On a stock Nikki, the only thing you're really changing is how the primaries respond.

In some cases, the accelerator pump will need to be adjusted after tuning the idle.

On Sterling Nikkis with the "flip-O-matic" secondary linkage, keep in mind that the AP will have to be adjusted when going from mechanical secondaries to vacuum secondaries, and vice versa.

Performance Tuning

 Let's dive right in to the stock Nikki so we can get familiar with the base jetting that gives us the stock performance. 

The Nikki carburetor jets are all drilled in metric measurements. Unfortunately here in The States, we've decided not to join the rest of the entire world in adopting the metric system. I for one would embrace the simplicity of the metric system, but as it stands now we'll need to do a bit of converting to get the stock Nikki jetting into terms most of us are more familiar with.

All we're going to concern ourselves with here are the fuel jets and the emulsion tube (air) jets. A more complete chart of North American production Mazda Rx-7 Nikkis can be found here , with the jet sizes also converted to SAE.

The primary fuel jets range from .91mm to .95mm, which converts to .036 - .037 inch.  While it is true that jets manufactured in increments of 1/100 millimeter  offer more of a range than jets manufactured in increments of 1/1000 inch, for the purposes of performance tuning the 1/1000 inch jets really are just fine. I doubt any appreciable gains could be had by using "in between" sizes measured in 1/10,000 inch.

The chart below shows the main fuel & air jets for all common North American Rx-7 Nikkis. Notice what sizes we're dealing with as a base for jetting: .036, .037 for primary fuel; .063 for secondary fuel; .035, and down to .024 for primary air, and either .055 or .063 for secondary air.

Once the conversion from metric to SAE is made for people that are used to dealing with SAE measured jets, the stock Nikki takes on a different light.

Immediately obvious is the very large gap between the primary size fuel jet and the secondary.

The configuration is good for the stock maximum flow of 313 cfm, but it won't meet the fuel demands of a Nikki modified to flow 465 cfm. There's lots of room for some improvement, but where?

There are a few different flavors of performance tuning. We can jet the modified Nikki moderately in the primary circuit and rich up top. That will give us great performance at WOT, and will conserve fuel when we're cruising along with the primaries just cracked open. We'll be lacking in the acceleration department, but we can compensate for this somewhat by adjusting the accelerator pump. This choice is a fair compromise between fuel conservation and performance and probably the best to use on the street if excessive fuel consumption is a concern.

We could jet the primary circuit rich for great acceleration, instead. this will get us into the secondary stage and into WOT faster, with less bog, too. No need to fatten up that secondary main too much because the primary will be supplying a rich mixture even up top. The draw back is, of course, power at the expense of a noticeable drop in fuel conservation. This is because even when you're just cruising with the primaries only cracked open a bit, you're running fairly rich.

This is the best jetting for track and auto-cross cars, without a doubt. They need that low to mid-range "power on tap" acceleration.

We could just fatten up all of it so it's supplying a rich power mixture all the way through, right up to WOT; - a real fuel pig! But even the drag engine can get drowned by a mixture that's too rich. Fact is, the jetting for a drag Nikki wouldn't be all that radical compared to the ideal configuration for a track Nikki.

 

	Primary Main Fuel	Primary Main Air	Secondary Main Fuel	Secondary Main Air
Economy Tuning	.032 ( smallest )	.027	.052 ( smallest )	.048	Configured for fuel economy. Very little power, and presents the danger of running lean (hot) @ WOT.
Stock	.037	.030	.063	.057 ( largest )	The average of jet sizes for all models. Configured for complete combustion of the fuel (emissions standards).
Street Performance	.048	.043 ( largest )	.065	.052	Configured for high-end performance and low end fuel conservation. Cruising mixture is set for less than 2K RPM.
Track Performance	.055	.027	.063	.045	Configured for full range performance with an emphasis on acceleration. Cruising mixture set above 5K rpm.
Drag Racing performance	.058 ( largest )	.023 ( smallest )	.067 ( largest )	.043 ( smallest )	Configured for power under load at any RPM over 5500. Very rich, and virtually useless for track or street.

In the chart above, I've laid out some very basic configurations to illustrate both the progression of jet sizes as performance increases, as well as the full range of jets we would ever really need for the modified Nikki. Obviously each configuration needs adjustments to suit application, environment, priority and driving habits, but the largest fuel jet (.067) and the smallest air jet (.023) on the chart makes up the full range. No smaller air jet will help, and even the ported 13b rotary is not going to require anything larger than an .067 secondary fuel jet unless the primary fuel jet choice is radically small for some unique application.

I'm going to concentrate most of my discussion on jetting configurations for street performance and track performance since that comprises by far the largest percentage of my customers. I refer to economy tuning and drag race tuning as "special application" tuning, but I consider them no less important in the discussion.

In the Sterling Nikki, the emulsion tubes are modified to receive the same Holley jets that work as Nikki fuel jet replacements. So for the full range of tunability of the Sterling for all applications, you'll need pairs of jets .023 through .067. Jets can start to add up as an expense, so it's best in my opinion to define your application, and then purchase jets according to the chart so that you have pairs that are a size over and under each one suggested.  With the Sterling Nikki, since the Holley fuel jets are interchangeable with the air jets, 8 pairs of jets, carefully chosen, can give you a very good range for fine tuning

Look at the size of the air bleed relative to the fuel jet for each tuning application in the chart. Notice the corresponding air bleed gets even smaller than it's respective fuel jet as performance is increased in each jet configuration. This is because air / fuel ratio needs to be a little richer for performance.

The fuel jet simply limits the amount of fuel that will enter the circuit when the air velocity reaches it's peak. With the same sized air bleeds, an .055 primary main is going to deliver the same air / fuel mixture as an .042 at partial throttle, running at 3000 RPM. This is because at that RPM, the carb is only going to have enough velocity running through it for a certain strength fuel signal. As the velocity increases the subsequent fuel signal increase will soon start to meet the flow capabilities of the .042 jet, and it will start to lean out. If the air jet is too small, the fuel signal would max out the air jet flow capabilities, and the mixture wouldn't lean out as much.

Below we see air / fuel charts from dyno run data demonstrating how small jet changes effect the A/F mixture at RPM increments of 500, going from 2000 RPM to 7500 RPM. We can learn a few important things right off the bat just by looking at the first four runs where there were no jet changes made at all. First and most important is that, even in as controlled an environment as we could get, there's a lot of room for leeway in what the air/fuel gauge hooked to the tail pipe is reading. A/F sensors are inherently finicky, and any sensor clipped to the very end of the exhaust system is going to have a few milliseconds of delayed reading, as well as a less than perfect accuracy of the A/F for any given RPM unless the RPM is sustained for an appreciable amount of time. In my opinion, true A/F readings should be averaged from several track runs. However, for our purposes of simply observing the effects of jet changes relative to A/F mixture, the following chart will suffice.

The second thing we need to realise is that any single slice of data presented here is as useless by itself as a quote from a politician taken out of context. The "whole" of the data must be consumed to fully appreciate how the jet changes effect the A/F mixture, for the reasons given above. As bad as singling out one piece of data from the whole compilation is not taking into account the conditions under which that data was collected.

These dyno runs were done with a prototype manifold, on a stock port with stock ignition, running a Sterling Nikki. The peak rear wheel horse power numbers are just under "flattering", in my opinion, and I think we could have done much better with a few other tweaks, such as advanced ignition timing. The following "mixture data / RPM" was taken during acceleration only. It's easy to see exactly where the accelerator finishes it's travel on the Sterling Nikki under load during these tests, as the mixture is consistently, dramatically effected at 4000 RPM.

In every test run, the mixture begins to richen significantly after 6500 RPM, and by 7500 it's in a steady accelerated decline towards pig-rich. The power begins to decline after 7000 RPMs, as well. This is due to the restriction of the stock manifold, and is unfortunate on so many levels. Obviously, the Nikki, and the Sterling Nikki, of course, would be much better contenders against the likes of the Racing beat Holley and the Weber & Dellorto carburetors if the manifold wasn't such a bottleneck. But even for the purposes of trying to demonstrate jet change results, had the manifold not begun to choke the intake at 6800 RPM, we would have a wider range of changes to work with.  We also were running the fuel pressure too high, but unfortunately didn't have the means to change it at the time. The results demonstrate how important fuel pressure tuning really is. Because of our limited time at the dyno, and the fact that the closest one to me is a 3 hour drive, we couldn't do a full tuning like I would have liked. Chances are, had the fuel pressure been properly tuned on the test car, the A/F ratios would have changed a bit more dramatically, as the emulsion system would have been aerating the fuel better, and the high end mixture wouldn't have been so rich. However, the data still serves as a useful demonstration of what effects jet changes have on the A/F.

As it stands now, one has to disregard the A/F readings at 2000 RPM due to the fact that every run seems to start off with the A/F wildly off course, and we have to pretty much negate the 7000 & 7500 RPM readings due to the manifold breathing limitations.

  Sterling Nikki Dynomometer Air / Fuel Ratio Readings

( prototype ported plenum stock manifold, Racing beat headers, stock timing )

 

Run	Pri. Fuel	Pri. Air	Sec. Fuel	Sec. Air		RW HP		Air - Fuel Mixture / RPM
								2000	2500	3000	3500	4000	4500	5000	5500	6000	6500	7000	7500
1	.053	.028	.063	.043		102.7		12.4	13.1	12.8	12.6	13.5	13.0	13.1	13.3	13.4	13.6	13.2	12.9
2	.053	.028	.063	.043		102.2		13.2	12.6	12.5	12.9	12.8	12.0	13.7	14.0	13.8	13.7	13.5	13.2
3	.053	.028	.063	.043		103.9		13.1	13.0	13.0	12.6	12.7	12.1	12.9	12.8	13.0	13.1	12.8	12.5
4	.053	.028	.063	.043		103.1		13.7	12.7	12.3	12.6	12.1	12.6	12.9	13.0	13.0	12.9	12.6	12.4
5	.053	.028	.065	.043		102.8		19.4	12.8	11.9	12.6	12.2	12.7	12.8	13.0	13.1	13.0	12.7	12.3
6	.055	.028	.063	.043		105.5		13.3	12.5	12.2	12.1	12.0	13.6	13.0	13.3	13.3	13.3	13.2	12.5
7	.055	.028	.063	.043		104.9		13.0	12.3	11.8	12.2	12.7	12.4	13.0	13.2	13.1	13.2	13.1	12.4
8	.055	.023	.063	.043		105.2		12.9	11.9	11.9	12.0	12.0	12.7	12.8	12.9	13.1	13.2	12.8	12.4
9	.055	.023	.063	.036		103.6		12.7	11.7	11.9	11.7	11.2	11.3	12.7	12.3	12.7	12.6	12.4	12.0
10	.055	.023	.063	.036		101.6		12.7	11.6	11.5	11.6	13.1	12.4	12.5	12.4	12.7	12.7	12.4	12.0
11	.055	.023	.063	.049		105.5		12.2	12.1	11.9	11.5	12.0	13.5	13.1	13.3	13.4	13.3	13.0	12.6
12	.055	.036	.063	.049		105.4		13.3	13.2	13.0	12.8	13.0	14.1	13.6	14.0	13.9	13.6	13.3	13.1
13	.055	.043	.063	.049		103.1		14.1	13.7	13.3	13.1	13.0	13.0	14.1	14.3	14.2	14.2	13.8	13.3
14	.055	.028	.063	.049		103.0		12.3	11.8	12.1	12.0	12.1	13.3	13.5	13.5	13.6	13.4	13.1	12.7
15	.055	.028	.063	.049		103.1		13.1	12.4	12.2	12.1	12.2	13.0	13.1	13.6	13.4	13.5	13.2	13.0
Stock	.037	.035	.063	.055

 

Run #1
Pri. Fuel: .053       Pri. Air: .028      Sec. Fuel: .063       Sec Air: .043
7500
7000
6500
6000
5500
5000
4500
4000
3500
3000
2500
2000
RPM
	11					.5					12