Modding the nikki

WHAT'S MORE FUN THAN TWO BARRELS OF MONKEYS?
...FOUR!

A carburetor needs adequate "air flow velocity" in order to have the fuel sucked out of the jets. If the venturis are too big, the carburetor will flow enough "volume" of air to feed the engine, but the "velocity" will be too slow to suck out the fuel through the jets. If they are too small, the velocity will allow enough fuel to be sucked into the carb, but there simply won't be enough air volume.

This is why oversized carbs disappoint people. There's no reason for the gapiing holes of a Holley 750 on a stock 12A rotary! It is also the reason why very small two barrel carbs will cause your rotary to have such a short powerband, yet a high peak horse power.

Because an engines requirements for air and fuel intake change with rpms, "one size venturi fits all rpms" does'nt really work to give us maximum performance. IDEALLY, (and someday I would like to try this, BTW!) there would be one venturi in a carburetor that acts like a cylindrical camera apperature that opens and closes in diameter with engine rpm....Not very practical.

Solution? Make more than one venturi in the carb, each with its own fuel jet from which the flow of air through the venturis can suck out. Each ventury has a fuel jet that is perfectly matched for its size to ensure that a rich mixture can be had with maximum air flow. (It's easy to regulate the fuel flow down from too rich). As one set of venturis begins to max out their air flow, another set begins to open.
In this mannor, effective velocity can be had over a greater range, ensuring a wider powerband. This makes your car "streetable".

The carburetor with 4 venturis is very much a design of "practicality". It allows the engine to breathe the perfect mixture over a wider range of rpms. If you take a good two barrel carburetor, and mate it to a rotary, there will be an RPM band up high where the engine is running with the maximum air flow, and perfect air/fuel ratio. But as you decrease rpms, the flow characteristics of that carb will not allow for the "perfect" conditions conducive to power output.

With a 4 barrel carb, you can have two points at which these conditions are met perfectly. When you have changeable jets and airbleeds, and therefore total tunability over the whole rpm range, you can make the transitions between those "perfect ratio spots" very subtle, so that the engine feels powerful all through the
power band. There are people who spend alot of time making four barrel carbs almost linear in their power band throughout as wide a power band as they can. Often there is a trade off, which is a bit of absolute high end power sacraficed for low end power.
Again...streetability.

The question remains often as a debate; but will ultimately be in the car owners hands as to which he needs most; An engine with a high peak horse power, but a narrow powerband, or an engine that has slightly less peak horse power, yet has a wide band.
Do you need a race car, or a street car?
People such as Paul Yaw, folks at Mazdatrix and Racing Beat, Pineapple racing, and many others have come close to obtaining both high end power usually associated with the Webers and Dellortos, and the wide power bands found in four barrels... in a four barrel application. But as yet in the world of carbs, there have always been, and probably always will be compromises.

Learn all you can.
Supidity is stupidity.
Not learning because you did'nt try is ignorance.
Niether is ever fast.


WHEN THE SECONDARIES OPEN...

Typically on a 4 barrel, the small "primary" venturis are opened for starting the car off, and cruising; just general driving. When it's time to pass someone, climb a hill, or just plain drive like a lunatic, the "secondary" venturis begin to open because the primary venturis have "maxed out" their airflow capabilities, AND/OR the corresponding "primary fuel jets" have maxed out their flow abilities, as well. Because the engine's still hungery, it's time to open up two more cans of beans. The same process starts for the secondaries.

A SMOOTH TRANSITION...

Ideally, we need the secondaries to "read" engine needs and cross reference that data with input data from the primaries. (Ahhh...screw it! Just get a fulie! KIDDING- I'm KIDDING!) Mechanical secondaries operate by having the secondary shaft linked to the primary shaft in such a mannor, that when the primaries are a certain
predetermined amount open, like about one third to half way, the secondary shaft will begin to open. As the primaries are slowly opened all the way, the secondaries are opening a bit faster than the primaries so that they are all finished with their stroke, and all four butterfly valves are straight up and down at the same time, allowing maximum flow.

In order to accomplish that, the linkage is hooked from an arm on the primary shaft to an arm on the secondary shaft; the linkage being a simple piece of heavy wire; the "arms" being more like cam shaped washers. But the length of the secondary arm is shorter than the length of the primary arm; and this is how the secondaries are able to finish their travel to WOT by the time the primaries do, even though they started to open
later than the primaries. For illustrative purposes, these cam shaped washers with different lobe lengths, and therefore different radii, can be thought of as a set of gears. The smaller diameter gear will turn more than the larger.

HOW THEY OPEN...

The stock Nikki carb is vacuum operated, but has its linkage set up in a mannor that is conducive to swapping to mechanical operation. The throttle shafts are honed flat on each side so that parts of the linkage can move the shaft, while other parts of the linkage can spin freely. There is a washer that can only move with the shaft on the primary shaft. It has a radial slot in the edge. There's another one right beside it, that can move freely, and has a tab that fits into the slot. This washer also has a small hole for one end of the linkage. There are two washers on the secondary shaft; One of these can move freely, while the other can only move with the shaft. These washers have tabs on them, as well. A heavy piece of wire links the primary shaft washer to the free-moving washer on the secondary shaft.

The throttle return spring holds the primaries closed. A spring is wrapped around the secondary shaft housing and hooked to the free spinning washer on the secondary shaft. The tab on that washer makes contact with the tab on the stationary washer, and so holds the shaft in the closed position. As the primaries are opened, the tab on the primary shaft attached washer slides along the radial slot in the free moving washer, untill it makes contact. Then, the primary shaft free moving washer, on which the wire linkage is attached, begins to move. This pulls on the spring loaded, free spinning washer on the secondary shaft, allowing the shaft to open.
But the shaft stays held closed!...By another funny business going on at the other end. And all this complexity is designed so that there is a certain amount of "free-play" in the primaries. That way, the secondaries are only allowed to begin opening when the primaries are already partially opened.

READING THIS SUCKS LESS THAN YOUR VACUUM SECONDARY ADVANCE BOX!...

There is an orifice in the front primary ventury that leads to a circuit that ends up at the box on the secondary side of the carb. In this box is a diaphragm that is hooked to a linkage that moves the secondary shaft. The diaphragm is spring loaded, and when there's enough vacuum present to overcome the spring tension, it sucks the diaphragm in which then pulls the shaft open.

The vaccum required to overcome the spring tension is substantial. Many people, including myself at one point, cut the spring in an effort to decrease the amount of vacuum necessary to prompt the secondaries to open. Removing the spring from the box alltogether basically renders it useless. The secondaries may then open with the slightest amount of decent vacuum present in the front primary venturi. The result could be similar to a large two barrel carburetor for some, while it will work great for others. There are variables associated with spring removal, including the condition of the diaphragm in the box. If is as old as the car, it may be no longer be as suple as it once was, and therefore you may need more vacuum than someone else.

Disconnecting the linkage from the box itself allows the secondaries to "free-float" when the primaries are moved far enough to remove the linked secondary shaft retaining spring tension. The secondaries are basically at the mercy of flow as a means to prompt them to open. A bit like having a small weather vane in your carb. This is why there are many varied results in doing this simple mod. A carburetor that is all gummed up will surely hold the secondaries shut where a cleaner carb is more apt to have the secondaries move. But they will not be timed, as they are not linked, and so will never open optimally, fully, or even consistantly.

Vacuum secondary operation is a great set-up for two reasons...Fuel economy, and a very, very smooth transition from primary operation to secondary. Often with mechanical secondaries, the transition is very noticable as a 'thrust' of power. This responce that one feels is often misinterpretted as power, when in reality it is the lack of fuel for a split second while the secondaries open. Forward inertia keeps the vehical moving, so the loss of power is only percieved when the power comes on. All this happens very quickly, and if there was a perfectly smooth transition, you would'nt feel the jerk of the vehical, but you would be ever-so-slightly faster. (we'll get to that.)

The fact that vacuum secondaries operate when your ENGINE says so is enough for most performance minded people to desire mechanical secondaries that they themselves control with their own foot. And converting the stock Nikki secondaries is very easy to do...IF you understand how they work to begin with.

CONVERTING TO...MECHANISISM?!?

You need to disconnect the throttle cable and choke cable before anything.
Now, yank off that vacuum box, along with the linkage. Well....don't "yank" it off, but remove it as if you were doing surgery on your dog. (You love your dog, don't you?) Don't destroy, or change anything that can't be put back the way it was. There are 3 holes on the carb where you took the box off; two for screws, and one where a short black plastic tube from the box went into. That hole is the vacuum hole, and should be plugged with perhaps a piece of rubber or plastic that you can remove later if you want to put everything back the way it was.

Study how the linkage moves on the "business" side of the carb. You'll need a wire twisty from a garbage bag. Remove the sheathing. This is what you'll use to wire the linkage together.
Now that the vacuum box is gone, there is nothing keeping the secondary shaft from moving when the primary shaft opens to the point where its linkage 'allows' it to. There's nothing prompting it to move, though, either. So you need to wire the two washers on the secondary shaft together, so that
the one with the linkage arm in it will pull the other one along with it.

DANCING BUTTERFLIES...TIMING IS EVERYTHING!

Ultimately, we want to end up with the primary and the secondary butterflies standing straight up and down together. This is truely "wide open throttle", and it's suprising to think that your carb may not be opening all the way up. Many people don't ever think to check, but after 16-20 years that ole throttle cable could've stretched a bit, and gone unnoticed.

When all of the linkage is just right, the secondaries will start to open somewhere between the 1/3rd to half-way point of the primaries' travel; and both shafts will finish their travel at exactly the point where all four butterflies are perfectly straight up and down.

Don't get cocky! There is a hidden danger here...If the linkage is not adjusted quite right, either on the wire that you put on, or the original link wire, a result could be that the secondaries don't actually close all
the way. This is no good, and will thoroughly screw up your idle. Check for any binding, and adjust your wire, as well as the original linkage wire (by bending or straightening it with pliers) as neccesary; And always double check that everything is timed properly.

Now you need an assistant to help with the throttle cable. Put the throttle and choke cable back on, and keep the nuts very loose. Have your assistant put the pedal all the way down, and you hand-operate the carb to wide open throttle. Snug up the cable, so it's roughly in the correct position. Have your assistant push the pedal down, and you can fine-tune the adjustment on the cable so that "wide open throttle" on the pedal really means "wide open throttle"! Be certain, of course that the cable is'nt keeping the throttle open during idle.
If there is that problem, then you'll have to do some pedal stop adjusting; but take serious caution not to spin the carb shaft too far, or irrepairable damage will be done to the components. (And, BTW, this is a real PITA if you have to do it! Just take your time, and get it right.)

Now drive it.

...So you drove it. It probably feels real powerful when you slam the pedal, say after 4500 RPMs. Anything under that, and you get a "bog", or "flat spot".
This flat spot is your engine gasping for fuel as the air velocity is suddenly interrupted by a "pie-in-the-face" rush of volume. For that fraction of a second, the engine does'nt have enough fuel, and is
running very lean, and with very little power. If we could maybe throw a surge of fuel in there, it would'nt suffer such a shock!

"I NEED A SHOT O' GASOLINE..."

The AP (because I'm tired of typing "accelerator pump"!) is a simple "squirt-gun" design pump in that it uses a check ball at the intake, and one at the outlet (under the nozzles). The operative part of it is linked to the "linkage headache" of the carb, and is designed to squirt a shot of fuel into the primary venturis upon their opening. If there were no AP, then the venturi butterfly valves would always have to opened very slowly to prevent stalling.

The stock Nikki AP set-up is fine for an economy sports car, and with the original vacuum operated secondaries, ...well, they opened so slowly, and infrequently, that there was no real need for an AP specifically designed
for them. But now that we've converted to mechanical secondary operation, we DO need an extra push when they open, because they will be opening every time you stomp the pedal!

Work the throttle shaft on your carb, and observe the squirt of fuel coming from the AP nozzles into the primaries. Observe, in particular, the duration of the pump shot with respect to the position of the butterflies and you will see, that if you were to be cruising down the highway with your pedal partially down, and then step on the gas pedal, there would be no pump shot...unless, of course you let completely off the pedal, and THEN stomp it again. That way, the linkage could travel back to closed position, and give
you a shot again on the way back to WOT. THAT does'nt sound like very much fun!

So - What we need to do is change the AP linkage to increase the DURATION of the shot so it will still squirt while the secondaries are opening. But then there's another lil' problem...There's simply not enough fuel to spread out over that long a stroke, and still be effective. So we need to increase the VOLUME, too.

We've fiddled with linkage crap enough for a while, so let's increase the volume of the AP first. For this, you'll need:
Carb gasket paper,
An exacto knife, (and I find that a hand held paper hole-puncher works
really well for screw holes, ect)
4 screws the are IDENTICAL thread pitch to the four AP housing cover screws,
but about 1/8th inch longer,
a strip of strong metal 1 1/2 inch X 1/2 inch X "whatever" (depending on the
material. I used a piece of 1/8 aluminum, but 1/16th steel would do it. It
needs to be strong, but drillable and cutable.)
a very small nut and bolt that will join this metal strip to the existing AP
lever,
...And a veriety of tools, including a file and a drill.

Remove the AP housing cover, and you'll find a diaphragm and a spring. If your diaphragm is a tough as an old shoe, get a new one. You need one that is soft and supple because you're going to "max-out" its movement to and
fro.
Take the housing cover and the gasket paper, and cut out 6 gaskets. Use a quarter, and trace out circles exactly in the center of each, and cut them out. Reassemble the AP using the four new screws, and putting 3 gaskets in front of the diaphragm, and three after. Pull that spring a bit so it's
about a quarter inch longer, too. That's it.
Now it does'nt seem like an awful lot, but it really makes a difference. The AP diaphragm can now pull more fuel in, and push more fuel out.

Now the linkage...

The original linkage is hooked up in a mannor in which the arc of the lever is so short, that it takes little shaft movement to complete that arc. What we want, is for that arc to be longer. Take the nut off the AP linkage arm, if you have'nt already. Take the strip of metal, and drill a hole at each end, so that the holes are about 1-1/14 apart. The holes need to be the same diameter as the one in the AP lever. Bolt the extention to the AP lever, and you'll see that the bottom will need to be bent slightly so that it is more perpendicular to the AP linkage arm.

Don't forget that spring on the linkage arm! Make sure there's good tention on the spring when it's all assembled by stretching the spring a bit. You may have to use washers as spacers on the AP linkage arm so that you can attach the nut, and still adjust it; Or, you can creatively bend or remake abetter extention.
But that's it!

Observe the pump shot, now. You should see that even if you stop operating the throttle midway, that when you continue moving it again, there is still fuel squirting from the nozzles as the secondaries are opening.

Accelerator pump tuning is not an art; but rather a purely "trial and error" proceedure. The key to being able to tune the pump is to make certain that the extention you put on is still in the threaded part of the AP tuninglinkage arm. Tuning should be done with 1/4 turns, and not in the driveway. The engine has much different requirements when it's trying to haul around a heavy car and your butt!

Also, more is not always better, and so it is entirely possible to end up with too much of an AP shot, with the results feeling exactly the same as not having enough. If you have too little, the is a lean flat spot. Too much obviously equals a rich flat spot. Your butt can't tell the difference, and that's why I say it's "trial and error". But with this set-up, you should be able to find a "happy place" where at over about 3000 rpms, you can nail it, and get no bog. It will take some doing, and this may even promp you to start experimenting with different fuel jets, ect.

You will still have a stumble if you try to nail it at under 3000; 2800 at best. If you drive like Grandma all the time, and suddenly want to nail it at 2500, well...get a supercharger!

Side note.........
There are two orifices in the front fuel bowl associated with the AP. I do not yet fully understand the set-up, but being of curious nature, I presumed that the top one was for AP over-pressure. I plugged that top hole by inserting a brass plug from the inside. I made the plug on my lathe, and I don't suggest anyone doing it by MY word, as I still don't know what it does (did).

The reason I mention this, is because I had to remove the airhorn and take out the brass threaded jet that covers the AP checkball, and put teflon tape on it, as I was losing fuel from that area. However, due to the fact that only one other person has tried this mod (that I know of...And he DID do the same thing, even though I said he should'nt, and had the same thing happen.), I don't know if the leakage was because of
the original mod causing extra pressure in the AP, or due to plugging that hole in the fuel bowl.
So, in short, you MAY have to remove the air horn to access that brass jet, and tape it up.

Please also keep in mind, that I have done this mod to an "already modified" carburetor. I use huge jets, and my carb flows about 1/3rd again what it did when it was stock. I've machined my own linkage set-up, and it's completly different than stock, so what I've just written is from memory.

I really hope this works as well for your stock carb, as it does for my carb. If you are not getting great results from this, my suggestion is to remove a pair of gaskets; one from each side of the "AP diaphragm sandwich", thereby decreasing the amount of fuel. But I have a feeling, it will be fine.

Feel free to Email me at dennisandmissy@mindspring.com with questions, and I would very much appreciate
feedback about results, too.

Dennis Williams, aka "Sterling".
P.S.
This is a compilation of information I've extracted from experimentation, The Mazspeed website, This site, Paul Yaws site, and several "rotary-wierdos"; including Speedturn, REVHED, peejay, Mikey D, and a guy named Steve.

I take full responsibility for any "Well hell...that-ain't-right!"s found in this write-up.
I only mention the above names to give credit where credit is due.

Original post from RX7club Member Sterling