karism

Helo Guys and Girls.

Last night ,i got the Nikki`s venturies back.
I sent it out to be machined bigger on Saturday.

The specs are as follow :

Primary stock :0.79 "
Secondary stock :1.10"

My machined venturies:
Primary :0.92"
Secondary :1.20"

Along with smoothed booster venturies,machined throttle shafts.Mechanical secondaries
Modded airbox

Fuel pump :
Bosch EFI prepump ,set at 2.5 psi with a Fispa pressure regulator (current fuel pressure)

The pump sounds insane ,but it gives a constant flow,with no pulsation.Its a rotary vane pump.(Its the pump that efi cars use before the big high pressure one)

So tonight,i`ll fit the carb,and start the tunning.

Karis

nosajwrx-7

Good Luck! Keep us updated!

karism

Thanks for the reply.Interesting that the rest of the guys arent interested.

Well, the first attempt to start didnt go good ,due to a semi-flat battery.

For some reason,i didnt change the primary fuel jets,and they are now way to lean for it to start.

Tonight i`ll change to bigger ones,and see if it helps

Karis

85rotarypower

I'm thinking of machining my own frankenstein nikki carb to make it flow better. I want to make it flow mabey even more air than a sterling carb, but we will see. I haven't even got to looking real closely to the carb yet, so I don't know the posibilities. I have to replace the carb on my car, so I will just use the bad one to experiment on. I want to hear the results of your carb though. I want to know if it will be worth the time on my part to try my own modded carb.

karism

I believe it will be possible to get the Nikki to flow more,than the Yaw carb,about Dennis`s (Sterling) I`am not sure.

The area`s of the Venturies differ greatly now from stock..

Primary venturies has 32% more area than stock
Secondary venturies has 19% more area than stock.

this,along with the cut throttle shafts,and so on,will yield a significant increase in flow.

Last night,i saw that i wasnt getting any fuel drawn into the idle circuit,and that was the reason for the previous no start situation.
I found that the idle jet was blocked,and cleaned it.Will have to see what the outcome is today

Karis

Rx7carl

Oh you can get more flow out of the Nikki, but the question is how much is too much? The next carb is going to flow more but will be for 12A BP and 13B SP and BP engines. The Sterling carb is right sized for 12A's stock or SP and 13B stock port engines. Beyond that, then you get into the realm of needing more flow. More isint always better. Remember velocity is just as important as flow capacity.

Sterling

You have to be careful when modding a Nikki for a heavily ported engine. You don't want to kill your low end...you already will have a problem there as it is because of the nature of the engine.

Karis, when you bore the primaries, state the diameter in mm.
State jet sizes in thousandth of an inch. (We like to be only half-assed on this side of the pond! )

So you're at 23.5 mm primary and 31 mm secondary.
That's good intuition, in my limited experience, and we'll get Carl's take on it, too.

We go 24 primary and stock secondaries, but you have to thumb through the history or visit the text on my boring, unfinished website to understand why we get the great airflow that we do- 452 cfm.
(Yaw's venturi diameters were the same, but he was getting 429 cfm.)

Remember to do the math. It keeps things in perspective.
When you calculate the volume needs of your engine, you'll find, for example, that a stock 12A @ 100% VE @ 8500 RPM will only be able to ingest 344 CFM.
You want to overshoot the volume a little, but not too much.
The engine's displacement doesn't change when you port it, but it's volumetric efficiency does.
For a Holley 600 to be an appropriate choice for a ported 12A, the volumetric efficiency of that engine would have to be around the 170% mark!!!
-Like I say, putting on such an oversized carb will kill the low end because there will be no velocity down low to cause enough pressure drop to allow adequate fuel into the main circuit. Plus you'll never reach full velocity up top because you're getting 170% VE no matter how much you want it!

Having that same carb with vacuum secondaries does nothing for you, either.
Just because the vacuum box only allows the secondary throttle plates to open only as far as needed has nothing to do with what's going on in the gapping venturis above them! Low flow equals low velocity equals low signal to the fuel outlet tube from the main circuit.


More math...
Remember area of a circle = pi x r squared...
Area of a circle = 22/7 x [1/2 diameter of the venturi]squared.

When you do this math, you will see that increasing the secondary venturi diameter by just one millimeter will yeild huge flow results when compared to increasing the primary diameter the same.

Since you want the primaries to have good velocity down low in the RPMs where flow volume is so little, it's a good idea to increase the secondaries instead.
--By all means open the primaries; but I wouln't go more than 24 mm.
By doing this math you'll see that with far less radical changes to the secondaries, you can increase the flow to what you need.

I can save all the guys who want to mod their own Nikkis some wasted time right off the bat...
The 79 carbs have a bigger choke plate, and thus a larger opening to the primaries. Common sense says that you should either use the 79 top or open up the later one.
That's one of the FIRST things we looked at, and Carl flowed the first Sterling Carb with a 79 top and a later top with the smaller choke plate hole.
-He got a 3 cfm difference! That's all. Shocked us, but we were also relieved because we were thinking about opening up every airhorn for major gains, and that would've been a SERIOUS pain in the ***!

Just a tid-bit for you.

Sterling

The banana doth speak in riddle!

Rx7carl

No, I'd say more in generalities.

You are the math genius ( ), and Im too lazy to re type stuff thats already plastered all over the net, including this forum. Now when were talking Nikki specific, I'll chime in.

Yea, kinda funny how myths get started. The old Nikki's we're nicknamed "bigtops" and everyone thought they flowed more just because it looks that way. But looks are decieving, and in matters of airflow, what seems intuitively right is often completely wrong.

karism

Hi!
The VE of the engine,is,as stated by Dennis,not that high.

One though though : Why do race 12A engines put 51 IDA`s on ?Because it makes more HP than the 48IDA`s!

I`am very aware,that velocity is needed in carbs.It IS useless to put a 600 Holley on a 12A stockport.
Dont worry too much,i`am not one of those people who put on oversize carbs,and try to make them work.My Rx7 is a toy,and isnt expected to be driveable,or smotth.Just just stupid,cant be expected from my porting

Anyway.I got it to start this weekend.Went,and put in huge fuel jets for the primary circuit.
0.035" Or 90 stock.
0.053" or 135 now.
Spent hours cleaning all the passages
I know its overkill, its the only ones i had handy.

Mmmm..It is alott crisper.It revs up much quicker when standing still,and in the gears as well.It sounds like a real j-bridge ,and not as much as a sp like it did before.I allso got the off-idle stumble sorted,and it drives alott niver now,with loads of off-idle torque.

I`ll be going down to 0.0492" or 125 main fuel this week,its closer to the area i need.

Any suggestions on the fuel jets for the secondaries ?

Take care

Karis

cdrad51

I guess it was an overkill for me to put 160s for primary fuel jet... and I have currently 190s for secondaries. What did you enf up putting for secondaries?

d0 Luck

a little off-topic, but doesn't the rx-2 nikki carb flowed the best compared to other RX carbs?

karism

Mmm,this thread came back from the dead..

cdrad51 :I thought about the fuel jets and couldnt come up with an answer,trying many different sizes

So i asked the guys who know.
Dennis(Sterling) and Carl(Rx7carl)

They told me that the Fuel jets size on the secondary barrel is more for midrange,and that is should tune my Main air bleeds.

So off i went to change the main air bleeds on the secondary barrels.
Changing them to a smaller size,will give a richer upper midrange and topend mixture.Seeing that less of the booster signal is bled off.

If you have enlarged primary venturies,you can put in larger fuel jets,but 160 is definately overkill.

d0 luck : Well i`am not sure which carb i have.As far as i know :The carb with the bigger choke flap opening flows the best,CFM wise

Dont quote me on this,i`am not 100% sure.Rx7carl can maybe verify this.



Ps : Read all the valuable posts of Sterling to learn about jetting,and the nature of the Nikki downdraught.Its in the 1st gen section Archive

He explains every circuit of the Nikki on his website.

cdrad51

Ok so you are using the stock 160s for secondaries?

Sterling

Carl did flow testing on the two different sized choke openings found on rx-7 Nikkis, and found that the difference was only about 3 or 4 cfm through the Sterling Nikki (at that time was only flowing 430 cfm).

With the improved version, the gain from the larger 79-80 airhorn might approach 5 or 6 cfm, but on a carb that is flowing 452 cfm it's a negligable amount.
You'll never actually be flowing that volume through the carb, so your gain at WOT might only be about 1-3 cfm...a concern overshadowed by the substantial restriction of some popular air filters.


This is a list of usual metric jet sizes found in Rx-7 Nikkis, and what their standard equivelants are:
1/100MM_____INCH

PRIMARY FUEL
91_________.0358 (rare)
92_________.0362
93_________.0366 (rare)
(Call all of these ".036"s)

SECONDARY FUEL
160________.063
(I have not found any other size in an Rx-7 carb. It's so large, that it's a bit like Hitachi simply slapped the largest fuel jet the carb could theoretically use, because afterall, it did have vacuum secondaries that rarely saw the fully open position; let alone the fact that a street ported engine with a mechanical secondary Nikki would be hard pressed to ever max out a secondary fuel jet of that size.)

PRIMARY AIR BLEEDS
60_________.0236 (very rare)
70_________.0276
80_________.0315 (rare)
90_________.0354

SECONDARY AIR BLEEDS
140________.055


When modding a Nikki, there is nearly nothing to be gained by cutting the secondary venturis larger for any 12A application. Most of the typical modification work that yields any gain is done to the primary circuit, and that includes fuel jetting.
The stock secondary fuel jet is very large for the needs of the stock or mildly ported 12A, and data collected seems to point to no gains from a larger fuel jet in heavily ported 12As, as well.

Jet configurations and recommendations are CARBURETOR specific, NOT port or engine specific.
What that means is that if Rototiller is running .08x in his secondaries on his Carter, he may get a beautiful result, but you can forget about trying that on a Nikki.
That's not to say that the Nikki can't perform as well as the other, either.

The two things that make the Nikki IDEAL for modding is the throttle body bore size configuration, and the outstanding emulsion system it has.
The primaries are just the right size to max out flow just under the peak power threshold for an unpinned rotary.
But without the ability to curtail the signal to the main circuit at that point, the carb would be either starving or flooding the mixture, and a smooth transition from primary main to secondary main would be very difficult.
Being able to change the air bleed in the emulsion tubes is KEY in carb tuning. The air bleed "bleeds" off signal to the main circuit (primary main or secondary main specific).
This is how the fuel delivery of two completely different carb models with very different fuel jets can deliver exactly the same mixture at a given rpm or volume.

Air jet selection (and fuel jet selection, to a point) depends on your driving style.
I drive in the power range of the engine all the time, rarely coming down below 5200 rpm during active driving. For this, what best suits me is a very small air jet because I rarely can "cruise" @ WOT, and am almost always on the accel or decel. So for my needs, I like the mixture a little rich throughout. (-I also get 12.5 MPG!)

But because of the ability to change airbleed jetting in my carb, I have created a slightly leaner band of part throttle that occurs (no load) at around 2700 rpm. Here is where I will learn to "cruise" and do some more "conservative" driving to see how far I can stretch the mileage. (Any load, such as an incline, and it becomes obvious there's little power. This is like a "flat spot" that I've tuned to be there on purpose to have the ability to conserve fuel.)
Ideally for this fuel conservation test, I will have to accelerate conservatively, as well. I can do all this and never open my secondaries.
But it's a misconception that you will always use more fuel when the secondaries open up.
For example, you could have the carb tuned so rich at the high end of the primary circuit that opening the secondaries actually leans out the mixture.

-All because of the emulsion circuit.

cdrad51

God damn that was some informative stuff. Thx Sterling.

karism

It`s obviously clear from Sterling`s post,why i asked him to help me with my setup.

Great info there Sterling!

(I have learned: I can never know enough of the Nikki)

Karis

lovintha7

I'm looking to mod my Nikki, but I don't have the tools. Can I send it to you, Dennis, to mod it? Also will need a jet and air bleed kit.

Sterling

Of course!
Cha-CHING! - another victim!

Relax folks, I'm kidding.
We don't make much money on these carbs because they are done in such small batches.

Just click the big button, and it'll take you where you need to go to find out more info.

A few extra jets will be supplied for tuning, and we hope to be able to have them for sale soon in a variety of sizes.
We will be selling them cheaper than other places.

You will need a 7 lb fuel pump, a cheap gauge, and a HOLLEY regulator (do NOT skimp on the regulator!).
I've just looked all this up on Summit Racing, and you can get it all for about 100 bucks.