I was looking into rejeting my Nikki. I was wondering what size jets you guys have ran for you primary and secondarys. Any info and how it would run after wards would be great. I have searched and have not found exactly what I am looking for, so please don't say search.


Thanks,
Chris

 

bump

 

i didnt know we could rejet our nikkis i think they have to be drilled for holly jets then you could go to town tring different sized primary and secondary
pm rx7carl or stearling for help on that one

 

Jets are available at Mazdatrix for a pretty hefty price.

I don't know anyone who's messed around with the jetting, myself. I'd be interested in the potential gains.

 

i've done it plenty.... there is NO NEED TO REJET unless you are boosting thru the nikki or have a ported engine. if you think you need more fuel then raise the float levels up a little or close off the secondary airbleeds. but seriously you will not gain any power or drivability from doing this.

why do you think you need to rejet?

 

if you wanna re-jet and dont wanna pay outragous prices
1. These jets are pocket items at junkcars.
2. Buy index and number drills.
3. Dril tehm out in small increments at a time and be sure to have a log book. You have to do this **** no matter what.

 

yea, if you're going to do this then get a set of "wire gauge" drill bits from ace hardware or something. or like wacky said. step up in the smallest of sizes because it will be noticable in how the car reacts to the fuel.

i just don't see the need for rejetting a nikki on a stock port. i've been there/ done that and tested this. no gains, UNLESSSSSS you have a cdi ignition setup and jet DOWN just a tad to lean the sucker out and you can gain a little better response.

 

Racing these things in ITA with free flowing exhaust and air filters and stock porting we left the primaries alone and went up about twenty points on the secondary. Its been over ten years but I think the secondaries are 160's and we went to 180's or 190's. Take your jets to a motorcycle performance shop they should be able to match up the design and sizes. Its very difficult to keep the AFR's in line on the top end without a properly designed intake system. Jetting down helps the mid range but hurts the very top and jetting up makes you a bit rich in the middle but perfect on the very top.

 

i have 140 primaries and 180 secondaries on my ported 13b

isaac

 

if you do back out some fuel on the primaries then you can always just leave the secondaries alone but close off the secondary air bleeds. i did this on a turbo setup and it worked great. the nikki is a small carb and with a 12a that has good compression it creates alot of signal from the boosters so there should be no fuel shortage on a stock setup

regardless, any tuning should truly be done with either a wideband, egt gauge, on the dyno, or at the track so you can truly tell if there is a performance gain. seat of the pants will hardly be noticed if at all for such a small gain.

 

A friend of mine had the same exact setup I have, but he had his carb rejeted and his car was a noticeable bit faster then mine and my motor had alot less miles. (80,000). I don't really understand what you are asking about why do I think I need more fuel. People but bigger carbs on 12A's all the time with noticable gains. I am not only talking about fuel, I meant to ask about more air flow also. I am just not good at wording things I guess.

 

Check some of Sterling's write ups in the FAQs that he has done on the Nikkis. Iirc, he makes a recommendation on jetting.

 

Cool man thanks, I have one more question, Has anyone picked up any gains from porting the intake manifolds. I know people have been back and forth on this alot, but a couple of peoples input on this would be nice.

 

ok, what i meant was... why would you need to jet the carb up? did you throw on a bunch of bolt-ons and think it's running lean? saying your buddy had an identical setup but was jetted up yet ran slightly faster could simply be driving or any one of many factors. you can't blind tune like that. people throw on bigger carbs yes, hell i did that. there's a difference tho, those carbs flow more air. more air needs more fuel. plus different carbs will create more or less signal which will also effect the jetting. there is no one cure all for jetting.

my main point is that if you think you need a little more fuel in the low-mid- or high rpms you can achieve this without re-jetting. jetting a nikki is not fun unless you strip it down naked to where you only remove 5 screws and can lift the horn off. i'm just trying to help save you some hassle.

 

No Rotor is correct. For a stock Nikki flowing only 313 cfm, there's really no need for larger fuel jets unless you have a free flowing exhaust and you consistantly shift @ 7000 RPM.
Driving style is a sorely underappreciated factor in carburetor tuning.
Two identical set-ups with different jetting may yield identical performance...until you swap the drivers.
Usually this is much more obvious with mech. sec.s instead of the vacuum sec.s.

No Rotor also made a great point about the Nikki having a very strong main circuit signal. It's a very well designed carburetor. But I highly recomend NOT capping the sec. air bleeds! This will only result in a siphoning of fuel into your engine. After only one test run, you'll need to clean your plugs.

I like to encourage people who want to play with their carb's jetting. The Nikki is a great carb to learn on, but you really do need to learn some air flow dynamics first to truly understand what it is the jet / air bleed changes you're making will be doing to the flow of fuel.

I suggest that you leave the air bleeds alone, and go ahead and play with the fuel jets. Don't get carried away, though. Small incremental changes are best, and be patient by driving the car for a day or two before making another change.

Holley Dominator air jets are available in packages of ten and many auto parts chain stores. They will probably have to order them for you, and they'll charge you about 30 bucks. If you find three bucks a jet too pricey, you can look online for Holley jets, but to get the savings you may need to buy far more than you intend to use.
If you buy the ten-pack, they are all the same size, so you will have to drill them out. Be sure to get size .023, as it's the smallest they make.

The threads are SAE 10-32, but are so close to the Nikki's metric fuel jet thread configuration that they fit right in.
When drilling, try to find a nut that fits the jet perfectly. You screw the jet into the nut and then clamp the nut in a vice. Use bees wax on the drill tip.

Not drilling jets in a lathe is very tricky business. By hand, it's all too easy to make the hole larger than the bit size, and when you're talking thousandths of an inch, a little is a lot.
(Remember you highschool geometry and crunch some pi(r squared) numbers, and you'll quickly see that small increases in bit size yields a counterintuitively high crossectional flow area.)
The other pitfall of drilling without precision equipment is that the hole may have a tiny bur or become slightly oblong. When this happens, the flow is impeded. The direction of the flow on one edge of the jet may be forced into the flow from the other edge, and the faster the flow, the higher the ratio of impedance.
In other words, if you have a bur on a jet drilled to .066, it may actually be maxing out @ the flow of a .052 jet. (...or worse.)

And then last, convert metric to SAE. There are a few freeware converters online that will make this a breeze. Just plug in the millimeter size and it'll spit out the SAE inch equivalent.
Holley jets are in SAE sizes, and the Nikki/Hitachi is in millimeters.
The primary fuel jet is either .92 or .93 mm, depending on your model, and the secondary fuel jet is 1.60 mm. These are .036 inch & .063 inch, respectively, I believe.
As you can see, the secondary fuel jet is quite large.
I think the only one worth playing with is the primary.

 

Why couldn't a simple fuel higher volume fuel pump and fuel pressure reguator be used to richen up the mixture a little? It's not like a jet change is really needed on a stock port. With free flowing intake k&N filter , free flow exhaust, and a littele timing I would think tweaking a psi or 2 more of fuel pressure shoud give you enough extra fuel for the above mods?

I don't begin to understand the dynamics of nikki's like the above gentlemen.

But during my rats nest remoal I accidently plugged off the air bleeds and noticed my car having a greater pull up top. It also flooded the car everytime I turned it off and made it run extrtemly rich. fouled plugs, crappy part throttle driving etc. Could a micro WOT throttle switch connected to a normally open vac relay/switch be used to close off the main air bleed at WOT only, leaving it to vent under normal driving but allowing it to suck in extra fuel under wot?

Just a thought. I'm sure I'm ten years behind most of the gurus here, and you will have ten reasons why the above statements won't work. But humor me I'm a newb!

Thanks!

 

The stock pump pushes about 5.5 psi, IIRC, and the carb has an internal regulator orifice drilled into the return line to ensure that pressure is up to about 4.25 psi max (3.75 on earlier models). Unless you're running a modified carb, the stock fuel pump should deliver "adequate" volume & pressure for most people.
Again, if you routinely shift @ 8 grand like I do, and you're running mechanical secondaries, you'll need a higher capacity pump.
In simple theory, you set the fuel pressure for the engine under the most demanding fuel needs, and that's the maximum you need. The stock pump will supply just enough volume for a header exhuast with a stock Nikki and a maximum RPM of 7K. With a stock nikki, even if you can meet the fuel demands for 8K, you will not notice a serious performance increase.

The higher volume / pressure fuel pump can richen the mixture under certain circumstances, but it's not consistant, and it's not the right way to do it.
As the fuel in the float bowls is used up by the carb, the float drops and the needles come down out of the seats and the bowls begin to fill up.
Ideally, the fuel pressure should be set that at max RPM, the fuel should be pumping in just enough to satisfy the engine's needs. This, in theory, will create a very narrow window of fuel pressure tunability that will have an effect on only the highest point of your RPM power band. Enter the emulsion system...

As the fuel level begins to lower in the carb, really only when you are maxing out the fuel pressure setting, the holes in the emulsion tubes begin to get uncovered. This leans out the mixture by effectively opening up another air bleed. Here is where there is a very fine line between leaning out, and having a perfect mixture with perfect atomization, or running rich enough but not optimally.
Only track veterans can do this very well. The rest of us have to "hit or miss" at it because rarely can we make test runs @ WOT for a whole mile.
Other than trying to get the very narrow tunability, the only thing too high a pressure will accomplish is rendering the emulsion system less effective as a fine tuning feature. This often happens as a result of dead-heading the carb's return line, and having the pressure set to the Dellorto/Holley standard of 6psi.

Raising the float levels will render the emulsion system completely usless, and increase your chances of siphoning at lateral Gs, an occurance that many autocrossers mistake for a leaning condition as their car coughs around left hand turns.
The float level should NEVER BE CHANGED because the emulsion tube holes that get uncovered as the fuel level lowers are perfectly in line with the midway point of the sight glasses. Raise it, and the holes don't get uncovered, causing siphoning. Lower it and the top set of tube holes are uncovered too soon, stifling aeration and leaning the mixture no matter how big a fuel jet you stick in.

As far as you having "accidently plugged off the air bleeds", I'm not quite sure how you could ever manage that. You may be mistaking other carb components for the air bleeds, I dunno. The emulsion tube air bleeds are simply drilled into the tubes themselves, and the tubes run down through the booster venturi anchor casting.

I come back to "driving style" again, because it's just so damn important in these conversations.
Some see "street performance" as being the street light race, and others, like myself, see it as twisty roads. These are two completely different jetting configurations.

What it comes down to is that the Nikki carb was obviously jetted with performance in mind. It's always a compromise between jetting for performance, and jetting to meet the US MPG requirements of the times, but the Nikki carb as jetted for the Rx-3,4 & 7 is nearly as close to the optimum performance configuration as they could get.

I love the Nikki because it offers so much room for modification. You can modify the venturis to satify a 12a bridgeport's appetite, and the fuel circuits will still deliver.
But without at least a free flowing exhaust, and some minor mods like mechanical secondaries & the accelerator pump volume increase mod, There's not really alot of room for jetting improvements.

 

thanks

 

i agree with sterling on all but one thing.. it is true that lowering or raising the float level should never be neccessary or touched but it CAN make a worthwile difference if done properly. and also it does depend on the driving you do. for drag type situations i have had success with lowering the float level and using a 1:1 fpr that pulls fuel pressure under high vaccum. i set the 0 vaccum fuel pressure to 7 or 8psi and when it's pulling 18 - 20 hg at idle and cruise i will be down to 3psi fuel pressure. this way i have a nice lean cruise and get good fuel mileage but when i go wot (mech. secondaries of course) the higher fuel pressure will kick in and raises my floats just barely above where they should be at stock and i get the fuel mix i need.

i have done this on many nikki's both n/a and boosted and it's worked great, but i warn anyone.... it's very very time consuming and quite a pain in the ***. hahaha

 

The demands at WOT vary too much for this practice be consistent for anything other than drag racing where virtually every run is identical.
Each gear presents a completely different load, and therefor has it's own optimum air/fuel ratio. Tuning optimally for WOT in 3rd usually means WOT in 2nd or 4th will not be all it can be. Just street driving alone presents a variety of loads on the engine that change the air fuel ratio demands.

Unless you've perfectly tuned every other aspect of the carb and car, I highly recommend leaving the float levels alone. If you do not, you really are asking for trouble.

 

I have a free flowing exhaust, mecanhical secondarys, modifed air box, 2GDFIS, and I rebuilt the carb back to stock when I first got the car. The car is mainly street use with some twistys thrown into the picture. The car sees 8 grand alot. So what would be your best advice on what I should do, should I just leave it alone for now or should i try to mess with it a little.

Thanks,
Chris

 

Your question has already been answered? From what I gathered just don't shift at 8 grand and U should be fine? U don't flow enough cfm through the OEM Nikki carb for that kind of RPM to be really useful? And if you messed with anything at all, just try opening up the primarys a tad. Just like stated above.

IMO w/o a WB02 or a dyno you're not going be able to tune it in properly anyway. The amount of power you gain from openign up the primarys a little probably won't be "seat of the pants" noticeable.

I'm sure I was mistaken... this is the port that I plugged originally. I was told by someone else on the board it was the air bleed for the secondarys. I'm sure I was misinformed. But the car did pull harder up top with this pugged? I suppose it could have just been that much slower down low that the transistion from primary to secondary felt quicker?



Anyway, thanks for all the killer info! U almost sold me on a sterling carb buy. I just don't think on a stock port 12A I can justify the power gain to the cost. Maybe if I was running a 14b or ported motor.... :cheers:

 

That's simply a carb inlet for the crank vent (oil filler tube vent). It offers low vacuum, and has absolutely no impact on the performance of the carburetor.

 

Why when mine was plugged did fuel continue to drip into the carb and not siphon back into the tank then? Not that I don't believe you but the two have to be connected some how? After starting the car if I block off that port I can watch the gas drip into the carb, as soon as I take my finger off the port it siphons back into the fue line? it made sense calling it the bowl vent... like so in this post...

https://www.rx7club.com/showthread.p...1&page=2&pp=15

 

Primary fuel jet is .92 or .93 mm, which is about .036 inch.
Secondary fuel is 1.60 mm, which is about .063 inch.

The secondary jet you won't max out. The primary can go as high as .055, but your fuel economy will suffer.
I suggets starting the primaries at a compromise of .048+/-.