I did most of the same mods to my race carb last summer. I bored and polished the venturis (without removing them) using a dremel. I didn't change out the butterflies in mine... I left the inner diameter stock and instead just opened up the top and made the taper slightly longer. I can't remember any of the measurements - hopefully they're still in a thread here somewhere. I also drilled out both sets of jets as well as the accel pump jet. I trimmed and shaved everything I possibly could.

A year later, I'm still really happy with it. Not only is the power increase substantial (butt dyno only, unfortunately), but the drivability is nothing short of amazing. It runs better than the stock carb did, rebuilt with all of the emissions and gismos intact. It's definitely a gas guzzler thanks to the larger primary jets. I see AFR's around 11.5:1 cruising down the highway. But at WOT it's consistent at 12-12.5. It's been great for autocross and has plenty of low end power. I think I've done about all I can with it besides seal it up and add boost, which is the next step for it.

 

Looking good Jeff



.

 

that a v-dub? and i've taken the venturi's out with a socket and small hammer. you just have to make sure the OD is the same.

 

I probably should have done this but now it's too late. I sent off both of my good rebuilder carbs to people who were supposed to do this for me. I'm thinking a trip to the local junk yard might get me a good used carb for mods.

 

Percent, yes that would be cheating.

You uh, you like the manifold you say? I already tried the manifld and this carb last month in the blue car. The results were impressive. It felt like it has a Camden or like Tom's green REPU did way back in the day when it had the RB Holley 550. Basically monster secondaries because there was a bog when they'd open (he stuck a screw in the linkage) before things would start flowing. This Nikki did the same thing. So hopefully the accel pump mod will solve that problem.

I'd like to leave the venturis alone in the boosted carb, just so I know it will have good drivability. Then again, these hogged Nikkis aren't as bad to drive as you'd think. This is one of those times where it looks bad on paper but real life proves otherwise. Plus I'd like to try my hand at venturi enlarging, but it seems like a fairly critical mod. In a boosted app, I'd rather play it safe. What do you think? We already know a smaller carb is better for boost than a larger one. Should I leave the venturis alone? Or enlarge a little? A lot?

Glazedham42, I missed your post in July. I was kinda busy. Nice job! And thanks for posting your mods list. I figured if anyone would pop a woody over the floats it'd be you.

You didn't mention it. How large are your venturis? Also, why did you remove the OMP tubes? My racing carb had them filled with a putty which I drilled out and tapped some OMP tubes in from a parts carb. No way I'm adding premix to every fill up when I don't have to.

By the way I see why you replaced the screw in the throttle shaft with smaller buttonhead ones. It's an FB only problem. The SA screws are smaller and made of brass. And only the primaries stick out. After a little grinding they're good enough for me. I have yet to mess with an FB carb this way. But I'll probably just grind the ends down and call it good. They only affect WOT anyway.

I think I'll go for an 84-85 carb next because Sterling mentioned they have a better idle circuit or maybe they were better candidates for boost prepping. Can't remember exactly. But I've got a decent looking one here just waiting for some attention. I hope it's a low mile unit without water damage inside.

LizardFC, good to know. Makes me want to give the venturi mods a try. I'll have to do a search for your name to find your info but it would be nice if you could remember a keyword or two and search (and find) the thread yourself, then post a link.

kentetsu, thanks! I know you're an accomplished Sterling carb user.

keeble, you are winna hahaha! I just sent a fake forum buck.

I have a parts carb to play with. Maybe I'll try tapping out a venturi or porting it in place. Hmm...

clubber, yeah once you get over the initial "first time" syndrome, modding these carbs becomes fun and not scary at all. I enthusiastically look forward to the accel pump mod especially because I had done a few before, but I was a little apprehensive about modding the boosters as I had never done it before. But the results turned out great. I'll mod another set on my next carb. Still not sure about modding the venturis themselves though. But I have a steady hand and some dremel experience. Plus I kinda aquired a feel for how soft the carb pot metal is while working with a file. I'm sure a dremel, if used right, can do what I need it to do to the venturis.

So keep your carbs. Don't send them off to folks who promise big and deliver little. Mod them yourself and save. Now I sound like a commercial (or presidental - same thing).

 

I'm going to run premix on Gus, so that is really the only reason I took out the OMP tubes. Sterling always said that they caused a little bit of restriction as well, but probably not a whole lot. I'll have to check on the venturi sizes for you. Off the top of my head 22 mm for the primaries and 28 mm for the secondaries seems right, but I'll have to check my notes. They both definitely get enlarged, and re-profiled though. I found Sterling's recipe online somewhere in one of his posts.

And, yeah. The throttle shaft screws on the FBs are HUMONGOUS. They are also a total PITA to get out. It was an exercise in super concentration to get those throttle shafts removed, milled, reinstalled, aligned, and the new button head screws loctited in. I finally got it all though. I need to put together some kind of throttle stop for the accelerator pedal now to keep from twisting the throttle shafts. I'm thinking an adjustable stopper on the driver's side floor that the pedal can hit.

 

If it's to be a "fake Stirling," you should call it a "Pewter."



Oh, and here's a really screwy off-the-top-of-the-head idea for you:

Wonder if it's possible to set the accel pump to run off a cam instead of just a linkage... a cam with a second lobe that strokes the pump again when the secondaries open?

 

Here are some drawings I made for milling down the throttle shafts. I took them to a machine shop that had a mill. They had to put together some custom fixturing for me, but got the job done and it turned out very very nice. I think Sterling ended up milling his throttle shafts down to about 2mm thick. I left just a little bit more meat to hopefully keep them from getting bent or twisted. You can see my drawings in the attached PDF. Feel free to steal, use them for whatever.

 

Hahaha. Mine is the Glazed Nikki.

 

Here's the thread with details on my carb work: https://www.rx7club.com/1st-generati...odding-964025/

Pics of my setup are on page 2 and 4, and there's great info all throughout.

And here's another on my dimpled manifold: https://www.rx7club.com/1st-generati...nifold-960175/

I called mine the "SuperNikki." I read through the thread and all I did for jetting was hog out the primary fuel jets with a hobby drill bit to about 100-105 from the stock 92 (1985 carb). I left the secondaries alone. I forgot that I did, which is gonna be kind of important when I boost it! Gotta start writing this stuff down.

Anyway, the bump in the primary fuel was enough to keep the mixture right for the entire powerband. For boost, naturally this isn't going to cut it. So I will be upping the secondary fuel jets as well. I'm not quite sure what numbers to start with yet. I'm also going to modify the linkage a bit so that the secondary plates open much earlier (with less throttle). The goal will be to get the extra fuel from the secondary jets into the engine before the turbo starts feeding it extra air.

 

Nice design, thanks! You should be good at that thickness. Mine are about the same width, only tapered. I've been beating on my carb for a good year now and haven't seen any evidence of it twisting. I think the most important thing is having a stop on the throttle cable side of the linkage to keep them from opening too far.

 

I find it funny that I started modding my Nikki 2 days ago and now there's a brand new thread on modding nikki's.

However to the point of my posting.
1. Cutting the venturis larger would allow more flow based on volume, but venturis act as an airfoils so optimizing the profile would probly give you better results, in theory of course and as everyone know theory and reality can have considerable differences.

2. I personally wouldn't cut the venturis in the carb, you want them to very close to identical or you'll pull different flows on each Venturi

3. Someone said something to the effect of the booster Venturi take up to much room and doesn't give very much flow around to the main Venturi on the primary side. I would think pulling the main venturi out and slightly shorting it then revamping the profile would be the best cause of action to increase flow. Remember Effieceny is everything. So giving the Venturi the most efficient profile and largest possible size is what your looking for. The beautiful balance we all looking for.

4.If you wanna play with Venturi cuts it wouldn't be difficult or very expensive to have a decent machine shop cut you a few sets. Personally I think it would be pretty funny to see people changing out venturis like they do jets

 

Also I would think that you would want them position near identical depths, got along with wanting the same cfm on both sides of the intake

 

Good points, fastorlast. Symmetry and consistency is important. My sorta redneck approach to this was to use various sized coins to make sure the venturis were evenly cut. If they fit down it tight at the same depths, they were even enough I also didn't change the inner diameter of mine, I only optimized the shape as you described. Although I still have the stock carb laying around, so I may build one with larger butterflies one day just to play around.

One thing Jeff and I have in common with our setups is an open-plenum manifold. This means that the fuel and air mixture from all venturis enters a common chamber in the manifold before being pulled down into the individual runners. This cuts down on the effects of inconsistent air and fuel flows through the carburetor. It also keeps pressure levels even. Gross inconsistencies can still have an effect though as air and fuel flowing from the right side of the carb is more than likely going to end up going down the right side of the manifold.

If you're running a setup where each barrel of the carb is directly connected to an independent runner, then any kind of variance is going to cause a drop in performance on one rotor. So having a machine shop do precision work would be very important in that case.

 

LizardFc Unless you changed you manifold after you thread you started then your not running a open plenum manifold. What your running is a duel plenum manifold, which will still need everything very close to exact. Or you'll need to machine a crossover in-between the to primaries. However yeah jeffs hogged out plenum will easily compensate, however it's still better to make everything even from front rotor to back.

 

2. It matters less than you'd think. You should see how uneven the FD LIM is! It's way less restrictive on the front rotor side, and the runner lengths are off by a few inches, too. The 12A intake manifold isn't quite as bad, but it's still not all that even (especially an FB manifold originally equipped with a shutter valve). I don't see too many people bothering with equal-length header primaries, either. As long as you get them pretty close to identical, you won't have a problem.

3. You want to be very careful with this. If you reduce the length of the booster venturis by very much you risk killing their vacuum signal, resulting in poor atomization and drivability problems.

 

I didn't mean the booster Venturi, I meant the main venturi pressed into the carb. I'm out of town currently so can't look at my carb directly, but there is a mathematic formula for the " proper shape of a Venturi and Mazda did a horrible job keeping with that formula. However if you remove the main venturis during your reshaping you might be able to shorten if to keep the booster and main Venturis from impeding eachothers airflow. However I haven't done the math. And like I said I'm out of town so I can't get dimensions on the stock venturis to see how much optimization is possible if you were to cut either the booster or mains short. But yes if you f**k it up your drivability is gone and it's not difficult to f**k up. Just make sure you did your math right first

 

Hmm as far as the fd lim I wander if that was done on purpose with the twin turbo set up for spooling or something. But idk Im never gonna have the money for an fd so I don't bother myself with fd problems lol.

And when it comes to equal length primaries on headers the people that do bother with also bother with doing a tuned exhaust and they normally see a few hp increase from there old setup. Intact I know alot of dsm guys that run tuned/equal lenth headers to keep reversion in the exhaust down and increase flow.

Along with most v8 guys but the header companies have for the most part done it for them

 

Sorry, I misunderstood. However, my point still stands. The booster venturi has to extend well into the main venturi, otherwise the main venturi isn't doing its job. The booster venturi is only there to increase the vacuum signal over that provided by the main venturi. That's why it's called a "booster" venturi.

No, it was done because the FD engine bay is too small and the twin turbos are thoroughly in the way of the LIM, so Mazda had to make compromises to get it to work.

 

fastorlast, please take it to a different thread. Thanks.

 

Put the pedal to the floor, then adjust the throttle cable length so that the butterflies are fully open. As long as you don't remove too much slack from the cable, you won't twist the shaft.

This is how it should be done on stock cars as well, but a lot of times the owner will see slack in the cable and think it's a bad thing.



.

 

Sorry jeff not trying to thread jack or start a argument just trying to give advise and some things to think about

Also don't know how this works but I think this would be a good candidate for an archive

 

Interesting stuff. A couple of things that I hope will help you all. First, if you think you can evenly and accurately machine the venturis with them installed in the carb with some sort of dremel setup, well don't expect even flows in the carb. And it's not simply the size of the bore that's important. Sterling wen't through many iterations of venturi mods, precision cut on his lathe which were exhaustively tested by me on my flowbench. If there was an easier way, believe me we would have found it. Early in the game I did the backyard mods like you all are doing and I never could get the damn thing jetted right. The fuel curve sucked and no tuning would flatten it out. YMMV, maybe you'll get lucky. Most of the other stuff you discuss is safe to do, ie- throttle shaft thinning and screw replacement, booster arm mod, etc. So my advice is to tread lightly and you'll stay out of trouble.

As far as secret stuff we did inside. Well they are there and they are not known publicly because.......................well they are secret.

Yea, I'd recognize that VW manual transaxle and the clutch arm anywhere lol.

 

Thing is, a venturi is a circular airfoil; it's literally a donut shaped wing.

Instead of using the vacuum that the shape creates to produce mechanical lift to move the airfoil, it uses the vacuum to create hydraulic lift to move fuel.

The amount of vacuum generated, and where it peaks over the surface of the airfoil (in other words, how deep in the carb throat vacuum is at max), is a function of the shape of the airfoil & the velocity as well as the mass of air flowing over it.

When you reshape a venturi, you are both changing the amount of lift it produces and the location of that lift, just like changing the shape of an airplane wing.

This needs to be kept in mind on projects like this.

 

Jeff did you thin the throttle shafts? I couldn't see from the pics. This was the mod that made the biggest difference for me (stock port 12A).





Also I had a discussion with sterling on cutting the venturis a few years back before I started selling modified nikkis, he recommended not to bore them out but merely to sharpen the inlet angles. I haven't experimented with boring them so I don't know how the results would compare.