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I pulled out the venturis of a parts carb that I have. The intention is to open them up a bit to better feed a new street ported 12A. I asked in another post if anyone can recommend for venturi sizes for a pure road race carb (i.e. don't care at all about performance below 4k rpm), but haven't gotten any responses. If anyone has suggestions on that, I would be happy to hear them.
The main question here though is whether there is supposed to be a hole through the throat of the venturi. There is a hole on the OD, but it does not appear to go through to the ID. I have tried poking a pin in there and it seems solid. I have tried soaking in acetone and still no go. There is clearly a hole in the primary venturis, so assume there should be one in the secondary too.
with a Weber, you can do 44mm venturis in a 48mm carb, 91%. at some point the restriction will be the throttle plate, and after that you're just removing response
I believe SA and earlier secondary venturi's had that hole drilled (my SA ones do at least) and FB carbs do not. That hole does nothing functional in RX-7 carbs, but it's useful as a landmark hole when cutting them - you want the choke point to be centered on that hole as it aligns with the bottom of the boosters.
I'm not sure about a size recommendation - I am wondering the same thing - although I'm going for 30mm secondaries on my street carb which feeds a 74 spec ported 12A. From what I've read here, it doesn't sound like many people go beyond that, although Jeff20B is really the person who would probably know that.
Even if you don't care about low end performance, still do the primaries. 24mm is good for really any carb. I think some folks have gone larger (maybe as far as 26mm, although I'm not certain), but I can't speak about that from experience. When my setup is done - and hopefully finally tuned correctly - I'll be running 24mm primaries 30mm secondaries, re-timed mechanical secondaries, trimmed and polished boosters, and a few other mods. Hope that helps and good luck on the work.
Thanks for the info on the "port" on the secondary venturi. When I looked at things closer, I could see that the primary vacuum signal is connected to the back of the slot on the secondary venturi to the vacuum secondary diaphragm. So it makes sense that there is not a hole through the throat of the secondary venturi. Similarly on the primary venturi there are holes in both venturis, but only one is doing anything (providing vacuum signal to the secondary diaphragm). The other is just leading to a dead end.
If anyone has any more recommendations on venturi sizes I would appreciate them. I am probably going to target 25mm for the primaries and maybe leave the secondaries alone (other than to clean up the inlet shape....maybe).
You guys have this thread handled pretty well so far but I can add this: stock secondary venturis will feel very weak if your primaries are hogged all the way out to 25mm... Think about that... Then consider hogging your secondaries. But only do it if you can cut them well. And I mean this more for the primaries than for the secondaries because secondaries only come on at higher RPM and are used part of the time compared to primaries which are always used any time you drive it. You can lightly fudge your secondaries but be extremely precise on your primaries. The secondaries can be easier to cut because they are bigger and easier to deal with. In other words, I am encouraging you to do your secondaries.
I machined (on a lathe) the primaries out to 24mm. Left the secondaries alone. Eliminated the locating arms on the secondary/tertiary boosters and cleaned everything out. The carb is back together and will get a trial when the rebuilt ported engine comes back. It will be interesting to see how the modified carb works (if it does). It basically only runs at WOT and never under 4k (at least when it matters), so I am hoping that relatively simple jetting changes will provide good race performance (A/F around 13 over the operating range). I have the old, stock (but "main fuel" rejetted) Nikki for comparison on the dyno. I will update with the results. Probably in about a month.
I machined (on a lathe) the primaries out to 24mm. Left the secondaries alone. Eliminated the locating arms on the secondary/tertiary boosters and cleaned everything out. The carb is back together and will get a trial when the rebuilt ported engine comes back. It will be interesting to see how the modified carb works (if it does). It basically only runs at WOT and never under 4k (at least when it matters), so I am hoping that relatively simple jetting changes will provide good race performance (A/F around 13 over the operating range). I have the old, stock (but "main fuel" rejetted) Nikki for comparison on the dyno. I will update with the results. Probably in about a month.
Thanks for the help.
Carl
Be very interested to see the dyno comparison of the 2 carbs back to back.
Back from the dead, maybe too simple, but I have to ask. I have four sets of venturies to play with. Could I use a 24 mm drill bit to hog out the primaries? I have been talking with my friend about doing a set and he suggested just drilling them out. Since I've got some to experiment with, just putting it out there. He has them and thinks it can be done.
It might make them too smooth? Or not funnel like? The top could machined to accomplish this.
straight taper inlet
upper edge as thin as you can make it (I go for box cutter blade sharpness just because I can)
narrowest point at the hole used for vac sec activation, which I use as a landmark
then a straight taper all the way down to the bottom
It wasn't dead, just waiting to go to the dyno, which I did today. My son and I broke the new engine in at Willow Springs on the last day before they closed for SoCal "stay at home". Pretty cool having the whole track to ourselves for a whole day. Were only able to test with the old carb. When I put the modded carb on it flooded and we couldn't quickly find the problem. I think I posted elsewhere questions about my flooding problems. Seem to have them solved with new (go figure) needle valves.
Anyway, the first dyno runs were with the old carb (stock with 110 primaries and 165 secondaries). Came out with 137hp, 120 ftlb, A/F = 11.9. Kind of rich, more so than it had been with the old engine, but whatever. Put the new modded carb on and initially had a couple of problems. Flooded when it first went on. Pulled the airhorn, poked at stuff and reinstalled. Worked fine the rest of the day. Apparently still need something. First run showed 1/2 the power of the old carb. Turned out the secondaries were not actuating. Figured a gasket was backward. It wasn't the one between the air horn and the main body, so I became the mechanical secondary actuator (to make up for the non-functioning vacuum secs) for the rest of the day.
First pulls with the modded carb and functioning secondaries came out with 150 HP, 128 ft/lb. No subsequent changes increased power, but did get better A/F. Initially had 130 primaries and 160 secondaries and the A/F was about 13.6. Changed secondaries to 170 and ended up with 12.5 - 13 (depending on how pre-warmed the engine was). Based on previous testing, I had determined that timing at 25 BTDC was best with the stock port so started there. Finished up the dyno testing at 27 BTDC, but there was no improvement so will stick with 25 BTDC to stay safe.
So summary: Street ported 12A does not really increase power unless the carb is opened up (stock port was 133hp and street port was 137hp with stock Nikki). The modded Nikki (primaries opened to 24mm and booster venturi alignment arms removed) added 13hp , which is almost 10%. Pretty satisfied with the results for the time being.
Also found out that the gasket between the main body and throttle body was backwards preventing a vacuum signal from reaching the secondary actuator and possibly also creating a small vacuum leak. I will take the car back to the dyno later to make sure the secondaries are actuating properly, but should be OK.
Interesting story. Now I'm curious what sort of setup you're running in addition to your hogged out primaries, street port, and I assume an exhaust. Was that 150hp at the wheels? What sort of dyno? We don't have - or at least I don't know of - a dyno around here so I try to estimate my power levels based on other folks with similar setups, hence the curiosity. I actually am almost done fiddling (again) with my carb; just have the t-body to do a little more work on. 30mm secondaries are still in the process of being cut, mostly because I'm lazy but oh well. They'll happen eventually - hopefully within the next couple weeks.
Something worth mentioning is that gasket in your photo(s). They're really thin and can cause issues (like you discovered). A while back Jeff pointed me to cutting my own gasket for that out of generic gasket paper. I used the thick blue stuff because it's what I had on hand, but really anything should work better than that wafer-thin one. Might be worth a shot if you're still having issues.
Yeah. That gasket is really thin. Don't seem to have issues at the moment, but will definitely keep that in mind. The exhaust is RB "streetable" (not road race) header with pre-silencer, followed by 14" x 4" Magnaflow (which I am pretty sure is not doing anything at this point) and a Mandrel Bending Solutions stainless steel turbo style muffler (only $39 and has been through many hours on the racetrack). I frequently race at Laguna Seca so need a quiet exhaust (<92dB). It is also nice when you are sitting in the car for 1-1/2 to 2 hours at WOT to have a reasonably quiet exhaust. 150hp at the wheels on a Dyno Dynamics.
As I have said before this is purely a race car so no need for good around town manners. I haven't had it out on track yet with the modded carb, but the car was 3 seconds a lap faster at Willow Springs with the new street port and old carb. That was on break-in day, limiting rpm to 7000 on our last session of the day. Looking forward to seeing how the modded carb performs. No indications yet that there will be any performance issues through the important range of operation. But I have been surprised before (like this morning when the secondaries wouldn't open on the dyno). We'll see.
The aftermarket carb rebuild kits come with this ultra thin black paper gasket. I've seen several of them with a pushed-in section around one of the secondary venturi holes. I'm like what? Why always here? It is awfully thin...
I tried the Mr Gasket Co charcoal gray gasket paper but found it to be too stiff. Then I tried their blue paper and viola! It seems to work. It is thick enough to confirm to any factory "nuances" of the mating surfaces between main body and baseplate. Yes, this paper is meant for coolant applications and allows for a little bit of oil soak-through over time, but it is pretty minor. Anyway, the blue paper works very well with boost and kinda mimics the factory beige thick baseplate gasket paper. I make my blue gaskets in production runs which I've found helps with quality control and accuracy. After the production runs, all I ever need to trim are the 4 barrel holes which I do custom for every carb. It is all about anti-reversion here, taking care to not allow any little bits to stick out in the way of incoming flow.
I believe SA and earlier secondary venturi's had that hole drilled (my SA ones do at least) and FB carbs do not. That hole does nothing functional in RX-7 carbs, but it's useful as a landmark hole when cutting them - you want the choke point to be centered on that hole as it aligns with the bottom of the boosters.
I've spent a fair bit of time reading about the Nikki carb control philosophy and just wanted to clarify that this hole did have a purpose on SA carbs. Its purpose was to bleed off some of the primary venturi vacuum signal to the vacuum secondary diaphragm to delay opening of the secondaries. I'm not sure if they compensated by having a weaker spring in the diaphragm for this or if it just meant the secondaries would open later on those carbs.
Last edited by Joekaistoe; 05-20-20 at 02:52 PM.
Reason: Grammar
I've spent a fair bit of time reading about the Nikki carb control philosophy and just wanted to clarify that this hole did have a purpose on SA carbs. Its purpose was to bleed off some of the primary venturi vacuum signal to the vacuum secondary diaphragm to delay opening of the secondaries. I'm not sure if they compensated by having a weaker spring in the diaphragm for this or if it just meant the secondaries would open later on those carbs.
Interesting. I haven't spent much time with the SA main bodies to know where that hole would go (besides apparently to the vac secondary circuit). Maybe I'll have to take a look here sometime and figure out the routing. Didn't realize it was there for a reason, although I guess why wouldn't it be. Neat info.
Curious where you found this reading material. One can always use a little extra Nikki knowledge if you happen to have a link or such .
My apologies Carl for keeping this slightly off topic, but it seems I found part of the 'hole in the secondary venturi' explanation on my own accord. Turns out the info was in the very manual I had skimmed through many times but never read in-depth; perhaps you can tell I am doing so now .
Anyway, here's a little info on what that hole does, and while I'm not certain if the SA carbs actually have the plumbing to make use of it, it would appear that some carburetors definitely did at one point. Essentially it's just what Joekaisoe said but I'll add a little more detail for those curious.
The primary vacuum port does still do the majority of the secondary opening work, however, the secondary vacuum port allows it to open later but also faster/more responsively. If there were only the primary vac port (which in FB carbs is the case, so Mazda evidently found a new solution to this; perhaps a stiffer spring as Joekaisoe theorized) the secondary diaphragm would begin to open earlier. This because the secondary vac port bleeds off some of the vacuum, therefore delaying the opening of the secondaries. That said, with the secondary vac port, once the secondaries to begin to open that port sees vacuum and speeds up the opening of the vac secondaries. When the throttle is let off slightly this secondary port also allows the secondaries to close quicker as it can bleed off the vacuum still present on the primary side. The secondary vac port is also smaller than the primary port due to its main role being more so focused as an air bleed rather than the primary vacuum source.
Carl, those hp numbers are wheel based right? So the street port nets you about 17HP if you can feed it right?
I had my car dynoed when it had the Dell (stock port) on it and it showed 134hp at the wheels. I know its better across the board with my hogged nikki but haven't been back to a dyno since.
Yes. That is "at the wheels".
The attached graphs show results from stock port (June 2012) with stock Nikki and street port with opened up Nikki. Interestingly for both tests the main fuel jets and timing were the same. I don't have A/F curve for stock port results, but my notes say it was 12.6. These were all on the same dyno.
Another question for you experts. Is it OK to use the gasket in the picture if there are no emissions doo-dads? I.E. do the holes in the baseplate/gasket/whatever need to be open? I believe not, but would like some confirmation. I have never used the gaskets before, but noticed that the flange is always wet when in pull the carb and then saw that there was a gasket in the rebuild kit.
I've never used those gaskets, although I do often see that surface wet after removing the carb. Pretty sure it's just a little spillage from removal as the leaks never show up on the baseplate itself while the carb is installed.
The only thing you'll lose (aside from smog stuff) will be vacuum advance, but it doesn't look like you were using that anyway (did you cut off the vac nipples?). There is actually a nipple on the carb itself that can be used for advance if you ever wanted to experiment with it (perhaps for a future not track-only build).
I'd think you could run it, although I probably wouldn't on my own setup. If you're having leakage issues you could try adding it, otherwise I'd leave it off.
Edit:
Just noticed you still have the shutter valve installed. Have you considered removing it? For a track build I'd think you'd want that out of the way, although I've never seen any number's proving whether it actually boosts power at all. Just a thought.
Last edited by Benjamin4456; 05-21-20 at 02:26 PM.
Interesting. I haven't spent much time with the SA main bodies to know where that hole would go (besides apparently to the vac secondary circuit). Maybe I'll have to take a look here sometime and figure out the routing. Didn't realize it was there for a reason, although I guess why wouldn't it be. Neat info.
Curious where you found this reading material. One can always use a little extra Nikki knowledge if you happen to have a link or such .
The carburetor manual (which can be found on foxed.ca) explains how it's intended to work, albeit buried in the dry stuff at the start. Based on your other post, I assume you found that information.
Fun fact, I believe the sizing of the venturi vacuum holes is listed as "Vacuum Jet" in the Fuel System section of the Technical Data (also on foxed.ca). The secondary vacuum jet is only listed in the 1979 and 1980 Technical Data. All others only list the primary vacuum jet.
Edit:
Just noticed you still have the shutter valve installed. Have you considered removing it? For a track build I'd think you'd want that out of the way, although I've never seen any number's proving whether it actually boosts power at all. Just a thought.
depending on the class the car is racing in it may need to stay, rules
I'd avoid using those aftermarket gaskets. They are only ok on old school phenolic spacers. The modern SA and FB spacers have a permanently bonded gasket and that is all that is needed, even if there is a little bit of wetness upon carb removal.
After resetting the gasket between the main body and throttle body I went back to the dyno (actually opened up the primary venturis on another known good FB Nikki in case there was something grossly wrong with the SA). Turns out the gasket wasn't the only problem with the secondary throttle actuation. With the primary venturis opened up to 24mm there is not enough vacuum to open up the secondaries. They would open part way, but only a little. Interesting comparison. With only the primaries opening, it made about 100hp. The the primaries full open and the secondaries cracked it made 120hp. Significantly less than 150 with everything wide open. So I guess that means mechanical secondaries. My plan is to test by wiring together the two tabs in the attached picture together. If that works OK at >4krpm then I will make an "actuator" that works without the wiring. I don't put any faith in the wire technique for longevity in an endurance race car. If it breaks on the street, you can just drive home with 100hp and fix it. On the racetrack you are screwed. It looks to me like the secondaries will start opening when the primaries are about 50% open, then both end up wide open at the same time. Does that sound right? This is an SA carb and I noticed that FB linkage parts are a bit differently configured, but haven't looked in detail at how they work yet.
I originally left the shutter valve installed in the intake, because it was required for the race class I was running at the time. I am no longer running that class, so I can remove it. I have hesitated though because that intake runner (at least at the inlet) is bigger to accomodate the shutter valve. I would be worried about uneven air/fuel flow (maybe negligible) between the two rotors. If I reworked both runners though that might make more sense.
I also did one pull with the gasket below the throttle body. That pull was a disaster (found out later due to insufficient vacuum) so pulled it and never tried again. As you guys suggest, there does not seem to be much point in it.
02-04-20, 11:18 AM
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