j9fd3s

iTrader: (3)

yep! we've been to the dyno like 8 or 9 times this year. if you want the fun one, run E85, mixture makes no difference in power.

the guy i race with is an engineer, and 98% of the time he's right, but that 2% is pretty damn funny. we got into some big argument about something, because he picked the wrong equation.

Jeff20B

iTrader: (13)

Thanks for posting wankel=awesome and j9fd3s. Just some quick stuff this morning.

The Racing Nikki, as I call it, was modified by a guy named Dave *******. Does that name ring a bell for anyone? I think it was supposed to be one of those 465CFM jobs from back in the day. It had a thinned shaft, which was twisted when I got it, and also covered in rust, so I had to swap out the baseplate. It has a transition circuit glitch at 2100RPM which is within parking lot speeds so it does get annoying to drive daily. But it is a racing carb! It also came with an open plenum modded stock spacer. I tried this open spacer but didn't like it so I swapped to a stock SA one which helped a lot (it's an SA carb after all).

As for the all FB carbs I've ever done, they all seem to have a 1700RPM glitch. This, fortunately, is below parking lot speeds and isn't noticed while driving. I think the accel pump covers for it. In fact I didn't even know it was there until I was slowly raising the idle and trying to hold it at certain points on the tach to feel how smooth the transition was, and noticed this annoying glitch. Nothing I did to the primary air bleeds fixed it, but driving it made it go away. wankel=awesome noticed it too and also made it disappear while driving. Now it's possible a Sterling carb didn't have this glitch. Or maybe just his 22mm carbs didn't have it. Or did they? I'm pretty sure his 24mm carbs had it because mine do. Anyone out there know anything about it?

I got a 25.3mm carb up and running yesterday on a different year baseplate. The main body is 84-85 but the baseplate is 83. This carb ran perfectly. The goal was to find a carb for the REPU that had lots of low end grunt but also good driveability and power. This carb is able to break them loose in 2nd gear, dry road, in primaries! The idle to main transition is great! I'd like to build more carbs like this for 74 ported 13Bs.

I mention this because a) I'm proud of my work and b) the baseplate was swapped. I remember Sterling saying something about the reason why his carbs didn't idle was due to accidentally installing the wrong year baseplates, and he publicly apologized for it. But the truth is all 81-85 baseplates are compatible! I mention this publicly on the forum because you may run into a water damaged baseplate but an otherwise good main body but you're still worried about the year of your replacement baseplate. Don't be. As long as it's in the FB family*, you're fine. Heck I just installed an 81-82 baseplate on an 84-85 main body the other day. Ran perfect! No secondary delay! This is compared to an original 84-85 baseplate that didn't seem to run as well for some reason, and did have a tiny sec delay (even at 3 grand). I can't explain it other than to say I think the original baseplate might have been higher mileage and just needed the shaft bushings to get a nice coat of oily crud to build up to block a possible vacuum leak. This will happen over time so sometimes you need to be patient. But I wasn't interested in waiting so I'll leave the baseplates swapped.

Yes, the screw heads on these carbs' butterflies are big. The threaded end sticks out kinda far too. It's too bad but I don't really want to mess with it. I guess you guys can risk grinding the threaded part down, and then maybe spider stamping the result. I did that on the Racing Nikki. It's probably worth less than a CFM though.

*FB family baseplate has one mixture screw. SA family has a big air screw and a small fuel screw. Easy to tell them apart.

j9fd3s

iTrader: (3)

i think Paul Yaw used a smaller screw for the butterflies. its needs it, but i admit getting the butterflies to seat properly after loosening them is not easy.

Jeff20B

iTrader: (13)

I found the post where peejay talks about his experiences with Sterling Nikkis. Start reading here at post #45. https://www.rx7club.com/1st-gen-non-.../#post11870575

It's interesting to read my post right below his, at #46. Wow, I've since solved the weak tip-in issue. I also know that going bigger than 24.9mm works, but don't go bigger than 25.3mm. And if you have a 12A, better keep it to no bigger than 24.5mm.

Next, read peejay's #52 post. https://www.rx7club.com/1st-gen-non-.../#post11871137 Compare what he says to Sterling's insistence on smallish venturis. Are they both right?

j9fd3s

iTrader: (3)

i haven't really played with venturi sizing much, however i have noticed two things. with the PP engines, the venturi Mazda wants is the same size as the intake runner. 43mm for a 12A and 46mm for a 13B. the throttle plate they use is either 48mm or 51mm depending.

the second thing is that we put Fungus Mungus's carb together with 38mm venturi's, and the thinking was, that he could start it, and get it running, but 38 would limit power and revs, so he could break it in without worry. we then put the 45's in, and we didn't really drive it much, so i don't know about 4000rpm up, but it did move the transition between the main circuit and the idle circuit.

so its really not a complete picture, but i'd size the venturi around the primary runner size, and call it good? i think you'll find that this is close to the .98" RB calls for.

at some point the venturi isn't the flow restriction anymore either, and then thats where you thin the throttle shaft and try a different screw. when i did mine, i thinned the throttle shaft, it was maybe 6mm in profile instead of 10mm, bending was still an option, but it should have been ok. i only ran it a couple times before going P port

Qingdao

iTrader: (3)

I'm running one modified myself.... not driving hehehe

I broke my 1/4 - 20 to Metric threaded rod adaptor thingy. But I have one more day off this week so a fixing I will do.

So far I've had very sticky floats, but nothing to report.

wickedrx2

I'm sorry if I don't follw all this about Nikki carbs but. How can a carb that is so small be better than a Holley

t_g_farrell

iTrader: (1)

Because the nikki was designed to work on a rotary and the holley was not. Its that simple and
when you modify it to flow almost as much as the RB holley 465 its hands down a better option.

Its not the size that matters but the way it flows and interacts with the primary and secondary
runners of the rotary.

wankel=awesome

Jeff, my secondary main air bleeds are now 120's. Only time I bog when I floor it is under 2k RPM, lol.

Also, im building one big enough for a 6 port (13b) now, any advice?

wickedrx2

I don't think a Nikki can have better flow , my Holley 750 annular set up is close to 600rwhp at 23psi and I'm planning to push it close to 30psi. And this is on e85. How can a Nikki do better than this.

heynoman

iTrader: (5)

I don't think it will ever be possible to to make a Nikki out flow a Holley (Larger the 465cfm). but I also don't think you can ever make a Holley have as good of drivability as a Nikki. I also think you need a smaller carburetor 750cfm is overkill on a boosted engine. I would not recommend anything bigger then a 600 on a boosted engine.

Jeff20B

iTrader: (13)

Mine doesn't bog at 2k RPM either! But I haven't tried flooring it any lower than that yet. Oh and my secondary main air bleeds are still at 80.

As for a Nikki on a 6 port 13B, I'm planning one myself. The venturis will need to be pretty big. I recently had great results correcting a set of bad SVT venturis that he lathed incorrectly. He made several of these badly cut venturis where the approach angle is extremely steep and it has the narrowest part of 22mm (his goal was to be like Sterling's 22mm cuts but failed spectacularly) that is 9mm below the little vacuum secondary hole. I use this hole as my landmark. The hole is in line with the bottom of the boosters where the narrowest point should be, but it can be slightly above the hole with no ill effects. Just don't ever let it end up below the hole like SVT did.

I don't know the actual angle but it is about the same as a flame shaped single cut die grinder bit or "christmas tree" as some call it. This bit is about 19mm across and just fits inside a stock 20mm venturi and is what I use to cut them from the bottom up to rough cut my 24.7 to 25.15 for 13Bs. It also does well for 23 to 24mm for 12As.

As for the garbage SVT venturis, one was smaller than the other so I had to hog that one out from the top down to the little hole until they were the same. I used the flame shaped bit and it worked perfectly to match the original angle he used.

Getting back to cutting from the bottom up, I kept going until the narrowest point was brought up to the little hole. The end result was supposed to be about 25mm but ended in a range of 25.98 to 26.03mm consistently across both. These are the most accurate I've ever cut, though a little bigger than I wanted.

So how do they run?

Better than the Racing Nikki's primaries which that Dave ******* guy cut to 26mm as well, however looking down into that carb, you can clearly see they are different sizes from each other and the upper angle is pretty inconsistent. Oh well, it was a racing carb so who really cares how its parking lot behavior is. It has a glitch at 2100 which you DO notice in a parking lot.

A Glitch? Do they all have a glitch?

Yes, but-

At what RPM do the corrected set of SVT venturis have this common glitch?

It is at 1900 but again you don't really feel it when you drive because the accel pump covers for it, just like it does with the smaller venturis I do, which usually are closer to around 1700. You do notice it sometimes, though, but it is far FAR nicer to drive than the Racing Nikki. I could daily drive this 26mm carb. The Racing Nikki? Not so much, but I have in the past and it wasn't terrible. You just throw a lot of fuel at it with big jets (before I had a wideband lol). Now I know to shrink the air bleeds down, which I did recently going from stock SA 90 air bleeds to FB 70s. Haven't test driven it yet...

So then how is the tip-in and how are the secondaries etc?

The tip-in is insane! Just a light tap of the pedal causes the car to lurch forward. It feels like a much bigger carb or it makes the car feel a lot lighter than it is. And the secondaries open perfectly without any bogs or delays like all my carbs do these days using the same modded air bleeds they all get. Usually wait until 3000, then punch it.

The 26mm carb was tested on a 74 ported 13B with an S5 turbo, which is all I've ever used. I've been told my boost tunes work fine for NA setups as well. Thus the plan is to install this 26mm carbed on an NA setup that might get a turbo some day. The NA setup can be bigger than 74 spec. So either a 6 port or a streetported 4 port. I have two engine candidates I'm thinking of for it.

The 6 port engine that might get this carb has 74 ported primaries and just stock ported end plates. The intake manifold began life as a 6 port 12A mani that someone hogged the hell out of and then welded it to a GSL-SE or RE-SI flange. It has so much plenum area it kind of hurts low end power below 4k. I have a second manifold option where someone welded a 79 12A manifold to an S4 NA flange. This one was not hogged out and has actual low end torque and ran great the last time I tested it with a long primary exhaust. It just needs some runner clean up work to match the single 12A runners to the dual 6 port holes in the flange. A third option, and one you might look into is to order a $45 aluminum plate from Mazdatrix that was scribed and cut out but not drilled yet, to fit the 6 port/T2 bolt and port pattern. Then drill/tap it to allow a 12A mani to bolt to it.

I'd say intelligent runner/port matching from the 12A pattern to the Mazdatrix flange would allow good low end torque and good high end power. And then a rather large hogged out Nikki to flow enough for a 13B would be good. Just don't screw up the venturi shape and you should be good up to 26mm but don't go beyond because there are almost not venturi shape left, and the transition circuit was never designed for venturis this big. I got away with it this one time, but I'm not interested in doing another. If you want to copy this giant venturi shape that worked on my setup, just cut from the bottom up and from the top down until the narrowest point is 26.0mm across right at the little hole. Oh and be sure to do 105 pri short slows. I'm going to do that next as the 118s seem a little big. Leave the 46 long slows alone as I was not able to gain anything by playing with mine (but your mileage may vary). And be sure your main primary air bleeds are stock 70s. They worked ok on my setup.

As for the streetported engine its ports are about half way between 74 spec and Racing Beat streetports. They say RB streetports close at 60 degrees. We know 74 spec closes at 50 degrees. So I'm thinking the mild streetport closes at about 55 degrees. I'm also going to use a reverse runner manifold, which has long primary runners that go down to the end plates while the secondaries are short and go right into the intermediate plate. The int plate has to be a tall runner Y or R5 for this to work. And they don't work all that well. Mazda sacrificed their secondaries to have more torque on the primaries. However to get over the tendency to want to wet-out along those long runners during quick stabs of the throttle, Mazda went with a slightly oversized carb of 22mm (not 20mm) and made the ports small 12A spec (40 degrees closing) and used TEENY air bleeds in the carb and slightly smaller fuel jets to compliment the small air bleeds.

The end result was a 1976 Cosmo made 110HP at 6k and 120TQ at 4k. All with 12A size intake ports and required less fuel to do it compared to the 74 spec REPU.

The 74 spec engines had a normal runner manifold and made 110HP at 6k and 117TQ at 3700. The carb had the same 22mm primary venturis but used larger jets and MUCH larger air bleeds.

The differences were something along the lines of 60 vs 80 pri mains and 90 vs 160 sec mains. Gasp! The smaller ones obviously being for the rev runner mani's carb.

This tells me that whatever carb I use on the rev runner mani needs to have really small air bleeds and somewhat small fuel jets to match. However the carb needs to also be kinda bigger than the application would call for if it was on a normal runner mani. So it's possible if I used the 26mm carb on a mild streetport and kept the air bleeds small, it just might work. Compare this to using a 24mm carb on a normal runner mani with only 74 spec ports. Just 2mm. That is a huge difference when it comes to Nikkis! And you and I know how sensitive these carbs can be on the primary circuit.

You want to know something? I think if it wasn't for my prior Hitachi carb experience, I never would have considered such small air bleeds in my Nikkis. But because I saw examples of them, it allowed me to make the leap of faith required for making boost prepping work. It was the "key" to making it work, if you will. I'm sure Yaw and Sterling never studied Hitachis to the level of detail I did, as I pretty much had to, to keep them running before I got into Nikkis. It was out of necessity due to running old school 13Bs.

wankel=awesome

For my manifold Im modifying a 4150 flange RB holley mani.

Itll be shortened, and the runners blended and opened into a plenum area below the carb.

t_g_farrell

iTrader: (1)

Note I was comparing it to the RB Holley 465 cfm carb. I doubt a nikki can do better
than the holley 750 but those are awfully large carbs, I hope they are on something
bigger than a stock 12A. You are comparing apples to pies here.

Jeff20B

iTrader: (13)

Just be sure to do channels like a 79 manifold where the primary and secondary can communicate. I don't recommend connecting across these channels unless it's like the small channel in the stock phenolic spacer, and then only under the primary side. Leave the secondaries separated.

82FanTC

iTrader: (9)

I have only driven around the block with the intake I have that was supposedly modified by Paul Yaw. It has the all of the runners isolated, the channel was filled in with some sort of filler. Car seemed to run well but like I said I don't have much time with it.

What does having the channel vs not channeled even do?

82FanTC

iTrader: (9)

Photo of the closed channel

wickedrx2

The 750 blowthru is on a 13b rew half bridge. And now is getting 13bre Cosmo. Runners. So yes I'm not talking a about a stock 12a, and the 750 is not too big for the motor. , I ran a 650 before and when I changed to a 750 I made about 70 rwhp. From just a change of the center section. At the same boost

heynoman

iTrader: (5)

I still feel the 750 is too big . I would expect a increase in power from the switch especially since you upgraded to annular boosters which provide more fuel and better atomization . Do you know what the of AFR difference was between the 650 and 750? I would expect the 750 to run richer since the annular booster usually require jetting down .also wanna say that Close to 600hp is great . What time are you running.

wankel=awesome

Thats exactly what I had in mind!

It shouldnt be that difficult to do.

wankel=awesome

Sexy. Although im not totally sure thats ideal without the channels.

wickedrx2

I kept the a/f on e85 around low to mid 12af. But I can richen it if I want to but the motor didn't like being on the rich side it made way less power , I'm running 2 power valves controlled by a Hobbs switch. So I can determine when I want the extra fuel to come in. , I'm going to shoot for 650rwhp. Around 28psi and this is a mustang dyno. But will see. This is on a 81 starlet. And I've only made 1 pass at the track. But The exhaust manifold came Loose because I had stripped the studs on the turbo manifold and I was loosing boost on every gear ,only ran 11s. But I'm shooting for way better time. , we'll see

wickedrx2

heynoman

iTrader: (5)

looks like you AFR is 12.3-12.5 when you go full boost at WOT which is a little rich as it is .that is assuming your AFR meter is setup to read gasoline. Any richer is way to rich that's why it looses so much power . Have tried leaning it out some?? Where you are at is just about perfect but you may be able to squeeze a couple more hp out of it at the same boost.

wankel=awesome

Dont lean it out! Youre already in teh static zone for fuel under boost.

Stay in the 12:1's. Do NOT be tempted to go lean with boost.