Thanks for the input! I think I got lucky Although as I said before, I didn't increase the bore, I only changed the shape. I have no way of bench testing mine, but my AFR's are right where I want them. Doesn't mean it's even, since I'm taking a reading from a collected header and not off each rotor. But it gets the job done. I think the open plenum helps a lot with this.

Good point, if you make the venturi too straight you'll severely diminish the vacuum signal and won't draw enough fuel, particularly at idle when air velocity is at its lowest. I still think there is some room to play with. You can open the inlet by at least 1mm, so also increasing the bore size by 1mm would still give you close to the same shape. After that, it becomes a balancing act of reducing flow restriction while still retaining enough signal to deliver fuel at low RPM.

Oneiros, those shafts are supermodel skinny!

 

I'm pretty sure that venturi vacuum generation at idle is minimal - - idle & low-speed vacuum is generated mainly at the throttle plate lip, governed by the throttle base opening angle.

That's why fuel feed at idle and just above idle is handled by the idle and transfer ports; there's not enough vacuum at the main/boost venturis to pull fuel up there at idle air-flow.

 

I would agree with that. The idle circuit on my race prepped carb is fine and starts up easily without a choke. The issue occours just above idle before the primaries are flowing. It has a hiccup that you learn to drive around. This probably wouldn't happen if the primary venturis were smaller than 26mm. However I'm just fine with the slight glitch between idle and primary operation as it is a small price to pay for the awesome free power of this carb.

I could probably tune it out if I knew which way to go on the primary slow air jets/bleeds #1 and #2. The #1 are 46 and the #2 are 150. I think this is stock on a 79 carb.

My 81 carb #2 air bleeds are 180!

 

Carl, thanks for checking out this thread and adding two cents.

This carb was already modified when I got it. But if I were to attempt to hog out some venturis myself, I'd probably just leave them in the carb and go at it with a dremel. Or just switch to a bigger carb like a Holley or something! :p

But at this point, what's done is done and I'm happy. Now I just have to decide whether to use this carb in the baja or someplace else.

What I need is low end torque. The air cooled 1600 was rated at 77 foot pounds at 2600. An old school 4 port 13B can't do that. It was rated (the 1976 Cosmo) at 120 foot pounds at 4000. But what did it do at 2600? Probably not a whole lot. But I've never seen a dyno of a stock engine before. I assume it's less than 77 at 2.6k hmm...

Plus to get greater power, you need a long primary exhaust collecting to 2.5" and direct fire ignition. The ignition is easy. The exhaust isn't. The exhaust does more for low end torque and power everywhere than probably anything else, except perhaps intake, so I'm kinda limited in what I can do as there is very little space back there for a proper exhaust system.

The ignition and intake are fine but exhaust is going to be the limiting factor here. I'm willing to entertain alternatives as long as I get to keep the rotary.

 

Nope. I only ground off the threaded portion of the screws. They are SA screws which are a lot smaller than FB screws.Well my carb is bored the largest I've ever seen on primaries and secondaries and it pulls hard from low RPM to high. It lacks finesse though, but it makes up for it with monster secondaries, which I actually like. Takes me back to the 550 RB Holley in a friend's REPU. But now I need to decide whether to use it in the baja or put it in something else.

 

Might also be a case where raising the top edge of the transition circuit would help... but that's strictly a 'theory' guess with no experimentation to back it up, so pursue at your own risk!

 

Are you talking about the #1 air jet's total length? If I remember from last time I looked at one, they are shorter than the air bleed emulsion tubes. It would be great if all you have to do is hack off a milimeter or two, but not sure what to do about recreating the tiny orifice. Solder and a tiny drill bit?

 

Interesting, I didn't know that. Perhaps there's a way to "tune" it then. I haven't experimented with the idle circuit yet. I had that transition hiccup on my stock carb but not on the race carb. If I make another one I'll have to keep that in mind.

Jeff, maybe wrap the exhaust around toward the front of the engine, under the trans, and collect it before a muffler on the other side? It's gonna be a creative setup any way you do it.

 

Nah, you can't go under tyhe trans in a baja. But if you have a 3: body lift, you can go above. I had access to a complete long primary exhaust from a baja that had a 3" body lift, and I cut it up for parts because it never provided the torque the owner was looking for, and I needed the parts for another project...

The owner decided to go turbo, and now I'm thinking of doing the same in my baja. The race prepped Nikki could go into a rotary truck instead. I bet it could outdo the old setup it had in the 90s. It might even rival the awesome 550 the green truck had.

Back in the 90s, the white truck was faster than a GSL-SE. It had a streetported R5 13B, Holley 600, aluminum flywheel (PercentSevenC now has that particular flywheel by the way, just some trivia for him if he reads this), and a fairly crappy single pipe exhaust with an RB collected REPU header (they stopped making these). The new setup will be 74 ported R5 13B, GSL-SE exhasut port timing, race prepped Nikki, reverse runner manifold, GSL-SE flywheel, complete long/medium primary exhaust. This new planned setup will launch so much faster, and still do fine at high RPM so should still be quicker than a GSL-SE. However an alternative is to go streetports and a Holley 600, while the rest stays the same. What do you guys think? It would let the race prepped Nikki go in my rotary MG Midget project which is also getting a full long primary exhaust.

Nikki or Holley? MG or REPU? hmm...

 

Ok I've got an update. The Nikki won't fit properly in the MG. Since a Nikki is such a tall carb, and it is mounted on a tall 13B manifold (even without a phenolic spacer), it runs out of hood clearance with an air filter installed. So, looks like the Holley stays in the MG. Not a bad end result for the MG.

I have some further fit/won't fit information regarding the racing Nikki. Looks like it might need to stay with the baja for a while because I just realized the hole I drilled and tapped for an oil return fitting in the baja engine's front cover is too small for most turbos. Most turbos use 5/8" hose and 1/2" NPT fittings or whatever the AN equivilant is. The point is the hose is typically 5/8" like a heater hose in an RX-7.

I originally drilled and tapped 1/4" NPT and installed a brass pipe plug so it would be sealed but compatible with Camden superchargers which use a 1/2" rubber hose on a 45 degree brass fitting that has 1/4" NPT threads into the front cover. Made perfect sense at the time to spend a couple minutes on this during the engine rebuild phase a while back. So just in case I ever wanted to use a camden, I wouldn't need to drop the oil pan to drill and tap, which is messy and you run the risk of missing some of the aluminum bits up under the front cover etc.

However now the only turbo that's small enough to get away with a 1/2" drain hose and large enough for a rotary that I know of is a stock S4 or S5 turbo, which I don't have access to anymore. And if I did, I'd still have to fab a custom tubular exhaust manifold to clear the stock intake manifold I have to use with a blow through Nikki.

So if I have to create an exhaust manifold anyway, might as well go aftermarket turbo, right? But then I have to use a 5/8" oil drain line which won't fit the front cover without going through the pan removal process which I don't want to do.

I wonder if it's possible to use a 5/8" hose but neck it down to 1/2" as it goes through the fitting into the front cover? Would it be too great a restriciton? By the way I'm not pulling the pan. It's sealed perfectly. I'd rather go NA than mess with the pan. Nice resolve, eh? It's because I was planning on throwing the turbo in the red truck this whole time and only recently questioned whether I should get a second turbo for the baja.

It seems something important like the lack of a proper oil drain fitting in the front cover is enough to drive the project in a different direction. And even if it did have the right size drain hole, I'd still question it.

So what do you guys think? Do you think my race prepped Nikki with its huge primaries and monster secondaries will have enough grunt to wow me in the baja? A vehicle with a bus trans but huge tires and winds up (pun) virtually with the same gearing as a sedan (bug) in the end and was set up for 77 foot pounds of torque. Will 40 degree intake ports close soon enough to keep adequate low end torque? Will the reverse runner manifold be the key? And if I went with a custom NA exhaust manifold instead of a shorty header, will that be the icing on the cake?

 

No, I was thinking of the actual transition (aka 'slow') circuit output port in the carb throat; it's a tiny vertical slot just above the throttle butterfly's closed position, near the idle port.

It provides the actual fuel/air mix into the carb during the throttle opening sequence just after the idle port loses vacuum due to the throttle plate moving up past it, but before the main jets start flowing.

My thought was that raising the top edge of that transition port a tiny bit would keep the slow circuit in play a bit longer as the throttle opens.

Probably a dumb idea, since if it doesn't work out the carb would be useless - - no way to go back.

Thinking out loud, mainly.

 

Oh, I see what you're talking about. I do have a spare TB I could potentially ruin to test this, but I gotta move on and get the baja going as soon as possible. I'd like it available for snow. You know, that stuff you've never heard of or seen.

 

it should actually do just that. in weber land there are some that have an extra hole, or sometimes people add the 3rd progression hole; its called, and it lets the idle/transition circuit deliver fuel at a higher throttle opening.

the idle and transition circuits work from the vacuum created by the throttle plates proximity to the hole, so just loosely it meters fuel based on throttle position.

oh and carb actually can be fixed, JB weld, solder or brazing can be done, although the transition holes need to be really precise.

actually the other fix in weber land is a throttle plate that has a different taper cut into it, so it achieves the same thing; larger throttle opening while running on the transition circuit, but without doing any crazy hole drilling

 

I lived in the deep midwest until I was twenty - - I'm all too well acquainted with a snow shovel, thanks.

 

Hey figured I would add to the thread again, just finished rebuilding one of my carbs and dropped it on a buddy's 85 gs. I can say it made one heck of a difference.

Cut the extra supports for the boosters and air foiled what was left. Swapped to mech secondaries, trimmed the omp lines back bored the secondaries 1mm and sharpened the inlet angles on both all venturis and polished everything. Mechanical secondaries kick in at 3/4 throttle and when they do you get thrown back in your seat hard.

Still gotta tune the AC pump cause it wants to hiccup when the secondaries start to open, but other than that it runs alot stronger all around. It all done to a stock port, straight pipe with stock exhaust mani and cleaned up intake.

 

^Oh, ok about the snow. I bet you and your back are greatful every winter now-a-days.

fastorlast, did you mod your accel pump the way Sterling suggests?

 

I put about a 1 inch extension on the AC pump, but didn't increase the pump volume. My buddies happy wit the results but I don't like the stumble so the next carb I test will have the increase in AC pump volume and a longer extension to try and avoid the secondary stumble. I've tried adjusting the nut on the pump and it seemed to help but not completely get rid of it.

 

Yeah, all the nut does is sets "end play" if you will.

As for where my carb will go, I've decided it will go in the VW pictured earlier. It starts up easily rain or shine and has proven itself in the blue car. I'd like to go turbo some day, but I need to get the car running and driving while working out the "bugs".