What about this...

Create a "block off plate" for the secondary barrels. installed between the carb and spacer and disable any secondary jets.

Maybe it's a stupid idea... but I'm just throwing it out there for a fabricator.

Also, what kind of fuel pressure would this 40 MPG carb be using? Stock fuel pump? What kind of ignition and spark plug setup is it using?

 

I find myself wondering if there are baffle plates inside the adaptor to route more of the OMP output to the rear rotor.

Based on the pix, it looks like the OMP line output strongly favors the front rotor, due to connector position and the internal structure of hte stock manifold; and since it is connecting below the venturis and throttle plates (instead of above, as in the stock Nikki) there's not a lot of atomization/mixing action that will be going on.

I'd worry about engine durability unless I had some evidence that the rear rotor was getting sufficient lube.

 

Looks like a Weber 34 ICT to me....

 

I understand. I'm saying that doing that did not disable your secondaries. There must have been another problem you were unaware of and inadvertantly fixed when you reassembled it.
I'm simply making sure that people don't get misinformed about how they can stop their secondaries from working.

There are no stupid ideas. But simply disabling the vacuum box link will cut off the secondary half of the carburetor.

I actually have been working on a pretty simple idea based on the older Hitachi carbs that preceded the Nikkis. They have a bleed jet in the vacuum secondary circuit. Changing that bleed changes the vacuum needed to open the secondary throttle.
Simply drilling and tapping for a Holley jet on the Nikki vacuum secondary box cover will accomplish the same thing. Two problems are that you're likely to have to drill your own jets because I'm certain that the bleed would need to be less than the smallest Holley .023. The second is that the jet would be sucking in unfiltered air, and could begin to dust up the guts of the box pretty quickly.

I'm trying to figure a way to use old Nikki valves and rats nest components to make this variable and controllable from the dash, like an OD toggle switch.
As "novel" an idea as that may seem, it really does come down to jetting & driving habits.

 

Isn't that similar to what I was suggesting above?

Doesn't the vac diaphram connected to the secondary linkage use an internal vac circuit for the signal? Could this not just simply be blocked off. Then drill into the doaphram housing and install a hollow barb. Then connect the barb to a vac source from the carb base. Install a ratnest (or any) vac solinoid connected to a switch in the cabin in the vac source line going to the diaphram.

on a side note while I've got the carb guru listening. I run most of my FI cars around 16.5-17 AFR under lean cruise conditions. With the rotray using fuel as additional lube are these AF setting to lean for a rotary? What woudl U suggest max lean cruise shoudl be?

Thanks!

 

If the goal was just to block off the vacuum secondaries from working, couldn't you just hand-cut a gasket for the diaphragm attachment pad that had no holes in it?

 

Yes, and an interesting idea that could actually solve a key problem. (I didn't see it.)
As it is now, the vacuum box signal actually comes from a hole inside the left primary venturi. It's a vacuum signal derived just like the main fuel signal, though the fuel signal is a bit stronger being inside the booster venturi.
The secondary linkage is locked closed until the primary shaft is about 45*, or half way to WOT. How soon the secondaries begin to open relative to the primary throttle position is determined by that linkage. It is what it is, and though anything can be modded, they open plenty soon enough. I doubt there's any performance gain to be had by making them open any sooner, and I'm sure that the transition circuit would need to be reconfigured.
How late the secondaries open up is determined by the diaphragm box spring tension. Pulling out the spring will cause the secondaries to open very easily every time the linkage is unlocked, but it's not always consistent. Simply cutting bits off that spring isn't all that effective. I've had success heating the temper out to soften them, but it can be a real PITA.
Keep in mind that on the vac. secs Nikki, they never fully open. That's why wiring them up makes the thing go like a raped ape up top.
So for MY purposes- that is to have a performance carb that can also be easily tuned on the fly for economy ([i]again, "relative" economy)- the thing to do is figure out how to keep the secondaries vacuum operated, but ensure that they do open fully at WOT, and then use either a jet or needle valve to bleed off the sec. vac signal.
You wouldn't want the secondaries to come on only at WOT because the transition circuit & AP just wouldn't be able to make that smooth. It needs to open gradually or there will be an incurable flat spot that you'd have to learn to drive around.
As it is now, I could drill & tap the Sterling to take a bleed jet, but since there's that PITA problem of the vac secs almost never fully opening, I don't currently send them out vac operated. I do, however, send them out with the box on and the link that mechanizes the secondaries can be flipped around to make them vacuum operated in three minutes. It would be easier for me if I didn't have to bother modding that linkage for that, and easier for the user, too, if I could just keep them vac. secs.
Maybe if the circuit was rerouted like you suggest, it could go to a much stronger vacuum source, ensuring that the secondaries open fully. Bleeding it down to a reasonable level is the easy part.I'm just a carb guy. You'll have to ask these engine builders around here about that. Hazarding a guess, I'd say that there are plenty of factors/variables that need to be considered when choosing a high air to fuel limit, like compression, RPMs, driving habits, OMP/premix settings/choices, ect, ect.

 

Disconnecting the vacuum box lever literally takes less than 30 seconds with some needle nose pliers.

 

To clarify. Gasket on wrong will cause vacuum leak in secondary vacuum circuit won't it? Not sure it will disable them, but the lever is definitely the easiest to do that.

 

I'm waiting for a set from Rob @ Rotary Shack. I dont mind trying it on and give you guys a report. Just to be fair, it will go on a 13-B streetport with full dual exhaust, aluminun\m flywheel and the carb will sit on a modded 76 Cosmo intake with reverse runners. It will be driven by my son who has a lead foot.

 

You sure that little thing will deliver enough gas Wacky? If it does, should be fine for my 12A. Let us know.

 

Yep- you guys are totally right. I stand corrected. Appologies all around, please pass the plate of crow.





What threw me was when autoxr was talking about the holes in the gasket. But that is a perfect photo of what can happen.
Now I know.

 

I know its not enough for a gas thirsty motor but what I'm after is the MPG. If it gives better MPG than a Nikki, then the 48dco will sit in the shelf collecting dust and there is no need for me to buy a toyota corolla.

 

I got 34MPG on my nikki in the city.

 

Noooo, the DCO! Poor thing.

 

Shaddap! You're not helping haha jk.

 

Lol.

Hey, are you CBZ?

 

Correct sir.

 

Ah, I thought so. I knew you were familiar!

Sorry to get off topic. Nikki = 4 barrels. Upgraded to a 2 barrel. Time to upgrade to a single barrel????

 

i used to get roughly 24 MPG with a tired SE motor and a Dell'Orto 48 DHLA as recently as 2004. back in the day i never actually measured my MPG, but i can say that a full tank of gas (1983 Rx-7) took me from the Bronx, NY to within 20-30 miles of Charlottesville, VA with a 12A and slightly modded Nikki - well over 300 miles, if i recall.

i think this project is a great idea for those that care about MPG, but ultimately, it's a personal choice. i wouldn't be interested simply because Rx-7s are too much fun when they're spitting HCs out into the atmosphere. i'd much rather get a Toyota, Honda or VW for better MPG.