Just tryin to do a bit of research on all the "possible" carbs you have used productively on a rotary. I'm also curious as to what "should" work well. Obviously intake options prevent alot of choices, so let's pretend for a minute that the intake adapter or actaul intake is feasable. I know that alot of people use the webers, the IDA, IDF, and DCOE. As well as thier counterparts by other manufacturers. Also the holley 465 and 600 and thier counterparts. Hurley offers an SU package. There's the Mikuni phhs. Other than those or the equivalent in another brand, what has been used with success? Also what would you like to see used, that "should" work? Just trying to expand my knowledge and understanding of the limits and possibilities.

 

I'm not really sure what you're asking. All of the carburetors you mentioned have been used with success. Some more than others. Webers are perhaps the most successful though, particularly the IDA and DCO. Dellorto were also pretty good carburetors.

I use a Holley on my car right now and plan to go fuel injection by the end of spring. I never got it to run optimally, but I have definitely made major improvements.

 

(quote)I'm not really sure what you're asking.(quote)

1.I'm trying to find out what carbs have been used on rotary engines, besides the ones I listed. What else has everyone seen or heard of.

2. I'm wanting input as to which carbs everyone thinks "should" work well.(that aren't commonly used)

**I realize that all carbs work "basically the same", meaning they all provide fuel to the motor. But I know different motors have different demands, so not every carb will work well on every engine.**

 

i've heard/seen just about any carbie you can think of. in addition to the ones you mentioned, i've seen people use Edelbrocks and Demons. unsurprisingly, the owners found them hard to sort out (like non-Racing Beat Holleys).
the carbies that are going to "work well" on rotaries are the ones that have the most versatility, as in they offer the most adjustability. i know you need it for all engines, but you REALLY need it on a rotary, especially when you start to modify the engine itself for more performance (i.e., porting). the bottomline is there's probably a reason why the ones that are not commonly used, aren't used much.

 

I'm sure there are alot of reasons. Among them ...
1. the r&d has been done and bugs worked out
2. they are readilly available and well liked
3. they are versitile ?sp
4. intakes etc. are already made to accomodate these

However, considering the number of carburetors out there, and the different "styles" and uses, I can't help but think that there are other carbs that would be "suitable", even "beneficial" on a rotary. The Hurley "SU" carb setup is a great example. I haven't ever heard of anyone using those till now either. But they have and according to thier r&d it nets good mileage AND awsome power. I'm not looking to "pioneer" rotary carburetion, I just don't like to discount the possibilities on the basis that " If it could be done, someone else has already done it. " and it obviously didn't work cause noone else does it. Now in the case of the Holley's etc., there are known issues in design, and that I understand.(though ironically they are apparently worth the effort to modify, since they also have a large following on the rotary) If there is any other carbs that have specific problems when applied to a rotary I'd love to hear them. Like the weber, the 97 was used on many different cars and engines, in fact many of the same ones. So, versatility doesn't seem to be an issue. Also like the weber, it is a "simple" carb.

I'm just trying to learn. That can best be done through exchanging information/ideas. The whole " do this because everyone else does " mentality leaves little room for improvement or change. I'd just like to know what the reason is for some carbs not working well, not just that there is one. Know what I mean?

 

The rotary has unique requirements at idle and partial throttle that makes
getting a carb pretuned from RE or RB or some other rotary tuning house
desirable. If you take holley 465 or 650 out of the box you may have to spend
a lot of time sorting it out. If you buy it from RB its almost plug and play. Same
for the Webers and Dellortos. Especially the Dellortos.

You can probably make any of the carbs you mentioned work, it depends on
your carb expereience, your time and how of both you bring to the project.

A lot of younger folks just didn't grow up with carbs and only have experience
with EFI and computers running everything. So that makes the project rampup
a lot harder for some folks. Not sure if your in that category but its one of
the reasons folks get frustrated with carbs and have to use presetup
packages.

Others of us are just lazy and don't mind paying for the R&D that
makes it a good solution

 

Well, I wouldn't say I'm an "old timer" at 43, but I grew up that way. Dad was a grease monkey, and pitman at the track on the weekends that he didn't race. I think I knew what a 41 ford was before I lnew what to do with girls! LOL I'm not afraid of a little hard work and experimentation, but I'm still learning the rotary and it's needs/demands etc. I avoid fuel injection and anything automotive that involves a computer like the plague. I just prefer to do all my tuning and performance modification with my tools and my faculties, not a pc and test equipment. I mostly just was hoping for info and opinions from people that have more "practical" experience with these motors. I'm not above learning from someone elses mistakes or experiments, it makes the r&d shorter and narrows possibilties. What I don't have is infinite funds or a machine shop, so I'll be doing everything the old fashioned way, trial and error. But anyone that doesn't ask questions and pick the brains of the guys that have been there and done that, is going to waste much time on failures before they get results. So what sort of "unique" requirements at idle and partial throttle does the rotary have?

 

told you many times, just buy a 48 IDA (or a 48 DCOE = has a choke for cold climate). you buy an IDA once and with a simple change in venturies and re-jet, it can be use from stock to mild to wild

 

The "problem" with a carb on a rotary is the odd vacuum pulse.
This said any carb with enough work will do the job, On stock or street port engines I love using the quadrajet but go bigger than a street port and lack of vacuum kills every function of this carb. Just as all the others the Qjet is not plug & play, it needs adjustment after any and all intake/exhaust changes.

 

EDIT
forget it, i think i mis-read your post.

 

i have nothing against trying to break the mold, but i do have something against beating your head against a rock. at this point in life, i'd just run a Weber IDA because they are proven in the characteristics that count. i had a Dell'Orto DHLA and it was great, but i felt i wanted something that could give just a little more. i think if i have to go back to carbies, then the IDA is the way i'll go.

 

Mel, I know you told me, and I listened/read/understood. But I have a couple motors started and a few about to be started. For the record I do have a weber on one already, a nikki on another. I intend to put an IDA on one of the others. But I don't see it as the "only" option. It IS an awsome "race" carb, what it is not, is an awsome "economy" or "touring" type carb. I have different plans for different projects, so I'd like input on possible options. Not just the fastest most commonly used ones. I also like the idea of a "tribute" to the early days of performance carbs.

Understood and respected, I don't feel a need to break the mold so to speak, but in experimenting and modifying we learn better what works and what doesn't and more importantly, why. I am building my Hot Rod, but going to work on a few "test" motors too with other ideas, and any modding that doesn't take into account the fuel delivery is doomed. Different carbs have different characteritics, I'm not soley looking at speed, so "IDA all the way" doesn't apply. I'm wanting to see what I can find that will work reasonably well in areas from mild to wild. So no one carb is the total package, many have things to contribute. In the end, whether I find another great carb setup or find that nothing else works, at least I'll better understand why and how to change that. Not to mention having some fun in the process. I like to know the how and why, not just the what.

Thank you all for your input, feel free to chime in anytime with any helpful ideas/suggestions/information on this subject.

 

A cam has a ramp to open the valve and the valve blocks flow acting like a shock absorber in air flow in 4 stroke piston engines. This along with 4cyls and up having at least one intake valve always open, among other things gives them a fairly steady vacuum pull with a plenum or with ITB a slow rise and transition to a wave of back pressure. A rotary with its ports ends up having a strong/fast rise in vacuum then quick transitions to a wave of back pressure, in a plenum setup this leads to some bounce in vacuum and with ITBs leads to a strong pulse.
There is more at work than just what i have put down but this can fill a book.

 

Makes perfect sense. I'd imagine that is why the webers and the holley work ok, better flow.

 

here are my thoughts. there are only 3 kinds of carburator, pick the one that fits your situation best

1. 4 barrel progressive carbs; nikki/holley/q jet/etc etc etc; ALL of these work on the same idea. you drive around on 1 or 2 small barrels, with good response and economy, and when you want to go fast there are 2 more that open up.

these carbs work great, there are 4 ports on the engine, and 4 ports on the carb, and also since we can do things like half bridge ports, where the port timing is different, we can get a carb with different sized throttles.

and out of the box holley does not work on an RB manifold, because the RB manifold is individual runner, and the holley in the box is setup for a plenum style intake. this is also why we need a 465cfm holley on an engine that flows half that. its more useful to look at the throttle and venturi sizes.

i will also argue that weather you have a holley/nikki/q jet etc, they are all basically the same, just different in detail, the nikki is small, q jet has vacuum secondaries for example.

2. constant velocity carbs, SU/zenith/motorcycles etc. not many people use these, manifolds would be the hard part. i see no reason why they wouldn't work very well. again throttle size is important, but a 48mm weber is about equal to the 1 3/4 SU isnt it? coming up with the right needle would be the hard part.

3. non progressive carbs, webers/ stromberg 97's/ dellorto/ etc etc. these are simple, we just pick the throttle size (48mm) and jet it and be done with it. works best on something with even port timing, as there is 1 barrel per rotor.

IDA vs holley, the holley is down on power, but probably is tamer in the 2000-3500rpm part throttle range.

all the webers are really configurable too, we can change EVERYTHING in the carb

something on a wrap around manifold (DCOE, Dells) will be more mid-rangy due to the longer runners.

lack of choke means nothing, get the timing/jetting close and maybe you have to fast idle it with your foot for 15 seconds. even my P port starts right up cold and idles with no choke.

so there you have it!

want full bridge port that gets 20mpg freeway? try the holley. wanna start with a stock 12A and work your way up to a P port? buy an IDA

 

It's the lack of a plenum that makes it different than other engines. It's a ram type system, and that is affected by tuning length and variables that a plenum system isn't. It also doesn't damp the intake pulses, so the signal has a huge effect on the carb. You can join the two runners and make a huge difference in idle performance. A simple rubber hose between the two is fine.
The Holley and the Weber are infinitely tunable, that's why they're favorites. But, put a wide open adjustable carburetor in the hands of someone who is not veresed in the basics, and it's a hammer.


gd

 

Installer67, just to clarify, this is really the point i was trying to make. i wasn't suggesting the Webers because of speed or power superiority. it's the fact that if you have a basic understanding of carburetion and basic mechanical skills and tools, you can have something that will run on a rotary of ANY port-type. also, parts are still easily sourced. as i said, i'm sure you can get any of them to work, and even work well/adequate/whatever, but chances are you will run into one impasse or another at which point your hands will probably be tied and no further progress will be made on the "average joe"-level.

as J9FD3S said, you have to state what you seek in order to get more useful input. despite the love i eventually developed for my old Dell'Orto, one of the things i missed was the "feel" of a progressive carbie and i think that's why i made it a point to keep trying with Holleys. to be honest, i'm considering getting my brother's Holley whenever we finish up his EFI system, so that i can temporarily use it on my current project and i'm prepared to spend time trying to figure it out this time. what i mean is i'm prepared to try things with it that i was afraid to try when it was the carbie on HIS car.


that said, J9FD3S, could you just give a quick explanation of the difference between non-progressives and constant velocity carburetors? i always thought they were the same.

 

Thank you guys! I realize you guys have been tuning and tooling these cars awhile, but being a neb to rotaries, there are alot of subtle and not so subtle differences that affect the way you would mod or choose a carb setup. The mods to carb/intake I'd make for a piston engine, can actaully be detremental to performance on a rotary. So, I find myself with the need to explore the differences in the affects and performance of the various carbs, in order to formulate a system I feel confident will perform to my expectations. I have some ideas, but as yet they are just that. The "pulses" of a rotary are something I've not encountered, and obviously need to be considered in any setup. I have ideas on porting sizes/shapes/matching as well, but that is a whole nother thread! LOL J9FD3S, I was thinking along the very lines you listed as far as carb choices, thanks for taking time to list it. I was actually toying with the idea of using a pair of "non progressive" carbs to function as a modified "progressive carb". In so doing, I'd have the ability to tune "primary" and "secondary" individually, making it quite versatile.
As to what I'm looking for, right now, mostly good low end and mid range. The majority of the driving I do is from 20 to 50 mph, on roads with very few straight stretches and many tight and 70 to 100 degree turns and lots of elevation changes. I do do hwy driving as well, but to get there it's 20 minutes of cornering first. So, WOT driving will not be happening alot.
I'm thinking of using the 97's style for there flow and simplicity to mod/tune. Using one smaller and one larger, with a longer throw throttle linkage on the larger to have it open at a slower rate, thereby running at idle and lower end on mostly the "primary"/smaller carb, while the larger progressively open at acceleration, for nice top end. The intake would have to be "open" at the top, with mid sized runners. (somewhere close to the sidesraft length, just shorter) with a longer turn at the base than the RB LIM and a more sloping rise in order to take advantage of the same height, with longer runners.

 

basic rundown of common carb tech, some only use one of the listed options others use more.

Vacuum secondary = a carburetor that has a set of valves held closed by a vacuum pot and adjustable by changing preload on the return spring, This set up offers arguably the best throttle response due to the design not allowing more airflow than the engine needs keeping it from bogging.

Spread bore = A carburetor with two or more valves, one or one set is smaller than the other"s." This allows for the engine to run off a small primary for low loads allowing for better fuel economy and throttle response then later allowing a larger valve to open giving more air flow for high load/rpm performance.

Progressive = A carburetor with two or more valves where one or one set opens before the other again allowing for better low load fuel economy and throttle response but allowing the total WOT airflow to remain high

Non-Progressive = A carburetor with two or more valves where all valves open the same amount at the same time. This makes for a carburetor that is simple compared to a Progressive, has good WOT performance and can be used as a direct ram set up where each barrel can feed a cylinder individually.

Constant Velocity = A carburetor that normally has a large slide valve and instead of "closing" inside the barrel it "shrinks" the Venturi keeping air velocity constant. This slide valve is almost if not always vacuum operated and the slide valve has a metering rod attached for fuel control, the more the slide opens the more air and the rod lifts out of the seat allowing for more fuel. They normally have a second valve as well for the throttle control. This setup is known for having incredible fuel control but if you are running more than one they are a pain to keep in sync *same vacuum across all carbs*.
http://www.omnilex.com/public/bmw78/cvcarb.pdf good info and good pics to help see the workings.

The Qjet is a fun one to play with as long as you don't go bigger than a street port. I used a number of check valves, Devcon, standard tools, some parts form a pet store and a vacuum tank and got one working on a bridge port, it got ~240 hp and ~26mpg. It also had the best throttle feel i've ever felt and had an awesome sound when the secondarys open. Its not easy and you will end up with lots of extra springs, hangers, and metering rods, but it was a fun project and had A+ results. The things that make me like the Qjet? Its a vacuum secondary, spread bore, **** ton of adjustability, once you figure out how it works its really quite easy and lastly it will work for any amount of required total airflow from 215CFM up to 750CFM its all how you set the secondarys to open.

 

maybe i'm using the wrong nomenclature. i'm thinking non-progressive like an IDA, its a fixed venturi, with no primary/secondary, like a holley. nillahcaz defined the C.V. carb really well.

that is really similar to how the stock carb (and efi) is setup. if you size it right, it should work really well. size it like the rotary engineering dual DCD setup (and look at the stock setup) and you'd really be in the ballpark

 

Q-jets and Edelbrocks are not "vacuum secondary" carbs. The work with a weighted "air valve," not off of vacuum, but mass or air forcing the valve or flap open. Both can be adjusted; Q-jet by spring force and Edel by grinding the weight. NOt dependent on vacuum.
The Q-jet has triple booster venturis that help to atomize fuel and better meter the fuel to signal. Edelbrock has similar ideals.
Nobody has dealt with a log runner type system. Is this because many have tried with no success?

gd

 

So if I'm understanding you correctly, you will get an improved idle if you join the two runners with a hose? What size? Also, if that's the case, why not use a "single" larger id runner to connect from the center plate? Wouldn't that do the same thing?

 

If the Q jet is a vacuum secondary has been a long on going argument. The secondary control valve is held closed by vacuum and helps control how fast they open, but as was said this vacuum pot is not the only control over this. The function of the secondary valves is not the only problem with this carb when you start getting into high overlap engines, THE ENTIRE CARB is vacuum operated the Idle and cruse circuit are vacuum dependent.

 

im trying and so far with sum small mods and playing around got a set of flatside carbs off a 1987 gsxr750 suzuki to work very very well on my 12a full bridgeport... so far testing has proving much nicer and crisper that a 48ida and rejetted nikki i have had on engine before. watch my thread for more finished photos soon

 

Sounds cool, what section is your thread in? Like to have a look at that setup.