Since my fuel economy is suffering...how easy is it to change the main jets and how readily available are the jets? On a Holley if you were too rich you would just start reducing the main jet. I know there are typically bleeds, air mixture jets and other crap like that. Do I need to only change out main jets to improve fuel economy, or is there another method of doing this?

Also, since my car is in the paint shop, I can't look at the carb right now. Is the vacuum secondary diaphram easy to change with the carb still on the car? I might change mine since I feel it is slow to open. Might be getting kinda stiff considering it is only 28 years old.

Another thing....when I was at Mazdatrix this summer I talked to the guy (who has been there a long time and really seemed to know his ****) about carb overhauls. He said he rarely ever pulls the carb but instead takes the top off the carb, cleans what he can from the top, change the accelerator pump and secondary diaphram and calls it a day. Anyone ever done this kind of "in place" rebuild on their carb???

 

Brandon are you formerly a V8 guy? Rotaries have nothing to offer below 3200 RPMs, where you would expect a V8 to show itself off. It doesn't matter what carb you have on there.

 

Yes, I will...
The 4 barrel behaves like a tiny two barrel until the secondaries begin to open, ideally just before the needs of the engine have exhausted the flow limits of the primary venturis at any given RPM (depending on load).
Your comparison would make some sense if all four throttle valves opened simultaneously. -But even then, the air velocity through those four venturis would be far greater than the air velocity through two large venturis of a two barrel of the same cfm, and air velocity is what #1) creates the depression that allows the fuel to come up from the emulsion tube and into the booster venturi, and #2) atomizes the fuel pre-mixture (fuel mixed with air in the emulsion system).

Sooooo...
Yes, a four barrel carb is better for low end and driveability. You can go as big as you need to accommodate the high end needs of your engine with a properly sized 4 barrel. Of course, going beyond those flow needs does absolutely nothing more for high end performance, but does begin to lessen low end performance because of the reasons above.

How many times have I gotten up on my soap box to preach this?
Here is the "problem" with the acceptability of what I have been saying, for many folks; Real world results compared to theory, sound as it may be, will always trump it. But enter in the HUGEST VARIABLE EVER in all of this, which for many, simply kills the science aspect of what I've been saying for years...
Everybody drives their car differently.
We've got guys right here in this thread who know they need to keep the RPMs up in a "window" of power, and then we have guys in here who expect to simply tromp on the accelerator and have instant power. We even have guys here that want the carb to be as automated as possible so that they don't have to give so much attention to controlling fuel intake. (Get FI?)

The rotary is well known not to make appreciable power at anything under about 3500 RPM, and that it continues to make more power right on up to redline. I shift at about 7400 rpm. Probably by the time I see that tach say 7400 it's closer to 8000.
I think it would be EXTREMELY beneficial to this thread to see what the shift points are of everyone with opinions on which carb is better and why.

 

Well said. And I do understand the theory. It's just that in practice the difference in low-end is fairly minor in my experience, as long as you're aware that driving a 2-barrel takes a little patience before you punch it, sorta like a mechanical-secondary Nikki. That's what I was getting at. On the other hand, my intake manifold also has much longer runners than a stock manifold. That could have something to do with it.

 

The velocity thru the venturii makes a huge difference in how well the carb works, regardless of carb design, number of venturi, or engine type/RPM.

I'm pretty sure the edelbrock 600 is overkill on a N/A 6P 13B, but it sure seems to run well, especially in the upper half of the RPM band.

 

New to the rotary game.
I just got my 1984 GSL home last night It has the webber 45 DCOE with stock down pipe into RB exhaust. When I filled it up today, to my surprise I was only getting 15 mpg.
I'm going to get in and see what is in there for a set up. If some one has a good starting point for jet size and such, It would be much help.
Or is 15 what I should expect with this setup??
Thanks
Paul

 

i rock a stripped nikki with mechanical secondaries. Love the set up and when properly tuned can get good gas mileage. I average 25mpg with full catless exhaust. Love how it kicks it at 3500rpms! Also very cheap and simple

 

It's all about: a) how well it's tuned, and b) how you drive it.

 

i've got the dcoe 45 setup on my 12a and i bought mine used and so i took a chance installed it and it ended up being tuned pretty good. it idles good, smooth transitions, smooth cruising at 2500-3000 rpms and great top end power not very much below 3500 and it seems like it does best when i don't go WOT until i'm atleast at 4500+ rpms. but i'm only getting about 160-175 miles to a tank from full to empty and when i fill up it holds right 10 gallons so i'm getting maybe 16-17 mpg. this is around town i have yet to get out on the highway on a road trip but i would think it would be a little better. plus i think my city mileage could possibly be a bit better as well but it's pretty much impossible for me in this car to not get on it at least once or twice every time i take her out. too much fun.

 

I'm running a fabulous Sterling. If anyone has not seen a sterling you can check my cardomain page in my sig and go to the last page. I believe mine is slighty more unique than the standard ones.

Naturally I'm biased, as most are after enjoying their new carb. It was fantastic out of the box, and once I get around to having a few bucks handy I'll be getting a handful of jets to get her tuned a little better. I know shes running rich, since I have a huge brown stain on my bumper and taillight above the exhaust. It won't even come off with the pressure washer and brush at the coinwash.

I personally have never had any bog issues other than trying to beat a car when turning at a light, and I stomp it, even though I know full well it'll just stumble and I'll sit there looking like an idiot. I don't do that too much, but every now and then I forget. This would happen with any carb save for the stock vac secondary Nikki. However the other side of that coin is the Sterling makes more power, and is more fun to drive. It honestly scared the hell out of me the first night I got her going. With the stock Nikki I always had the issue that arises due to the accel pump. I'd be cruising along and want to gun it, and I would have to come off the pedal, than back on. Silly little carb.

One thing I love about the Sterling is when cruising on the freeway, and I need to pass someone, I just ever so slightly dip into the secondaries and enjoy the acceleration that comes with little effort. It has also given me great highway mileage. On a trip from here to Winnipeg MB, I averaged about 700 KM/tank, which I think is about 440miles, traveling at an average speed of about 75MPH. Around town I don't enjoy the same mileage but of course, I've always got my foot right into it. It also sounds fantastic.

I am considering buying a second carb, probably a 2-barrel, just so I can see what all the noise is about. I think it would be kinda fun to have 2 carbs to change between as I felt the urge.

 

I LOVE FLAT TOP ROTARIES!!!!!(alternator on the side)

Very very nice.

When i had my Rx7,i could compare the Nikki to the Dellorto.The funniest part about it : It was a 40 Dellorto Side draught with 36mm venturies etc etc.Wasnt supposed to work,but it worked great!!

Chalk me up for sidedraught!

 

Glad this thread got brought back up. My friend put the RB intake and a Holley 465CFM on his 12a with a RB header and an exhaust that consists of a glasspack silencer and a turbo style rear muffler.

Problem is his car always seems to be loading up at idle. I have adjusted his idle mixture several times. Seems to run good as soon as i adjust it, but soon it loads up again.

Another problem is we cannot get the idle below 1100 RPM on a stock engine. I back the idle adjusting screw all the way out and 1100 RPM is the lowest.

Also a third issue is everytime he shuts down the car, a second or two later it backfires. It backfires worse if he has been ******* it.

All these issues concerns me because I was considering running a setup like his.

Any ideas on how to correct any of these issues???

 

my idle seems best at 1000 on my dcoe 45.

 

+1

 

I'm having the same problem with my SP 12A. I've asked around here and have not gotten any real help. I called RB and they were no help. I found on my car that I have a vacuum leak around the secondary throttle shaft. The carb is has been doing this since new.

I read that they have teflon bushings on the Holley secondary throttle shafts when the carb is a vacuum secondary carb. I emailed holley to ask about this. I also asked if they sell the bushings.

It would be great to see what you find out.

 

Weber 48DCOE on a 13B 6 port motor. Never lets me down

 

I realize this, I just got the car and will start tinkering
I checked the jets this morning and the mains are 180 and the others are 60f8
Don't know if this is to big so I'll get a few different ones and play with it.
It runs very nice now and idles good, has a small flat spot if you whack it, but if you roll it on it's not bad.
May switch over to a Sterling, we shall see
Paul

 

Hey does anybody have any experience with quadrajets and just because someone has told you they suck is not what im looking for i need someone with actual experience im looking for one with small primaries and huge secondaries for when the turbo kicks in

Trying different setups for drivability on a heavily ported engine for experienc and knowledge for the masses!!lol i wish i had my old 650 back

 

It's been a while since I messed with quadrajets but I may be able to help. What are you wanting to know?

 

Yeah 36 chokes is what you need for stock 12a to run well otherside it won't have the velocity you need at idle or thru the prgression to be smooth. This is a 48 Del from RB but the chokes are around 36, I'd have to take it apart to verify.

Karism I've been following your beetle build up with interest. I want to take this same engine and eventaully swap it into a fat chick. Right now I'll just work on getting a SP 12a ready to put in here later this year. Keeping my fingers crossed.

 

The main problem everyone is having with big carbs is when they punch it it bogs due to lack of vacuum at low rpm cant get fuel ect.

I dont know anything about quadrajets except the one on my old truck got great gas mileage compared to eb 650 i replaced it with
it also had (i think ) very small primaries and huge secondaries

it was on a 84 chevy truck if that helps

which one do i need or what type was on the 84 truck with a 305
i wish i still had the old truck just to see which model # was on the carb

I believe with small primaries it would help with low end punching vacuum troubles

and thnx for the honest reply!!---StumpDrummer

usually those that laugh get smoked at the track!!

 

I wrote a long article describing all the different carbs available back in the year 2000. Things have not really changed much since, except the prices have gone up some.
Go to this link, then click on the "Carb Facts" button

http://www.mazspeed.com/faq3.htm

 

My '79 has a Dellorto on a Atkins Aviation manifold and is attached to a '85 GSL-SE motor. It's just for autoxing. Great power. Driveability is unknown since I don't drive it on the street.

My '81 has a stock Nikki on it. This is my rallyx car. I used to drive it on the streets. Driveability is excellent but definitely not quick.

My '83 has a twin Weber 36 DCD setup on top of street ported 4 port 13B and my '84 the same setup on top of stock 4 port 13B. Good power and excellent driveability on both. No bogs anywhere. Any gear, any throttle position. When I had this on my '84 when it still had the 12A it got better mpg than the stock setup. I have noticed in the last couple of years that the '84 stumbles in sustained high g right hand turns.

Other setups that I've driven or owned:

I've had a Dellorto with the 2 piece manifold on top of GSL-SE motor. Great power but definitely prone to bogging.

Custom Holley on a street ported 4 port 13B. Excellent driveability and decent power.

Unknown Holley on a stock port 4 port 13B. Pretty much sucked in driveability and power.

Complete RB Dellorto setup on a 12A. A friend of mine had this setup. A little side by side action showed that the car was not as fast as my twin Weber 36DCD setup on the 12A. This was done in the late '80's.

 

The pump the other owner put on the car, screams like a $2 ***** trying to get a $5 tip.....Will a stock fuel pump push enough fuel to run a Weber 45 side draft??????
It has a fuel pressure regulator and Gauge.
Or what's a good quiet pump??

 

[QUOTE=StumpDrummer;8754468]The main problem everyone is having with big carbs is when they punch it it bogs due to lack of vacuum at low rpm cant get fuel ect.[QUOTE]

On my Holley setup to fix this problem I went with a AP jet to get a larger squirt in there. On the qjet you should be able to do the same.