YellowJacket

Question for all the GURUS in here.....
I just acquired a Holley 650 carbuerator from Ebay (only $200) Now I do know that to use a carb such as this requires some modification to the carb itself. Does anyone here know the nature of the mods needed to make this carb work with the Rotary engine.
Thanks for the help.....

peejay

Let's analyze the problem.

Holley carbs are designed to run on a multi cylinder engine with multiple cylinders feeding off of each barrel (or rather, each primary and secondary). The vacuum signal to the venturis in the carb is nice and smooth because there's always at least one cylinder sucking air.

Our stock intake manifolds are semi-independent runner - each primary and secondary barrel sees only ONE rotor. At low RPM the vacuum signal is very staccato - suck-stop-suck-stop etc. At high RPM this smooths out.

What happens when you slam a stock Holley on a stock-type intake manifold? You have to richen up the mixture a lot for the engine to run right at low RPM because the vacuum signal sucks. At high RPM the engine will run EXTREMELY rich. You can fix this in two ways - dick around with the air bleeds and jets to correct the fuel curve for operating conditions that Holley never intended... OR... use an intake manifold that allows the barrels to see the vacuum signals from BOTH rotors!

A fellow rotorhead on the Mazspeed forum stuffed an Edelbrock carb on his half bridge ported 12A and the engine didn't want to run past about 6500. At some nameless dude's suggestion ( ) he decided to cut a passageway between the two secondary barrels so they see a smoother vacuum signal. Instant success - the engine no longer choked on excess fuel and it screamed to dizzying RPMs like a bridge port should.

FWIW a 650cfm is just right for a built 13B *if* you keep the manifold independent runner... once you go to a plenum type setup it will be WAY too big. Probably a 390cfm or 450cfm carb would be best. 390cfm double-pumpers are rare as hen's teeth though, they're only good for dual carb setups on small-blocks, and for people racing 2.3l Fords... so not many people want them, so there aren't many around, so finding a used one is difficult (and buying a new one is out of the question!) still looking for my 390 tho!

- PJ (duuh, what if I put my 750cfm vac-sec on my engine and lock out the secondaries, so it's just like a 375cfm two-barrel?)

peejay

BTW I forgot two points... Holley carbs are NOT easily tuneable beyond playing with float settings, fuel pressure, and changing main jets. The air bleeds are cast holes into the carburetor - non adjustable. (Some race carb manufacturers modify them to take miniature jets, but those carbs are EXPENSIVE!!! Cheaper to just get a Weber 45DCOE and be done with it) Same with the power valve circuit restrictions (the PVCR's are the "jets" for the power circuit), they're non adjustable, although some people on the BLOWTHRU mailing list modify them by carefully drilling them larger, or drilling the holes out and tapping them for jets.

Basically, unless you're a real techie geek and don't mind dicking with adjustments constantly until you get it right, Holleys suck for anything but the American iron they're designed for. And some people even then swear against Holleys and use metering rod type carbs like Edelbrock/Carter and the almighty Quadrajet.

And, uh, $200 for a 650? I hope it was new in box... typically used carbs go for $50-75-100 for common stuff like that. Seeing as it'll be better to find a smaller Holley (or something altogether different like a Weber or a Yaw-modified Nikki) can you return it?

YellowJacket

My last Rx HAD a Holley 650 siting atop a Racing Beat intake manifold (fed by a Holley Red fuel pump) and this thing HAULED ***!!!! Im talkin tire spinning launches and chirping the tires in 2nd and occaisionally 3rd gear!!!!! Now that set up I had gotten from Racing Beat and mounted it to a stock 12a with Racing Beat headers and freeflow exhaust system!
So what you guys are tellin me that in this case this particular carb would NOT be a good thing to use in my current Rx??? Cause it would be my 3rd holley carb on a rotary and the previous 2 didnt have any problems (save for some
gas-guzzle-itis) but they both produced GOBS of power.
At any rate if worse comes to worse I'll re-sell the carb. Any other details on how to get a normal Holley to run on a rotary?? And can someone give more detail on that whole vacuum signal thingie????
Thanks!!

1stGenRacer

What would be a good aftermarket carb for a stock 12a engine?

RC51DRAG

what gains can be attained with the Holley carb?

StumpDrummer

Crap Can the holley!!(theres a reason for this its personal) and use an EdelBrock 650 w/Vacuum secondaries ,Back off the spring that holds the secondaries closed one turn, use an open plenum intake like peejay said, or use a 1 inch spacer to help at low rpm's when you back the spring off, i have found out the secondaries open more smoothly and start opening a little earlier you'll also need to put an extension pole on the accellerator pump so you dont get way too much fuel dumped in when punching the carb from low rpm's get ready for some mind blowing fun! use a good header with 2 pipes coming off of it run 2, 2 inch exhaust pipes into 2 smaller 2 inch mufflers free flow are better.
you should feel the engine pulling to around 9-9.5k now!!and i mean pulling
go ahead and buy the rebuild kit now you'll need it soon hehe good luck

RX-7 Chris

iTrader: (19)

wow this is an old one.

Jeff20B

iTrader: (13)

I like where peejay mentioned to cut a channel connecting both secondary barrels. I have a Holley carb adaptor to fit stock 12A and 13B manifolds with a channel connecting both primaries. I believe this channel won't help or hurt anything because there should be a PCV system already in place inside the carb. What I'm curious about is whether to cut a channel connecting both secondaries, like peejay said. I have a Holley 390 and I'd like to try it on a 12A or 13B.

PercentSevenC

At least they're searching. Still, very interesting posts by peejay.

Jeezus

iTrader: (4)

Lol at pulling hard at 9-95oo rpm.

Jeff20B

iTrader: (13)

I want some opinions on what I should do with my carb adaptor/spacer thingamajig. Should I cut a channel connecting both secondary runners like peejay mentioned? Or completely hog out the middle and make an actual plenum? The 390 Holley is right out of the box so I'm leaning more toward hogging out the middle, but I'm little worried about 'over-doing' it.

Fake dual plane or total open plenum? Anyone?

Rotary Powah

iTrader: (1)

I've heard that the nikki if modified is as good or better than a holley.

84stock

iTrader: (5)

I would connect the primaries only. I did have success with a 1" open spacer on a 13b though. My upcoming plan involves modifying a 3310 series 750 vac sec 4160 series holley into a 4150 series along with a proform mainbody swap and then blowing through the carb and charger (twin charging). This is still a long way off though, completion next year if I am lucky....

Jeff20B

iTrader: (13)

Thanks 84stock for the info. Since my adaptor is already connected at the primaries, this brings me to my next question. Should my manifold have completely seperate runners like a Racing Beat, or should I use one with channels connecting primaries and secondaries like a '79-'80?

84stock

iTrader: (5)

leave them seperate. If you wanna see the diff just add a 1/4 or 1/2" open spacer.

Jeff20B

iTrader: (13)

I'll start out with my seperate runner manifold and make a 1/4" open spacer. Thanks 84stock. You've been a tremendous help.

84stock

iTrader: (5)

I think an ideal n/a setup, and I mean "ideal" set up would be a half bridged 13b with the primaries left "stock". Take a RB intake, add a 4 hole quadrajet to holley spacer and join the primaries only. Then mount a 650 holley "spreadbore", preferrably vac secondary, and dial in the secondaries just right. The huge secondaries would feed the bridgeport well and the primaries would contribute well to great throttle response and help with economy. If the secondaries are blocked off it should help tame the bridgeport idle to some degree.

Here is a pic of the bottom of my 850 spreadbore which contributed well to maintaining 12+ lbs of boost with the camden on my 1/2 bridgeport, this being said a 650 would be ideal for a n/a setup. Notice how the primaries and secondaries are kept separate while the primary to primary (and secondary to secondary) plates are channeled.

Rotary Powah

iTrader: (1)

Wow those secondaries are mega

StumpDrummer

Hey can you let us know how the carb works out im looking into the quadra bog thing myself i think the one im getting is a 750 but i think the primaries are the same on the 750 and 850 not sure

theres a couple of good links to get off google about tuning a quadrajet you might want to do a search they were very detailed

StumpDrummer

They come in real handy about 8500-10k when the turbo's really kicking in...No starvation problems with vacuum secondaries

Qingdao

iTrader: (3)

I don't know about holley, but I do hate the off center jets on the edelbrock. Lean/rich on turns SUCKS. and Nikki doesn't do that. SO modded nikki is better in my opinion.

The old man is always trying to push the Q-jet on me. I think a quadrajet would be AWSOME on a supercharged 13B. If nothing else the bowels are centered so no lean/rich on cornering. and the supercharger would be cool too. If I find buried gold or strike oil I might try this.


I know its a thread from over a decade ago, but I couldn't find any other reference to anybody even coming close to using a Rochester carb on a rotary engine.

wankel=awesome

Actually,the rochester would be pretty well suited to run on open plenum and with some sort of draw through turbo set up.

the supercharger on rotary thing is too expensive and doesnt seem that impressive.

Jeff20B

iTrader: (13)

Exactly.

Qingdao

iTrader: (3)

Yeah I looked at some superchargers last night. The camdens, while easy to install, are VERY expensive. and the eton's are very big and I dunno if they would easily fit on the right side of the engine (M45 might work). The Mazda millinia Miller supercharger might work, but it doesn't look very powerful (I suspect its just the factory kind of helper amount of boost)

I had the chance recently to poke and hold (similar to share and tell) a Q-jet. and its surprisingly similar to the nikki, only spread bore and 750 cfm. From what I've read and been told the Q-jet has issues, at least early models, with warping. You wrote draw through, but couldn't you blow through it with the same ease of blowing through a nikki. Or would the warping GM carb not withstand pressure?