QuadraJet - Which secondary metering rod to use?
 

I'm considering changing the secondary metering rods on my rebuilt '72 Chevy #7042210 QuadraJet on my 400ci to see if it improves my ETs. I haven't taken the carb apart yet to measure the thickness of the current rod tips, but I see Summit offers the following secondary metering rods:

EDL-1951: 0.0410" Letter code CE for identification
EDL-1954: 0.0667" Letter code CL for identification
JET-201010: No size given. Fits stamp DA

Not sure what the letter codes stand for? Any ideas?

Am I correct in assuming that the larger the diameter rod, the less fuel is delivered? So, if I find that I have a larger diameter rod installed, I should go down in size (like the rod that is 0.0410?) for more fuel when the secondaries are at WOT?

Thanks!

Jim.

your questions have the correct answers. the other consideration is the hanger. A = richer Z = Leaner

i did a quick search on google.

https://www.carburetion.com/rodshang.asp

http://www.hioutput.com/tech/qjetrod.html

obviously before you make any changes, you need to find out what you already have installed.

usually around a G hanger and DA rod should be a good starting point if the Main circuit is setup correctly.

Initially years ago I followed Doug Roe s book now I use Cliffs Qjet book.

I stock AX and DA secondary metering rods. Your carburetor originally came with DA's. The AX rods will richen it up a tad over the DA's.

https://cliffshighperformance.com/pr...-metering-rods

one of the things I really appreciated about Cliff's book, was the listings at the back to identify via the letter codes all of the secondary metering rods - super handy!!

Thanks everyone for the excellent replies! :) I've bookmarked Cliff's site and may order from him soon after I check my current metering rods...

CE metering rod is the richest long power tip rod originally made by GM for any quadrajet, meaning the power tip starts flowing maximum fuel at 70-80 degrees of throttle opening. The dimensions on it are .135-.096-.041" long tip.

CL metering rod is a long taper rod that is on the lean side at .135-.108-.067" also with long power tip. It will reach the power tip at the same time as the CE rod, but has a .026" larger tip. that would be like jetting down about 4-5 jet sizes.

a good approach would be obtain as many of the long power tip rods as you can find, and use them like you would use jets in a Holley. From leanest to richest they are:

DD-.105" (leanest secondary rods GM ever made for Qjet, period)
BS/CZ-.095"
CG/AS-.077"
AT/CL-.067"
BZ/CX/AY/AZ/CD-.057"
AU/CV/CK-.053"
CE-.041" (richest long power tip rods GM ever made)

keep in mind these are metering rods, the smaller size is richer.

there are medium and short taper rods that are richer than the CE rod, but they'll delay the power tip to wider throttle opening point. the richest triple-taper rod GM ever made, was a BV with a .030" short tip.
so you can take ANY rod, turn the tip down in a drill or mini lathe to .030", and have the same WOT function as the BV. or...cut the tip off, and you have the richest metering rod, because the tip is completely gone, and the secondary jets have no restriction at WOT at all, like the AD rod.

if you're racing, you can just REMOVE THE RODS AND HANGER. we did. it works. you can also remove the primary power piston rods, and just run jets. I did, it worked.

the limiting factor for a Qjet carb is, beyond a certain point, the single needle valve can't flow enough fuel to keep the bowl filled. at that point you need a dual feed, dual fuel bowl carb, like a Holley, or Carter Competition Series, etc. I did reach that point, and had to bid the Qjet goodbye. If I had run 1/2" fuel line from the tank sock to the carb, or a fuel cell up higher in the trunk, I may have been able to cure that problem...instead I threw the towel in on the Qjet. The engine would just suck the Qjet fuel bowl dry, even with 2 fuel pumps, an electric pushing into a mechanical. I was running a '70 Buick 800cfm center, '69 Pontiac 428 baseplate, top. Back to back the 800 center was worth 3 tenths, and a phenolic spacer was worth another tenth. 4 tenths difference between a flush mounted 750 Qjet, and the 800 center Qjet w/spacer, on a bone stock 400 RA IV cammed fresh motor, with iron intake, #12 heads, headers, 3640lb w/driver, automatic street car. Later on with the bigger motor, dropped a Holley 3310 on the motor, fuel starvation problem instantly solved.

A medium power tip starts at 80-90 degrees throttle, and a short power tip starts at 90-100 degrees throttle. it's a good starting point. there are many richer metered rods at WOT than that, but they are medium and short taper, meaning they'd delay the fuel flow to wider throttle opening points. the richest of all was the AD rod, which had no power tip at all. i.e. at wide open throttle, it flowed through the open secondary fixed jets with no restriction, into the secondaries. you can make any rod into an AD rod, by grinding the tip off.

FWIW, if you have enough motor, in the 1/4 mile the metering rods in a Qjet don't even matter. 25 years ago we were at the track with a friend's race car, running a 428 Pontiac. It was running 10.50's with a Qjet. He changed the metering rods a few times, but it didn't make any difference. I suggested simply REMOVING the rods and hangers, and do a test run. it ran 2mph faster, and 2 hundredths slower on the next run, with no rods or hanger at all.

you want to meter the secondaries on a Qjet to give the engine all it's got as soon as it's opened, provided that's not too rich for the CID, cam, intake, heads being used. Drive around part throttle on the primaries and get the sharp throttle response, and good drive-ability, mileage. In a way you can think of the primaries as the idling and city/highway cruising mileage carb, and the secondaries as the power carb.

the vintage Carter Competition Series 1970-74 carbs were a Qjet on steroids with dual floats, dual float bowls, and phenolic center section. If you like the spreadbore design and Qjets in general, find the vintage Carter Comp series and purchase it, and tune it up. you won't regret it. it's identical in venturi, throttle plate design to a Qjet, just much better fuel delivery, and about 50cfm more flow, i.e. 850 cfm. They were advertised as 1000 cfm when new, but that was stretch advertising. it will support a higher level of horsepower than a Qjet will, due to having dual needle valves, one per side, but with the same street driving manners.

keep in mind this is relates to what hanger is used. a B hanger pulls metering rods out of the jets in secondary about .095" further, than the leanest letter V hanger. When I was using, building, selling a lot of Qjets, back in 1982-1998 era, I would only use B-C-D-E-F-G-H-I-J hangers. those are the richest ones made, in .005" lift increments, from a B with .520" drop, to a J with .560" drop. You can just bend any of them to the B dimension. If you use a V hanger with a .615" drop, it will negate the effects of the richest AD rod to some extent, because it will keep the metering rods .095" further down in the hole, compared to a B hanger. so....I'd just avoid the lean hangers altogether.

NHRA Pontiac SuperStock 400 racers used to run a J hanger with an AX rod with .040" short tip.

There are secondary metering rods with long tips smaller than CE, the rods from the 301 turbo carburetors are one example.

Since I build and supply parts for folks who race in quite a few racing classes, including circle track , most carbs are supplied to them with a set of machined secondary metering rods machined from the same cores.

These rods will then have the same upper section diameter, taper, tip length and included angle leading to the tips. This is the ONLY way to make predictable metering changes with a Q-jet without also having to mess with hangers.

Some racers are more meticulous than others and want a full set from about .028" to past .060" in .002-.003" increments. I think .005" is close enough, but if you are trying to be the very best and data logging, using DA, etc I guess precision is king.

The later model Q-jets are 850cfm, not 800 cfm. All early versions are 750cfm. Most don't flow to full potential as the factory limited the open angle of the secondary air flaps and sometimes the secondary throttle plates (1968 FB's come to mind here) to limit CFM and engine power.

I do agree that the biggest stumbling block for the Q-jet is one N/S assembly, but even with that going on we have customers feeding big power with them and running well into the 9's. Several ways to skin that cat. Some use smaller N/S assemblies and high fuel pressure, other larger N/S assemblies and less pressure. ALL will use at least 8AN fuel lines, and a rear mounted pump at 140gpm or larger fed from a sumped tank or fuel cell.

As far as tuning with secondary metering rods your aren't chasing much there, assuming the carb is well dialed in right to start with and staying FULL on a hard run. On quite a few occasions early in my learning curve I've changed secondary metering rods from so rich the engine was huffing a little black smoke to so lean it started to surge slightly on the top end and didn't see more than about a tenth and a half for ET difference and less than 2mph.

So if you thing you're chasing half second or so in vehicle performance save your time/funds. Personally I prefer to run a tad 'fat" at WOT. EGT's are lower and less chance of putting a piston in the oil pan just to show a few hundreds quicker on tthe time slip.....FWIW......

agreed....if the difference between using metering rods, and no rods is so slight...why bother if racing. but you need enough motor to run a Qjet with no secondary metering rods without being too rich. if it's a handicap bracket class, jets with no metering rods would be more consistent, less complex, less can go wrong with fuel just going through a hole, with no hangers, rods, flap tension spring, power piston spring to consider at all.

super stockers don't run fast because they have the best carburetor. they run fast because they have really good chassis setup that hooks, and puts it all to the ground.

take any Super Stocker, and start an unlimited single carburetor class, all other rules being equal, the unlimited class would have aftermarket Holleys, Dominators, etc. and be faster.

let's be honest here. the first thing an honest Qjet builder will tell you is,
you will always make more HP with a Holley, inch for inch.
but the Qjet does a damned good job for what it was, and is.
in the case of SS class racers, that's the carb they have to run,
because of the rule book,
not their carb of choice otherwise.
if Qjets were the best carb made for maximum HP,
NASCAR would have been running Qjets instead of Holleys back when they still ran carbs.
and the GT-40 would have ran a Qjet at LeMans, instead of a Holley, back in the 1960's

:D

the early Pontiac SD455 and Buick 455 1970-74 carbs were always listed as 800cfm rating, not 850cfm. Are you saying the later 1975-up carbs flow 850cfm ?
Where's the extra 50cfm come from ?

the early Carter Competition Thermoquad, was obviously a high end copy of a Qjet from day one, it had the single venturi booster like a 455HO carb, bigger primary bores, dual floats, dual needle valves, phenolic center. One look at the top, you can tell they copied a Qjet, and improved the fuel delivery. A single inlet line splits into 2 feeds internally. Not to be confused with the production Mopar Thermoquads, these used unique jets, and some other one off parts.
Back in 2000-2010 they were on Ebay for $75-$125 each. I sold all my Qjets but kept these. Picked up half a dozen over the years....everyone was kicking them to the curb to buy Demons, Holley HP, etc. Oddly enough many Buick GS guys sold me their old Carter Comps for a song. Now they bring a lot more just for a core. The float bowls are on the sides, not in the middle like a Qjet.

too bad they stopped making them like this.
gas crunch hit 1973, drag racing and demand fell off a bit.
they were discontinued.
Carter actually did make a better Qjet IMHO, improved linkage, albeit more complex.
but it was expensive, and now somewhat scarce, and pricey.
I look at these, and realize they had to pay some sort of licensing fee to Rochester,
to make a carb so similar looking on top and bottom...
Qjet was the cutting edge of carb technology back then,
Carter figured, why reinvent the wheel. Just modify and improve it a bit.

check out this HD...on a parts carb vintage Carter Comp series.
the huge jets are the primaries.
the smaller jets with ends sticking out, are secondaries
needle valve threads in back of carb.
Buick guys would run 93 jets in front, 125 jets in rear
this parts carb has 89 jets in front. jets are screwed in upside down into towers.
I believe that's an idle jet in the side, pressed in, for primaries.
on top, very simple. one air bleed primary, one air bleed secondary, and opening for primary metering rods. these do not run any secondary metering rods. it's been years since I looked at this stuff.
this center section is an 850, with venturi in bore.
the "1000" has no venturi, it's straight through on primaries.
secondary tension adjustment is all on the side, with washer locking screw.
no allen underneath like a Qjet.
one carb with a spare center, easy swap from 850 to 1000cfm. use the same lid.
jets, needle valves, floats, metering rods all stay with the lid. pull it off, and change the metering. leave the base, center on the car.
primary booster venturi has only one ring, for more flow.
all air bleeds in the lid covered by a hinge pinned cover
these carbs, were badasz

idle jet (?), fuel distribution tower to primary, secondary nozzles.
the round tube casting sticking out of the lid, is the accelerator pump housing

After reading all your posts, I just noticed this one, and think I can offer some assistance;

The 1971 Pontiac (except 400/auto & 455-D-port/auto) used the same 750cfm main body casting, but with the lower booster ring removed - which is why they have a small bump in cfm.

The real 850cfm used a totally different main body casting with a larger primary bore;
I believe that this was a Buick casting which I also believe was only used on a couple engines;
There is a pronounced hump in the side wall of the primary bore.
If you have Cliff's book, flip to page #31 - it's shown on the top.

I believe that Pontiac opted to use this "Buick" bowl casting for all their Quadrajets starting with the 1975 model year.

I when I was looking for a picture to add to this post, I found THIS post elsewhere...

may as well finish up...idle jet in side of main well tower,
jet seals on main well, idle jet feeds cavity around main jet,
idle jet pulls directly from the float bowl, not off the main well
idle passages crosses to other side, goes down side of main well tower,
up into top of carb, picks up idle air bleed, and goes forward to front edge of lid,
then straight down through front of phenolic center section,
into the baseplate, very similar to Qjet idle discharge, transfer slot.

downward through phenolic center, into baseplate, to idle discharge passages/slots

primary power piston/metering rod vacuum source

idle air bypass for front barrels, on this parts carb not drilled. I've had to drill them through before.
these Carter Comp carbs will drizzle on the main nozzles, just like a Qjet, idle bypass air is part of the cure.

rear idle air bypass, not drilled on this

"the early Pontiac SD455 and Buick 455 1970-74 carbs were always listed as 800cfm rating"

The 71-74 Buick 455, 73-74 Pontiac Super Duty carbs are the only large castings made before 1975.

All front inlet 1975-up automobile carbs will also be the larger castings.

Some side-inlet Chevy units were stil the smaller castings thru 1980.

All of them would flow 850cfm if the stop were ground a bit to mimic the open angle as used on the Edelbrock 850 cfm Q-jets.

The 1971 Pontiac carburetos missing the outer booster rings flow 827cfm.

Over the years folks just got in a habit of calling any large primary bore q-jet with a "bump" in the castings "800 cfm". Most were less than 800 cfm as the stop for the air doors limited the full open angle some.

As far as making power once you dial in any carburetor for the application and it flows adequate CFM nothing else is going to outrun it. I've proved this over and over again on the dyno and at the track back to back testing. IF you are using a dual plane intake designed for a spread bore carburetor it actually has an advantage over a square flange carburetor on the same intake as the plenum areas are wider, the intake has more volume, and the carburetor bores are lined up right in the center of the plenum areas.

This is how I continued to outrun every other type of carb tested against my 1977 Pontiac Q-jet for so many years. We ALWAYS made more power on a dual plane intake than anything else bolted on right after it for the next run.

My Q-jet didn't fair so well on single plane intakes and power was typically WAY down unless we used at least a 1" spacer to get the huge secondary throttle plates up and out of the plenum area.

As far as running w/o a primary power piston and metering rods it works quite well and eliminates any moving parts plus more precise fuel flow thru a single hole vs a hole with a metering rod hanging in it........

this has 89 primary jets, 116 secondary jets.
that's in thousandths, actual size.
want to jet it up ? drill out the secondary jets with an 1/8" drill bit
not kidding ....:D that's what the Buick guys did for 30 years....
Buick intake manifolds are flat like a workbench,
the big 1000cfm carb would slow down velocity,
so the fuel could make the sharp turns, resulting in higher flow,
and make more HP

accelerator pump diameter is huge, connected to squirters with a small hose, slips on between those 2 fittings.

the primary and secondary main wells, also contain the air bleeds pressed into them from the top, the jets cap them off. the main well is drilled from the top to accept the primary metering rods. there's also additional air bleeds in front of the primary bores in the corners- they do both pull air in during idle/off idle, and discharge fuel at high speed as well, thereby shutting idle fuel if memory serves...

Carter Comp on left, QJet on right
Carter has 1.5" primaries, 2.250" secondaries
Qjet has 1.375" primaries, 2.25" secondaries.

1/8" bigger primaries on the Carter
the phenolic center section of the Carter doesn't have a shelf in the front of the primary bores, it's more open than the Qjet
it's a Super Qjet on steroids...

Cliff,
It would be great if those Qjets flowed 850 cfm, but they've been called 800 cfm by some very reputable sources for a very long time- such as Doug Roe, back in the day when they were still being put on cars new, i.e. 1973.
If we're going to now second guess these sources, then we can second guess all the carburetors made out there, and add 50 cfm at will. You know as well as I do, the people who started using the 850cfm number, were Edelbrock, when they copied a truck carburetor exactly. Opening the air valve or throttle a hair more, doesn't add 50 cfm. I'd have to see it on a flow bench at 20" to believe it.
I remember specifically seeing from several sources, information stating the throttles should NOT be opened into a vertical position, they should be in line with the baffle- and it was designed that way for a very good reason- not to open all the way.
If you have enough motor and rpm, yes indeed it could flow 850 cfm. But that's not what it would flow at 1.5" HG, or 20" water, the traditional testing standard. The guy at Stealth Carburetors explained it this way, when I asked him what their carbs flowed:

"it is dependant on the engine

a 1.250 venturi carb (650) on a 350 engine at 4000 rpm flows 400cfm
a 1.250 venturi carb (650) on a 430 engine at 8000 rpm flows 880 cfm

the actual "flow" is dictated by engine size and rpm not carburetor size

the true CFM is not determined by the carburetor ---it is determined by the engine, but in the retail world this is how people determine CFM with a carburetor:

1.6875 throttle blade--1.250 venturi =650
1.6875 throttle blade---1.300 venturi=700
1.6875 throttle blade--1.345 venturi = 725
1.6875 throttle blade--1.375 venturi =750"

so...can you add 50 cfm to a Qjet ? sure, if you have a monster motor, and rev the piss out of it, until it pulls that much through the venturis by sheer force.
but the standard of the industry still is 1.5" HG/20" water. not what the biggest engine made can pull through it.

@'relic;
I don't really know what you're trying to do here.

The OP was asking for advice on modifying their Quadrajet's metering, and you started up talking about these other carbs;
I was following, because frankly I thought what you were showing looked neat - even if it had no bearing what so ever on what the OP (tremo) wanted to glean from the membership here.

But now you're trying to argue with what Cliff has shared?

Please don't try to antagonize the man;
I for one appreciate what he and a few other Quadrajet experts share.
I am grateful he still comes to this corner of the internet.

Doug Roe's book was good for it's time, and still offers some great information if you can find the original printing (HP-35; 1973);
The original printing was so much better than the latter condensed version, but even some of his tips have been proven over time to not offer the gains suggested;
The thing that immediately come to mind from Roe's book was the drilled out idle tubes;
I sold his latter book probably over a decade ago, but I seem to recall that even that printing was still touting the drilled out idle tubes shown on pg 256 of
Roe's original book - Cliffs book offers why this is a bad idea on page 104 of his book.

Beside's unless I am mistaken flow measuring is done on a bench designed to measure airflow, not a motor.

Oh wait... I just noticed this post:
Ok, you got me.

https://i0.wp.com/www.skepticalrapto...80%2C230&ssl=1

Doug Roe published all the Rochester carb specs for 1-bbl, 2-bbl, 4-bbl carbs,
back in 1973. Qjet with air valve wide open 90 degrees, is 800cfm
not 850cfm.
the logic oh, Edelbrock said a truck carb repop flows 850,
therefore ALL 800 Qjets in the past 50 years also flow 850,
is a stretch.
what's the flow bench say at 1.5" HG ?

Those are the fuel pick-up tubes for the secondary POE-system.

FWIW

"CE metering rod is the richest long power tip rod originally made by GM for any quadrajet"

Whatever published source you used was in error, I suggest that you get your micrometer out and do your own testing.

"the early Pontiac SD455 and Buick 455 1970-74 carbs were always listed as 800cfm rating"

Incorrect again, the 1970 Buick 455 carburetors are the smaller castings. Two errors in your information. Not trying to be critical but IF you are using research as the foundation for your the information you are posting it is inaccurate. How accurate was the Doug Roe testing from 1973? Did he actually do the testing, and did they use an adjustable stop or vary the open angle of the secondary air flaps during that testing?

As for the larger castings being "800" vs "850" cfm, I had the smaller primary bore castings tested, the 1971 455 HO castings tested, and the larger castings as well. My source came up with the same numbers that Edelbrock did for the larger castings when the air door was opened to the angle listed in my book in Chapter 6 about Edelbrock Q-jets.

I didn't make a big deal about it in the book because it is NOT easy to do that sort of testing and we also noted that companies who do "rate" their carburetors for CFM don't all use the same standards/test methods. Even so I mentioned it briefly a couple of times in the book and in this thread and already regretting it seeing how one person got their panties all wadded up about it.......FWIW.

I really think that the dyno and drag strip testing I did is more important and shows us the potential of the different Q-jets. Back to back to back at several private track rentals I tested a 1970 Pontiac RAIV carb, 1971 HO 455 carb, 1977 Pontiac 17057274 carb with the air door allowed to open the same as the Edelbrock 1910, and a Holley 4781-2 850DP carb with downleg boosters.

The dyno showed that all the carburetors were dead even to 4500rpm's before any differences started to show up. At the end of all the pulls the 1977 Pontiac Q-jet made the most power, followed very closely by the Holley 4781-2 carb, then the 455 HO with the Ram Air carb finishing last.

I backed up the testing at the track and the 1977 Q-jet ran .02 seconds and .3 MPH quicker than the Holley 850 which backed up the dyno differences to a "T". Remarkably the 455 HO carb was almost the same as the 850 Holley and for some reason showed slightly more MPH. To this day I attest that to the fact that we were using a dual plane intake with wide plenum areas designed for spread bore carburetors (They are lined up directly in the center of the plenum areas) . The 1970 400 RAIV carb lagged behind close to where it should have by looking at the dyno results. Nearly as I can remember is was down almost 2mph on top end and almost a tenth in ET. Sadly I lost those test results when my last hard drive crashed so don't have the details to put up here for comparisons.

Hope this helps some instead of continuing to fuel the "witch hunt"......FWIW........Cliff

at the time of publication that was the richest metering rod with long taper made by GM,
Doug Roe was not in error, he was building these carbs and writing publications back in 1973 when they were new,
and he listed all the rods from leanest to richest in the metering/capacity flow section.
I did notice there may be some typos or transcription errors, because rods listed as medium taper that I own, have fairly long tapers.

speaking of errors...hey we all make mistakes...opening the air valve further than the stock setting, will cause the bottom of the air valve to progressively block the secondary main fuel nozzles. that's why they are offset the way they are from the factory, take a look sometime. if they are opened completely 90 degrees on the bottom, it will block the fuel nozzle stream severely and cause a lot of turbulence. it will also cause the top side of the air valve to progressively block incoming air, because of the way it's designed.

take a look, I happen to be building a carb for a guy with a '76 Firebird, 400 block, 421 crank, 60 over, 434 cid today, using a Buick 455 800cfm center section, and '73 Pontiac 455 Qjet top.

and one more thing...the throttle blades should be in line with the steel plate baffle, so the air flows staight over both smoothly, not offset from each other. otherwise that too creates turbulence. the correct setting should look something like this, and beyond that, it's not gaining anything..

this one I drilled bypass air holes to .193" , 73 jets, no metering rods, because the cam is huge,
and he's already having an issue with another Qjet carb he paid good money for from a well know Qjet building shop, we've all heard of...
that one is not idling the big cam correctly. his existing carb has primary nozzle drizzle and smoking out the exhaust too rich.
this one I opened up idle tubes to .043" and the idle down channels are .055" stock form leaving them alone- on the Buick 800 center.
FWIW the idle tubes can be drilled wide open to .068" and the down channel will become the restriction, being it's smaller.
I've done it before and it idled at 1000rpm in park, and 800rpm in drive TH350, with a 250 @ .050" solid flat tappet cam, on my own car 20 years ago.
that also was a Buick 800 center section, with Pontiac top.
the Buick 800 centers have smaller air bleeds, so they are richer.

A lot has happened since 1973 or whenever the Roe book came out.

The angle of the secondary airflaps is well covered in my book with recommendations plus the maximum open point as used on the Edelbrock 1910's.

I verified the maximum open angle by removing the stop and drilling the airhorn for an adjustment screw to find the point of no return or maximum effective angle where going further doesn't improves nothing or has diminishing returns. This testing was done on the dyno and backed up at the track with drag strip runs on several different outings.

When I wrote my book I didn't use any reference material, and have actually never owned the Roe book or have even looked at one since my high school days..........

I am enjoying the information being presented here…..

I also enjoy a variety of publications on the Perfomace modifications of qjets and Pontiac’s in general ….

McCarthy, Roe, Ruggles, Hand….. they ALL offer something. Do they cover every “trick in the book”? No…. You still have to get out and try some things for yourself….

Thanks to those willing to offer their experience and wisdom….

GM Qjet secondary metering rods available c. 1973, from richest on top, to leanest on bottom
and...how the rod tapers work
knowledge is power...Merry Christmas

jets and primary metering rods

metering area chart for jets, metering rods

FWIW, secondary hanger specs by letter code.
yesterday I just bent an "N" hanger, to "B" hanger specs,
and put it the 800 with CR rods just to start the motor.

flow rate capacity for Rochester carburetors, including monojet, 4G, 2G, Qjet.
the handwritten numbers for 2G are flow at 1.5" to compare to a 4-bbl at equal pressure drop. Unless you're using a '66 tripower intake with the big 2bbl carbs, the early tripowers with small center carb really aren't worth running, unless it's a numbers restoration or show car. A single 800 Qjet, or 850 Holley flows the same, or more- and is a lot less maintenance and complexity, more equal fuel distribution, and better intake runner layout. With the availability of 1000cfm Holley carbs based on the 850DP, and Dominator carbs, there's really no reason to run any tripower at all- except for nostalgia, looks, the sound, or the hell of it cuz it's there on your garage shelf.
regardless, if you run the '66 tripower intake with the 3 big carbs, it'll have 900-920cfm total capacity...measured at 1.5" HG like a 4-bbl is.
the downside is, eventually rebuilding 3 carbs instead of one, and the tangle of fuel lines, progressive linkage.
20+ years ago a friend from Detroit area obtained all the flow numbers for various factory and aftermarket intake manifolds, connected to a head on the flow bench.
the tripower intakes actually flowed very poorly. the vintage aftermarket intake manifolds flowed pretty lousy as well. the best flowing were the Victor, Nash/Warrior, Torker II, Performer, RPM, Street Dominator, Torker 1, and GM iron 4bbl 1967-74, more or less in that order.

1973?

No wonder there are so many errors in your outdated information.......a LOT happened in the next 50 years, including J, K, L, M series primary metering rods, quite a few secondary metering rod offerings, and CFM testing using the later castings allowing the secondary flaps to open past 90 degrees.........FWIW.......

How far should the flaps open for max flow?

If you have my book the maximum open point is discussed in the Chapter on Edelbrock carburetors.

When we did cfm testing the entire stop was ground off and an adjustment screw installed so we could continue to adjust the flaps past 90 degrees to find the optimum open point, and when going any further made no improvement or diminishing returns.

I backed up the cfm testing with dyno testing using the adjustable "stop" and it confirmed what the air flow testing did exactly. Going too far open not only doesn't improve airflow, it starts to impede/redirect fuel flow from the secondary nozzles as well. This testing was done over 40 years after the 1973 information was printed.

It would be unlikely that any of that sort of testing was done way back in 1973 and why the cfm ratings are less than what we came up with.......IMHO......

SDBob, your right down the road from me.

When i dynoed my 482 ci with a 7043240 Qjet and ended up at 586 hp, the vacuum at the end of the pull was 2". So we were some 15% over the rating at 1.5", whatever that number really is

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You have a good point here and there is some logic in what you say. However, Cliff has been an expert with these Qjets for years and his data is solid and recent. One thing about the net, the BS out there is incredible. I'm a data person and knowing Cliff and having work done by him makes me trust his judgement. Experience is the key, IMHO. I have a 69 273 Qjet that was rebuilt by a renowned carb shop in California back in the 90's. Highly recommended. That shop is now defuncted BTW. I needed service for it 20 yrs later and Cliff took it in. Cliff found a lot of problems with the internals to my surprise and brought it back to factory specs. The RAIV engine has never run better. Now everyone is entitled to their opinion. But Cliff's research and knowledge with Qjets is golden from my experience with him. There is and always will be people claiming to be "experts" with anything. Do your research so you don't get burned!!!

Tks for the kind words.

As far as cfm ratings I'll add this. Edelbrock made their 1904, 1905 and 1906 from the late model castings with the large primary bores. They rated them at "795" cfm. Interesting that the secondary air flaps open EXACTLY to 90 degrees same as the testing done in 1973.

The Edelbrock 1910 uses the same castings, but has the airdoors open considerably past 90 degrees with a much shorter stop. Their is a good pic of it in my book in the chapter on Edelbrock Q-jets. Assuming Edelbrock tested their Q-jets prior to selling them their information is exactly what we found here flow testing the same units.

Also +2, 3, 4, 5 and 6 about the amount of BS out there plus outdated and inaccurate information, especially on the Quadrajet carburetor. There isn't a single day goes by here I don't get a call from someone wanting a "bottom plug kit" because their engine fails to start after the car has set for a week or so. They went on the Internet and every poorly informed soul regurgitating old/outdated/inaccurate information responds to their complaint telling them to glue up the bottom plugs with some useless epoxy that doesn't hold back fuel in the first place. The real problem is either a swollen or failed accl pump cup either not putting fuel into the engine when fuel returns to the carb or it so tight in the bore it woln't let fuel past it to fill the accl pump well or some sort of fuel delivery back to the carb issue. Either way you get a no-start scenario and have to dump some fuel into the engine to get it to fire.

Here are the facts. Rochester fixed the leaking bottom plug issue clear back in 1969 when they made the castings thicker at the plugs and pressed in solid aluminum plugs and swaged over them. From 1965-68 ALL of them will be leaking because they used small brass cup plugs driven and staked in place. Lead was common for the front plugs and we often see those leaking as well.

It's pretty rare to see a later model carb leaking compared to every single early one, but in any and all cases leaking bottom plugs will NOT keep your engine from starting after a long period of not using the vehicle. In most cases they actually cause hot restart problems after 15-30 minutes of sitting after a hot shut-down as they leak fuel into the intake flooding the engine............

As mentioned, I study many different sources for info…..there are MANY guys out there that are very very knowledgeable, yet may never have chance to pass that knowledge along. I take all into consideration.

At the end of the day, its just a carburetor…….If I screw it up, I’ll build another one without a single worry

I have many reference books in my library on Pontiacs written by authors I do trust. I see your point there. However, the internet has opened the door to "supposed" experts that can ruin a job and/or parts you may be working on through half baked and ignorant suggestions. You see that here and especially on Facebook in the Pontiac groups. If your goal is just to get a carb or whatever part to work properly, no issues with your point. However, if you have a rare $6000 original RAIV 273 Qjet and it gets screwed up, there is plenty of worries man!!! You just can't "build" another original as you mentioned. It all depends on what you want done. When you own a rare car and the parts on it are difficult or almost impossible to replace, you normally are a bit selective on whom you want working on them. That is my logic anyway and Cliff R. fit my requirements.

This is exactly my point. Half backed ignorant suggestions to lost souls that don't know any better. The internet is full of them so beware!!!

...and that was why I was remiss when Cliff left - and I'd hope he wouldn't do that again.