The Stovebolt Site Forums The Shop Area The HiPo Shop Vacuum/ Carburetor Selection

I’m running a 283 , presumably rebuilt. I have no access to the history of the engine. I certain, by the running characteristics, that the cam is non-stock.
The carburetor is an Edelbrock 1405 , 4bbl , 600 cfm.
Manifold is Edelbrock dual plane Performer.
Vacuum is 13” , well below optimum. (All vacuum lines , brake booster etc. are new and intact) I attribute the low vacuum to the camshaft. Engine performance is excellent in acceleration or cruising. The only fault is that it’s twitchy at idle speed; frequently quits abruptly when shifting neutral to gear. I do have a new 2500 rpm torque converter; the reduced the problem but did not eliminate it. (that converter sure did improve low end acceleration…)

I’m thinking that there is a relationship between the low vacuum and the unreliability at idle rpm.
My reading suggests that the normal max cfm for a 283 is 480 - 500. My carb is 600 cfm. Is it possible that I would benefit from a 500 cfm carburetor? To put the question in other words: would a 500 cfm carb “cope” with the low vacuum better than the one I’m currently running?

My horticulture degree did not prepare me very well in this area.

Standing by,
CK

You will probably get more of a benefit from advancing the ignition timing at idle a little, rather than messing around with swapping carburetors. Just one problem- - - -for every degree you add to the base timing, you'll need to subtract a degree from the centrifugal advance. That's done by limiting the distance the advance weights are able to travel- - - -NOT by installing stiffer springs. Drop me a PM and I'll be glad to walk you through how to modify the distributor by email and/or telephone. The procedure is a bit complicated to describe with a keyboard. On engines with enough of a cam to give a lumpy idle, I've found that around 15 degrees of initial advance or a little more is usually necessary for good off-idle performance. That also makes a noticeable increase in idle manifold vacuum.
Jerry

Hi Waveski, I can't remember if that Edelbrock carb is a spread bore or square bore, I would think the idle should be ok with each option. At idle, only two of the carburetor barrels are in play, those being the two front barrels. Do you have a tachometer and what is your idle speed at 13 inches, is that in drive or park? You could disconnect and plug all of the vacuum ports to confirm that you have no vacuum leaks through any of the components. My 283, 2 barrel manual trans idles at roughly 550-600 RPM with 17 inches of vacuum, the cam is a stock lift/duration with smooth idle. Typically, when you have a cam with a lot of overlap and/or long duration, it will idle rough and will need a higher curb idle speed to stay running. My guess is that whoever built the engine thought it was a race truck and the manifold/cam combo was geared for HIGH rpm use, the manifold certainly was not a good choice for daily driver/ street use. The Performer is a nice street manifold, the carburetor should be ok with it, the cam could be way overkill. If your buddy still has the Quadra-Jet carb, it would be a good experiment to install it and see what the results are, the Q-jet has very small primary barrels and is an excellent street/economy carb, the small primaries result in high velocity and great fuel atomization at idle. The Q-Jet will bolt on directly to the Performer manifold due to it being drilled for square bore and spread bore carbs. Edit-try HRL suggestions first. we were typing at the same time.

The cam I have in my 283 came out of a 355 that I built for my 1984 crew cab 3/4 ton years ago. I didn't like the characteristics of the cam in that application and pulled it. I installed a Performer manifold and Performer cam/lifters/chain/sprockets, still running the 800 cfm Q-jet (original to the 454) and 3 speed automatic, 3.73 rear end, it was an amazing difference in operation.

The advertisement for the 1405 calls it a square bore carburetor, the picture looks like the primaries may be slightly smaller than the secondaries. If the carb is in good condition I would think you would be able to make it idle and run ok with a street cam, try the timing first.

I’m around 850 rpm idle in neutral, set it up there to mitigate the problem.
Buckshot- why do you feel that the manifold is a bad choice?
I should state that I installed that manifold , in place of a single plane high rise.

On another note , while keeping in mind what HRL said, and knowing that this has been hashed out here before, would those smaller primaries of a QJet possibility work better in my situation?

No, it won't make any difference, the cam has to much over lap. Change the timing and see how it acts before messing in the distributor, drive it and see if it detonates under light load and heavy load. Keep increasing the timing if the vacuuum improves, but keep driving to check for detonation. You may find no other action is required with a few degrees of timing increase. Also move the vacuum advance to full time vacuum if it's not now, that will help also. You don't want so much timing it get hots or detonates, so you will just have make a change and test it, keep the timing light in the truck along with the wrench required to move the distributor so you make changes as needed. If you find the vacuum increased but it detonates, then limiting the mechanical and/or vacuum advance may be needed, or better grade fuel .

If you don't mind lifter noise, Rhoads Variable lifters will solve the problem, they bleed off at low rpm creating a much smaller cam, thus increasing vacuum. Around 2000 rpm ( depends on valve spring pressure ) they pump up and work as normal lifter do. You get better low rpm torque and vacuum, but still retain the higher rpm preformace. I run them in my 250, it sounds like a mechanical cam with lifter clatter. They also decrease the load on the cam lobe at idle where oil splase is the lowest, so oil choice is't as crucial. My 250 will idle in gear around 19" of vacuum.

The cam over lap and piston ring seal determines amount of vacuum, carburetor size has nothing to do with it. If you scale down the carburetor size enough to increase vacuum, you will be running a very rich mixture.

Carburetor size matters at full rpm.

Very convincing argument Joe, thank you.

The single plane manifold was a bad choice for street use, the Performer dual plane is great! My long post probably was not very clear.

Good, thanks.

The two most common mistakes street engine builders make is "Too much cam" and "Too much carburetor". Any cam with an "advertised" duration of more than 260 or so will probably produce low manifold vacuum, a lumpy idle, and less low end and midrange torque than a "stock" cam. I've proved that on the dyno more times than I can count. Where those long duration/high lift cams shine is at RPM ranges few street engines ever see. About the hottest cam I would recommend for the engines my high school students wanted to build was a Comp Cams 268H. Along with a fairly low rear end gear, and a slightly higher than stock stall speed torque converter, some of my kids became kings of the neighborhood stoplight drag race scene- - - -and they had the speeding tickets to prove it!
Jerry

In the course of weighing my options I should explore the possibility of pulling the cam without pulling the engine.
It might be possible if I disassemble the ‘54 grille.

Or, I could just exhaust all timing options and live with the results.

Boy - old trucks are … fun …..

What type of distributor are you running? One of the offshore HEI "clones" would make a good test bed to tinker with the advance curve, at a fairly low cost, and without messing with the original one you have.
Jerry

If I recall correctly, my distributor is an older Delco Remy with a new cap I put on last winter.

While I certainly agree with the ignition comments, an engine is composed of many parts, and the performance of the engine is a result of how well each of the parts like its neighbor!

Consider this FACT:

When Mr. Duntov built a race only 283 CID engine for the 1957 Corvette, he chose a single Carter type WCFB, number 2493s.

The Carter 2493s was rated at 380 CFM !!! And you have 600 ?????

Even the 2493s would probably require some modification to work well with your specific engine.

Jon

Aha -
A voice for carb reduction!

Care to elaborate?

I guess not.

This is what is going on: https://www.stovebolt.com/ubbthreads/ubbthreads.php/topics/1570224/the-carburetor-shop#Post1570224

I completely agree with Jon, who, BTW, is recovering from major heart surgery. A 600 CFM carb is WAY too big for that engine. To calculate the theoretical (not actual) CFM of an engine, use a very simple formula- - - - -divide the cubic inch displacement by 2, and multiply that figure by the maximum RPM you plan to run. Then divide that figure by 1728 (cubic inches in a cubic foot). Since most street engines struggle to achieve more than 90% of the theoretical airflow, multiply the theoretical CFM by 0.9. Now you've got a realistic baseline to shoot for when figuring out what size carb to use.

Example: 283 divided by 2 = 141.5 cubic inches. 141.5 X 6000 = 849,000 cubic inches.

849,000 divided by 1728 = 491.3 CFM- - - - -at 6K RPM! 90% of that is 442 CFM.

Why would you need any carb bigger that 500 CFM capacity for street use where high RPM is seldom, if ever a consideration? Or maybe you just really love getting speeding tickets? I'd suggest using a Quadrajet calibrated for a 305 Chevy V8, or one for a 307 Oldsmobile engine. I believe there was also a 260 Olds with a QJ carb. (Nope- - - -all the 260s had DualJet 2 barrels) Both carbs have very tiny primary venturis with triple-stack construction to give extremely efficient low speed performance and HUGE secondary barrels to give unlimited high speed capabilities.
Jerry

Jerry - very well put, but there is more.

Waveski - shaking (getting better) now has me as a "two finger typist" and not a very good one. Sorry for delay.

Would be happy to explain more if you wish to call: 573-392-7378 (9-12, 1-4 Mon-Tues central time non-holidays).

Jon

Did the little Olds/ Buick aluminum V8s ever come equipped with a Q Jet?

They were gone after '63 and I think Qjets came along later. I think Rochesters were used.

The first QJ I can recall was on a 1966 Chevy 396. It was a dismal failure due to a really oddball needle valve with a rubber diaphragm attached to it. Those were mostly recalled due to massive flooding problems when the diaphragms ruptured.
Jerry

The initial production of the Rochester quadrajet was for the 1965 Chevrolet 396 engines.

These units had LOTS of issues, and richly deserved the "quadrajunk" moniker assigned to them.

The 1966 versions were better.

The 1967 versions were much better, and pretty good carbs.

The 1968 versions continued to improve.

When Rochester changed the float pin placement over the 1969~1971 model years (not all models changed), the carb became (opinion) excellent. For a street carb, (again opinion) second ONLY to the Carter thermoquad.

The 215 did, in fact, use Rochester 4-Jet carbs.

CAUTION MUST BE USED IN THE SEMANTICS OF THE ROCHESTER 4-BARRELS!!!!!

Early Rochester literature referred to the 4-G series (square-bore) as a quadrajet.

Later Rochester literature referred to these as a 4-Jet.

The model most associate with the term quadrajet are the later 4-M (spreadbore) series.

All (opinion) are GOOD carburetors. (Note there is NO Rochester 4-B series ; )

Jon

Welcome back Carb King!
I too am familiar with the inside of cardiac wards, great to hear that you are on the recovery side of the equation.

My basic reasoning tells me that smaller primaries may be beneficial to my situation. ( I’m back on piston engines now…). Some comments earlier in this thread made me think I was barking up the wrong tree. Carb Jon and others now lean back in the direction of QuadraJet.
The Quad does have much smaller primaries than my Edelbrock…

As to the question of why am I running a 600 cfm carb on my 283 , the answer is- it came with the truck. I did swap manifolds , as mentioned earlier.

That’s it for tonight.
Again- welcome back Carb King.

1) So the 215 came with a 4GC, not what we today commonly refer to as a Quadrajet? 4 barrel sizing for a Stovebolt is an oft-discussed topic here. Would these 215 4bbs be a decent candidate?

2) What was the smallest CFM Thermoquad?

The 4GC or 4-Jet was/is a square bore carburetor, square bore is defined as all four barrels/bores/venturis being the same diameter. The 4GC is basically the joining of two 2G's. The common notion of the Quadra-Jet is a spread bore carburetor, the primary venturis/bores/ barrels are considerably smaller than the secondary bores, these small primaries have the job of inducing fuel on start-up, idle, off-idle and mid/transition fuel delivery. The small primaries of the Q-jet have high velocity vacuum and do a good job of fuel atomization for the for-mentioned duties. The very large secondary bores of the spread-bore carburetor are designed to deliver a large volume of fuel and air when high horsepower is demanded. The Q-jet had a good 20+ year run and was still in use just prior to modern electronic fuel injection, it was GM/Rochester's best all-around carburetor for basically every GM vehicle with a V8. If you are thinking about experimenting with a 4 bbl., the Q-jet would be the easiest to acquire vs. the 4G, and would probably offer better low rpm benefits due to the small primaries, just stay out of the "go peddle" and secondaries.

JW:

(1) From a standpoint of size, - yes. From a standpoint of economics, - no (prized collector item, unique rebuilding kits).

(2) Short answer - 520
(2a) Longer answer (A) 520 (late smog), (B) 700 (earlier smog), (C) 800, (D) 850, (E) 1000

When looking at "replacement" units, unless one is independently wealthy, one should consider not only size, but market value, and replacement parts. The Rochester 4-G is very much in demand for those restoring, or racing, the 215. Result: higher than anticipated prices. One should also consider replacement parts (the accelerator pump in the 4-GC used on the 215 is unique, so the local FLAPS rebuilding kit isn't going to fit).

78Buckshot - the primary bores and the secondary bores are not always exactly equal (1 9/16 primary, 1 11/16 secondary is common) (1 7/16 primary, 1 9/16 secondary less common but exists).

Jon

I have gathered from reading other ancient threads (HAMB, mostly)…that there were a couple 400 CFM versions of the AFB.

Would those be relatively available and a more economical option that the 215-application 4GCs?

I suspect those small AFBs might be somewhat coveted for dual quad setups?

Jon, does that refer to the 4G, and does that also correlate to Waveski's Edelbrock? I don't have an Edelbrock to examine, the pics on the net look like the secondaries could be slightly bigger than the primaries. Almost all of my carburetor experience has been with the Q-Jet due to my 1967, '73, '74, '76, '77, '84 GM trucks were daily drivers, all had V8's with the Q-Jet. My '78 Vette still has the original Q-Jet. I did swap a Thermoquad on to my '67 for a short time since a Mopar buddy let me borrow it but that was back in 1972 and I was still in high school and learning! Aside from a couple of Holly's and installing a new Edelbrock on a friend's truck, it's been Rochester hands-on experience. Thank you for all of your knowledge, stay safe and heal up.

When considering a replacement 4-barrel, the FIRST item to consider is application! No, not whether the engine is a 235 or a 261 or, or, or.....

The application here is: (A) functionality on a street-driven vehicle; (B) race-only, (C) eye-candy.

If the application is (B), chances are good that the best SINGLE race-only (1/4 mile) carb is a 2-barrel, NOT a 4-barrel.

If the application is (C), then "beauty is in the eyes of the beholder".

If the application is (A), and the builder is looking for "best for he/she" carburetor, then lots of items should be considered.

Some items to be considered (the individual should prioritize the items, rather than having someone else prioritize them).

(A) Carburetor primaries applicable to "cruise" or normal driving RPM
(B) Carburetor secondaries plus primaries applicable to WOT
(C) Expertise of the builder (or the mechanic working for the builder) on the carburetor selected (it ain't gonna run right out of the box)!
(D) Availability of the selected unit
(E) Availability of replacement parts for the selected unit
(F) Cost of the selected unit
(G) Cost of replacement parts for the selected unit
(H) Ease of modification (this really is a sub-issue under (C) above).
(I) Availablity/price of intake manifold (one can always be fabricated)

Some have mentioned the Q-Jet as a good replacement for the stove-bolt; I respectfully disagree. I consider the Q-Jet to be the second-best carb (the first is the TQ) for FACTORY 4-barrel installations for street vehicles, so I am NOT throwing rocks at the Q-Jet. But tuning the secondary on a Q-Jet for use on a stovebolt is difficult, unless one has an excellent stock of rods and hangers (if one doesn't have that selection, that grumpy Missouri hillbilly has all of the hangers, and can fabricate all of the rods, or custom rods, for a price!). Yes, the Q-Jet has small primaries (most are 150 CFM), but I like to tune the primaries for cruise RPM, and then tune the secondaries for WOT

I would really suggest looking at a small square-bore carb. As an example, the Carter WCFB 2214s is a 295 CFM carb (147.5 primary / 147.5 secondary). So the primaries are roughly the same size as those of the Q-Jet. But the secondaries added then give a tunable addition for WOT.

The 2214s is simply one that would function well, there are others.

Jon

Buck - the square-bore with either the same size or slightly different sizes applies to all carburetor makes.

JW - there are a couple of aftermarket 400 CFM AFB carbs (9400s, 9410s) that as you suggested, are in GREAT demand (read EXPENSIVE) for 2x4 set-ups.
Additionally, there were a very few 380 CFM and 400 CFM AFB's produced for factory installation (one was the Sprint option on the 225 CID MoPar, another the 271 CID Barracuda). Most of these are so rare, I have never even seen one! As far as price is concerned on these, the last one I had brought $2500 several years ago. After the customer paid, he told me he had been looking for more than 3 years, and mine was the first one he had ever seen. He sent his private plane to the local airport and I turned over the carb to his personal pilot. Most folks do not wish to play in this ballpark, which is why I don't suggest these in my posts.

Take a look at (D) and (F) in my post above.

Jon

Most of my reasons for asking about the 4BB carbs are purely curiosity. But there’s another, slightly more practical, reason as well….

My horizon in this hobby is probably another 20-30 years (Lord willing). I am trying to think about what life might look like a decade or two down the road, and the availability of carbs/ tuning parts is one of those. That internal thought process encompasses certain aspect of all your points above, A through I.

The reason I tend think about 4bb carbs is they are common and ubiquitous enough to perhaps be BETTER supported for LONGER than some of our common 1BB options. Most “car guys”, outside of some VW and early Ford enthusiasts, probably picture a Holley or Edelbrock/AFB in their mind when someone says the word, “carburetor.” The largest remaining repository of tuning expertise and parts availability is around those two platforms, perhaps followed closely by the Stromberg/Holley 94 crowd.

Honestly, if I project myself 15 years into the future, I wonder if the 390 CFM Holley on a Clifford intake is the “best” (least bad??) all-around candidate to address MOST of points A through I. Still in production. Not particularly cheap but not outrageously expensive. Replacement parts as common as dirt. Probably weak spots in points “B and C” on your list. A little bit of a square peg in a round hole that would need a fair bit of tweaking to work optimally on a Stovebolt. But perhaps pretty sustainable once dialed in. Something I intend to experiment with, someday. I don’t love the Holley design, but there are certainly countless numbers of them still running up and down the road.

The WCFB is something I hadn’t given much thought to. How tunable would that platform be? Now and into the future?

Fuel Injection, anyone?

I don’t love the fuel injection idea, or the currently available options, but I don’t dismiss it out of hand for all the same reasons listed above.

We might have to get more comfortable with EFI concept as time goes on.

Just tuned back in here.
- I wish to point out that I have no interest in efi for my rest-mod rod 283.
- Regarding the discussion of the last 2 days, there has been occasional reference to carb options for Stovebolts.
Has there been a tangent of the discussion that has veered to stock Stovebolt engines , or have all comments been in regards to my 283?

Almost anything you can do with the Holley (EXCEPT go around corners at more than 1 g side forces) I can do with a Rochester or Carter on 40 percent LESS fuel! (well, close to 40 percent!)

Realistically - there are tuning parts available in more places for Holley than for Carter. Could that be because more Holleys than Carters need tuning parts more often??? LOL

How often will you need parts? The two Carters on my shop truck ran flawlessly with no intervention for 21 years until I decided I wanted factory air, and the shop installing the A/C had it for 21 months. During this time the floats stuck, and the carbs had to be cleaned.

So once you get it dialed in, maybe one or two rebuilding kits for the shelf???

No comment on efi

JW - call me some time if you are interested in discussing this, no obligation at all.

Jon

Not everything in the thread has been specifically focused on the V8's and not your 283. I can't remember if the GM Heritage site has any info for the 265 V8, I know that my 1957 283 was offered with the stock 2G two barrel and an optional 4G four barrel that raised the horsepower rating by 10 hp. Mine has the 2G with vacuum/centrifugal governor. I had thought about experimenting with a stock factory 4 bbl. manifold and Q-Jet to see if I could improve the off idle drivability and gain fuel economy, it wouldn't cost much since I have the parts. I like to keep things as original as possible so I haven't proceeded.

The 1957 Truck info kit has the specs on the 265 V8.

Here's an excerpt. Says it came with a Rochester carb for the trucks with carb numbers listed. Maybe the car info kit has different info.