kips70cutty

Needing carburetor replacement for my 77 403. Curious as to the airflow needed for street usage. Has dual exhaust, otherwise stock. Currently running an Edelbrock 1904 that is rated at 795 CFM. Was there when I bought it, so don't know any different. Not looking to race, just something that will run well when demanded with decent economy when not... Thanks for your input.

Octania

I would recommend the 800 cfm QJet from a 77-79 Olds 403 engine.

olds 307 and 403

Since that is Qjet get a kit and see what jets and rods are in it. My 403 carb had 44K primary rods with 73 jets. I believe CV secondary rods with an I hanger.

oldcutlass

What intake manifold is on there? Does the car run poorly with the current carb? A rebuild kit for that carb is relatively inexpensive.

MDchanic

You're looking for a post I made last week.
Just plug in 403 in place of 350:

"350 cubic inches = 0.2025 cubic feet
Since your 350 is a 4-cycle engine, it requires 2 revolutions to cycle all of its cylinders, so
0.2025 cubic feet ÷ 2 = 0.1013 cubic feet per revolution.
0.1013 x 5,000 RPM (probably a bit higher redline than reality, but I'll use it anyway) = 506 CFM
506 CFM x (very roughly) 80% efficiency = 405 CFM the carburetor air flow you need."

If you do the math, you will find that you need 420-466CFM depending on whether you rev to 4,500 or 5,000 RPM.

- Eric

CutlassDad

MDchanic's calculation is right on.

You have to ask yourself the question how much time will the engine spend at your chosen high RPM?

Bigger (CFM) is not better!

Gerry

kips70cutty

To answer a few questions, it has the stock intake, my garage guy that does a lot of work on my business vehicles told me that the shaft bushing is worn and leaking. The Edelbrock carb rebuild kit does not include bushings because they are not serviceable. Mechanic said they would have to drill out and replace bushing and by that time I could buy a new one cheaper. It is not a complete dog, but does start really hard and stumbles on starts at the stop sign. I figure the bowls are leaking down and need sealed and the floats need to be replaced. I suppose I could attempt the drilling out, but don't want to end up with a paperweight if buying a new one will take care of the issues. Edelbrock's run $350-400 and I did find a place on line that has rebuilt qjet's for around $200.

oldcutlass

With a stock intake I would run a Quadrajet. Then you can get rid of the spacer.

olds 307 and 403

Being your car is a cruiser get a reputable place to build you a Qjet or get a Street Demon carb. Don't bother with an Edelbrock, old and inefficient with hot start issues. The Street Demon is based on the Carter Thermoquad with all new 3 barrel castings, main body in either aluminum or aerospace polymer and the problem areas fixed. They are around the same price as the Edelbrock carbs.

cutlassefi

Quick Fuel 750 "Slayer". $299.00 all day, electric choke and extremely tunable.

olds 307 and 403

Mark, how rich out of box are those carbs? We are talking a 7.8 to 1 403 with a peanut cam. They do seem like great carbs for the money. I just don't see them being as efficient as spreadbore cruising around with a sophisticated metering system like the Qjet or Street Demon.

cutlassefi

Don't assume, that'll get you in trouble!
Read this. I've found the same thing to be true on these as well.
http://www.carcraft.com/techarticles...retors_tested/

Imo the Slayer is the best deal out there, hands down.

olds 307 and 403

It sounds like it would be a good cruiser carb, either it or the Summit carbs sound decent for the square bore carbs, dollar for dollar. I was considering a wideband for tuning, still might pull the trigger. Of course I zero Holley tuning parts, so that adds to the cost. I am surprised the Qjet was running so rich but also wasn't blown away by the other carbs in power made during testing. I just remember how the Holley on my Sister's truck ran, great power but was very temperamental. I want something that runs well at freezing and in the summer heat without messing with it just to get it to start and idle.

grampy

The stock 403 quadrajet is the best way to go. I have run modfide 403 carbs on warmed over 455. for a street carb part throttle responds is better then any other brand of carburetor.if you spend 98 percent of time at part throttle on a street car why put up with a lazy part throttle on a aftermarket carb.

cutlassefi

It would be of help to a lot of people here if you could qualify your findings. This one did this, that one did that etc.

Thank you.

oldcutlass

While the slayer series is a good carb, his stock manifold is set up for a spread bore carb only. Please note, he would have to put the adaptor back on.

J-(Chicago)

I have an 800 cfm qjet here if you decide to go that route. It has a brass float and the accelerator pump has been swapped for ethanol blended gasoline.
I tried using it on a 350, but it was way too rich. I think someone modified it for a 455 buick.

captjim

I had Holley's on several Olds and a Buick 350, they were far from lazy, very responsive with decent mileage. A carb is a metering device, it does not matter which brand is used, if it is tuned incorrectly, it will not run very good.

I also think those calculators work in theory, but not in the real world. It has been my personal experience that these engines "like" big carbs. IMHO.

grampy

On stock engine the small primary's and triple booster of quadrajet is more sensitive to air flow than any Holley's larger primary's and single booster period.smog quadrajets are a little lean but fatting the idle restrictions a little and up jet a size or two will make quads run like fuel injection. If anyone has done before and after on a retune carbs the quads part throttle always drive nicer.

captjim

You really should take a little more time reading, I NEVER said it was "better" than a Q-jet, I said it ran good, and was not lazy. Q-jets are fine carbs, but I disagree with the "fuel injection" statement, I have heard that before, it is nonsense. A well tuned Q-jet is a pleasure to drive, but so are other brands which are MUCH easier to tune and easy to procure. That jet change alone is no simple task, not to mention the myriad combinations of rods and hangers on the secondaries. IMHO.

grampy

As for experience on carburetors I have been a mechanic since the 1970s when every car had one.I have worked on thousands of car and trucks.I remember when cars had point and condensers single barral carburators ,three or less vacume line on the engine,single master cly,no power anything.

captjim

That is fine, so have I ( I remember when CCC systems came out, we all went crazy) but what exactly has that got to do with anything? It does not make your statements (or mine) more or less accurate.

grampy

Hey let's lighten things up .Chrysler lean burn electronics and ccc quadrajet in the early 1980 put a gun to my head . I still get nightmares! I am not trying to bust some *****. I just give my opinion take it or leave it.most carburetor problems are dirt first ,wore out bushings,malfunctioning acceration pump and last poor rebuild kits. Are the average problem with carburetors in stock engine repairs.

captjim

I agree. The worst American car ever made was the 78 Cordoba.

cutlassefi

You're kidding right? Isn't the Qjet one of the carbs that ultimately needed electronic assistance in order to meet emission and fuel economy requirements? Then only to be Replaced by EFI? And now you're going to tell us your Qjet is as good as EFI?! Really? Then you're comparing it to a really crappy EFI.

grampy

See it easy to get along .I hope the guy that posted this don't get upset that things got off track. It is good to get a wide of opinions and make your choices that you can feel is the right one for you.trust me the internet has enlighten my mind to think outside the box.there is some sharp guys on here that have merit in their opinions.even if you have your douts, that why every one thinks the earth is flat right! Lol!

grampy

Cutless efi arrogance has no place when your lively hood is hand and hand it is how you make your living .let me give a a wise man quote don't sh_t where you eat. It will eventually leave a bad taste in your mouth.

cutlassefi

I'm not the one that makes unsubstantiated claims. You get all bent out of shape the second someone challenges your statements yet you seldom back them up. I asked you politely to give examples of your different experiences with the various carbs that you claimed you've tuned etc. I think that was a fair question but so far you've dodged that like a bullit.

Thank you.

grampy

My years of experience in the automotive feild is my proof.here is a fact most average joes buys a carb and just slaps it on and goes,they don't have a clue how to tune it .aftermarket carburetor have a generic tune so they will work on any displacement.they are always a little fat and that make them a little lazy as I call it throttle response is not as crips as it should be.on a mild set up I say stick with the stock quadrajet it is set up for the engine. There set a little lean for emissions.but ideally a bump in jet size or two will get it in the ballpark.quads with apt can fine tune things.adjusting the choke pull off timeing and secondary wrap spring pressure will make for a almost flawless transition opening of the secondary. Thus this is where I make the claim of like fuel injection you just have to drive one to feel it.on many stock engine quadrajet of the early 1970 drove great.the only drawback was choke warmup was not as good as fuel injection but when warmed up they were great.this post started out as a guy with mostly stock car whats every one opinion on carburetor .my opinion was and still is there is nothing to be gained there is nothing wrong with the stock quadrajet just rebuild it and tune it .put in a 74 or 75 jet and use the apt to dial the primary's in ,the CV rod will work. Just ajust the choke pull off timing and secondary spring tension for no bog or sag.(looking back now he has a aftermarket quadrajet ) keep it just tune it might be a little fat now try down jetting and ajust the apt for best throttle responce.

grampy

Here is another experience I have seen. Guy with a mostly stock engine race his car at the track get some numbers.he want to upgrade go faster he buy a new aftermarket caburator put it on take it back to the track expecting big gains but it ET and mph is about same or worse. Even if the guy new how to tune it I have never picked up more than two Mph on a reject tune on a mild build anyway.ET is more of a traction issue.I will also say if you had a pile of aftermarket carbs of the same size and ran them one car the mph would be real close to each other.the real difference in them is idle quality, part throttle responds and secondary transitions.some high end carbs have more adjustments air bleeds and things. if you don't know how they work and make the adjustment and just slap it on and go where is the improvement.BTW 90% of the time you run down the track the carb is wide open if your overall air fule ratio is correct.the real improvement is tuneing the first 10% which is primary's, pump shot and secondary transition.

cutlassefi

Some of the other stuff is correct but your attention to detail in the highlighted area is second to none. Wow.
Anybody can write what you did there. So why would we believe you more than the next guy? Your posts unfortunately typically don't convey the claims of your vast knowledge. So just a suggestion, maybe learn how to spell, punctuate and describe your findings/experiences with a bit more detail and less opinion. I for one would certainly take them more seriously.

Thank you.

grampy

You are just a salesman with a big ego .I really don't know why you got it for me .I really don't care.who appointed you the final word . i guess someone that say work with what you got you wont be able to sell him somthing is a threat to you.sure you sell a few camshafts .efi are self tune now even a monkey can do it ( the guy who wrote the software is the smart one). I bet you take your own car to the dealer because you don't have skills to fix it yourself.if some one ask me to build a engine with a unlimited budget I could assemble one that make good power. New parts have the engineering behind them ,its not the guy assemble them.you sell the parts not engineer them.

MDchanic

Grampy, even though Mark may have handed you a shovel, you don't have to keep digging.

Mark builds engines, he builds them according to experience and sound principles, and they run well after he does.
He also sells camshafts and EFI, and gives excellent (and free) advice about them, whether you buy from him or not.
He also is capable of consistently producing clear, comprehensible sentences and posts.

It looks like you have good experience with tuning engines. The best way to communicate that to the members here is to clearly and carefully explain what you mean, and to speak in specific terms, and not in generalities... And also to not attack members here who have been members for a long time, and have helped many other members, and have proven through their actions that they are worthy of respect.

Why not take a breather and go at this differently?

- Eric

cutlassefi

Eric, thank you very very much. I try to give honest and worthwhile posts. If others don't see that then unfortunately there's not much I can do about it.
I've been on the bad side of advice given by someone who claims this but is actually that. That's why I'm so opinionated on stuff like this.

Grampy- I've tried repeatedly to give you sound advice regarding your posts. You don't seem to grasp what I'm trying to tell you. I don't know how else to put it. The more you attack the messenger the worse YOU look. Sorry for your difficulties with this.

grampy

I never go out of my way to be the agresser but mark dose on anyone that don't share his opinions.. I guess i dont comucate well in my post and ramble on i am old give me a break.I not your average mechanic. I spent a few years working for a engineering firm as a prototype technician. Taking production trucks striping the engines to short blocks retrofitting protype heads,intakes,turbos superchargers,alterative fuel systems. That had data logging sensors in them sometime up to 100 channels of thermocouple ,pressure transducer and oxygen sensor data. this was some precise work my tools had to be calibrated every 15 days.every thing about the build was documented and verified . I am no sloch as a engine builder/ assembler.anyone could achieve the the results mark get on his builds on there own .he just has working relation with venders and outsourcing to fill his needs. I think he just change dyno operators ,a good operated that has extensive experience is invauleable if you got the money they can dial everything in for you all you need to bring is your checkbook. I agree mark knows a lot ,if you spend some time with these guys you will learn a lot.I am old school we did not use dynos have wideband sensors.we use commsence and trail and error.reading plugs using are ears ,eyes and running road test .this is not fast or easy but you can get good results.I post here for the average Joe that want to do it them self without a big budget.