Seeking carburetor guidance, vacuum vs mechanical secondary

HFolb23

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Looking for some carburetor guidance.

I have an 81 C30 with a 454 and turbo 400. The truck does have a cam but I am unsure of what other motor work is done to it. To quote my mechanic “it does have a pretty heavy cam in there”. The truck has an edelbrock carb on it now and my mechanic suggested we change it begin solving a low idle issue where the truck wants to stall on takeoff.

My mechanic is suggesting a Holley 750 with mechanical secondary. While researching it seems the mechanic is correct that the mechanical secondary’s are superior, however they’re more fussy and need to be carefully selected and tuned to match the motor. I see the vacuum secondary’s are more forgiving and universal especially on a heavyweight vehicle. Am I correct in thinking that a vacuum secondary may be a better option given the unknowns of the motor work?

I know I need a manual choke and dual inlet, it’s the secondary type that I am unsure about. This truck is not ever going to be a race truck. I want a nice fun cruiser where reliability is the most important thing. I don’t want to have to worry about taking the truck somewhere and it dying at an intersection or not starting again when we get where we’re going. I appreciate any and all advice.
 

AuroraGirl

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Looking for some carburetor guidance.

I have an 81 C30 with a 454 and turbo 400. The truck does have a cam but I am unsure of what other motor work is done to it. To quote my mechanic “it does have a pretty heavy cam in there”. The truck has an edelbrock carb on it now and my mechanic suggested we change it begin solving a low idle issue where the truck wants to stall on takeoff.

My mechanic is suggesting a Holley 750 with mechanical secondary. While researching it seems the mechanic is correct that the mechanical secondary’s are superior, however they’re more fussy and need to be carefully selected and tuned to match the motor. I see the vacuum secondary’s are more forgiving and universal especially on a heavyweight vehicle. Am I correct in thinking that a vacuum secondary may be a better option given the unknowns of the motor work?

I know I need a manual choke and dual inlet, it’s the secondary type that I am unsure about. This truck is not ever going to be a race truck. I want a nice fun cruiser where reliability is the most important thing. I don’t want to have to worry about taking the truck somewhere and it dying at an intersection or not starting again when we get where we’re going. I appreciate any and all advice.
vacuum Is superior
By definition a qjet would be able to do what you want better

Your 454 probably would like a edelbrock with Powerblaster to aid in fuel atomization, and you are at sea level right?
Is your edelbrock adjusted right? They have a whole thing about that

Hows your timing? Regulator on your fuel pressure?

Why do you want a manual choke? Do you have one now?
 

85K304SPD

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I would say that the Edelbrock carb should be good, if tuned and jetted properly. You aren't going to gain anything by going to a Holley for a daily driver. You might have more problems with the Holley. A double pumper will use more gas than a vacuum secondary. Run a return line and filter to avoid vapor lock. There is a reason that the GM engineers chose the Quadrajet. That might be your best bet for dependability.
 

Bextreme04

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Looking for some carburetor guidance.

I have an 81 C30 with a 454 and turbo 400. The truck does have a cam but I am unsure of what other motor work is done to it. To quote my mechanic “it does have a pretty heavy cam in there”. The truck has an edelbrock carb on it now and my mechanic suggested we change it begin solving a low idle issue where the truck wants to stall on takeoff.

My mechanic is suggesting a Holley 750 with mechanical secondary. While researching it seems the mechanic is correct that the mechanical secondary’s are superior, however they’re more fussy and need to be carefully selected and tuned to match the motor. I see the vacuum secondary’s are more forgiving and universal especially on a heavyweight vehicle. Am I correct in thinking that a vacuum secondary may be a better option given the unknowns of the motor work?

I know I need a manual choke and dual inlet, it’s the secondary type that I am unsure about. This truck is not ever going to be a race truck. I want a nice fun cruiser where reliability is the most important thing. I don’t want to have to worry about taking the truck somewhere and it dying at an intersection or not starting again when we get where we’re going. I appreciate any and all advice.
If you are going to be racing and reving it out to 7k RPM, then sure 750 double pumper with vacuum secondary would be right for that truck. If you are driving it on the street and not reving it above 5k RPM, its too much carburetor. The mechanical secondary is only needed if you are drag racing and have a big cam that isn't producing much vacuum. Manual choke is also really just a personal preference... to me it's not great for a street daily driven vehicle.

I have a holley 4160, single feed, vacuum secondary, electric choke 650cfm unit on my Buick 455. Its a 1970 high compression 455 from a 1970 Riviera, so it's WAY more peppy than just about any truck 454 is going to be, and it has no issues with that carb.
 

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My take is that your mechanic does not want to spend the time to properly tune your carb for you.

I am about to put a Holley 570 Street Avenger onto my 454 with a mild RV cam in it as my Q-jet has significant wear on the throttle shaft bushings.
It should be plenty for this version of the engine.

When we put a bigger cam in it and better heads, then we will step up to a Holley 750 with vacuum secondaries. It should be just spiffy then.

But with either option, we will be going through the efforts of tuning the carb to what the engine wants so it will be happy.

I would also strongly recommend getting an Air Fuel Gauge for the truck to aid in the tuning process. It will let you "see" what is going on with the engine and you can tune from there.
 

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Keep in mind a heavy vehicle with a large cam and big carb is going to eat fuel, A LOT of it and also have poor power at lower RPM's. You didn't say what the intended use was. A mechanical secondary carb would be my last choice for that application.

Really sounds like the motor was built by someone who was uninformed.

Here's my suggestion on a heavy vehicle like a 1 ton. First, get big block heads with the small oval ports [and intake] they flow better at low RPM. GM knows what works, all the big block 1 tons our company owned were that style head. Then a very mild [if not stock] cam. Top it with a Q-jet or smaller CFM Holley or Carter.

Economy isn't ever going to be that great but with what you have would surprise me if it got 5 or 6 MPG.

One thing most people aren't aware about a Q-jet. They flow around 750 CFM. Reason they work so well and have decent economy is the very small primary venturis.
 

HFolb23

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I appreciate everyone’s input so far, great advice and I knew I should’ve asked here first.

The truck is a toy. It’s not a daily driver. It’ll never do any work, it’ll never tow or haul anything, and as for MPGs I have absolutely zero expectations as for good fuel mileage. I really only intend to take it for a fun drive on the weekends. We’re after smiles per gallon with this truck, green crowd be damned.

I live at around 1000ft above sea level and have a camp around 1800 feet.

My mechanic feels that my issues are a combination of a possibly bad carb, the major exhaust leak, and possibly needing to adjust timing. To his credit, he really hasn’t gotten to dig deep into the truck doing much diagnostics yet other than a test drive as I had hired him to do other work and had him give his opinion while he had the truck.

The truck has a manual choke on it now, and while I don’t plan on using it I’d like to have the option since the cable is already ran.

I have no idea if the carb is adjusted right. I would suspect it’s not because of how the truck wants to stumble, but it also drove 50 miles without incident to my house as well when the seller delivered it to me. I have contact information for a local guy who does the carbs for all the local modified and dirt track racers. For what he wants to rebuild and tune the edelbrock carb I could put a fair amount towards putting a new one in there. I’m not knocking the guys price at all, you pay for the knowledge, it just seems overkill to have a race car carburetor specialist build a carb for a truck that’ll only ever put around getting ice cream.

I will crawl around and try to figure out exactly what im dealing with under the hood with this truck. I’ll see if I can pull numbers off the block and I’ll check out the heads but when I bought the header gasket I did notice they were small round exhaust ports because I mistakenly bought the square port gasket.

The Q Jet seems like possibly the best option here.
 

Matt69olds

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It sounds like whoever is working on the truck wants you to pay for his lack of knowledge or ambition. With some reading comprehension and basic troubleshooting skills, you should be able to improve the way the truck runs with what is there.


It’s hard to beat a well tuned Q-Jet on the street.

This is a perfect example of how a universal aftermarket carb is “universally” wrong for 99% of the applications they are sold for.

Aftermarket carbs are calibrated rich. They have to be. The manufacturer has no idea what engine the carb will be installed on, or what modifications have been made.

Let’s say you have a completely stock small block engine. You buy a aftermarket carb calibrated for a mild engine. You bolt it on, set the idle speed, the as supplied calibration works well for that mild engine. Everyone is happy, win-win!


Now, let’s say you bolt that same carb on a big block with a pretty aggressive cam, headers, heavy vehicle, etc. The engine isn’t going to be happy, in fact with the mild calibration the engine could suffer damage from running lean. Nobody wants that.

To be safe, the carb manufacturers have to assume the worst. It’s up to the installer to make the needed changes. Which brings me to my pet peeve, the same money spent on buying a aftermarket carb would be better spent on PROPERLY rebuilding the Q-Jet. The time spent fiddling with tuning a Holley or Edelbrock carb is time that could have been used on the factory carb.

If I was building a all out race car with zero concern for drivability or economy, I’d probably use a Holley. But since most of us drive on the street, and don’t have unlimited fuel funds, the more efficient carb is the better choice.

Lastly, there is ZERO reason a street driven, unloaded truck should get 6mpg. If it does, either it’s leaking fuel, the odometer is off, or the engine way out of tune.
 

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HFolb23

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I just went out and checked out the truck. I didn’t climb under the truck to get a block number as it’s pouring rain, but the carburetor on it is an Edelbrock 1407 9238.

Reading online it seems it’s already a 750 cfm carb, and it’s an air valve secondary which until just now I didn’t even know existed.
 

85K304SPD

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You can do it yourself, since its just a toy. Change the plugs, wires, cap and rotor. Take off the Edelbrock and take the top of it and clean it out, re install with a new base gasket. Put a gasket on the exhaust. Set the timing and then see how it runs. You may not need to change the carburetor. You need a good baseline before you start changing things that worked well, previously. Just my opinion.
 

HFolb23

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You can do it yourself, since its just a toy. Change the plugs, wires, cap and rotor. Take off the Edelbrock and take the top of it and clean it out, re install with a new base gasket. Put a gasket on the exhaust. Set the timing and then see how it runs. You may not need to change the carburetor. You need a good baseline before you start changing things that worked well, previously. Just my opinion.
Distributor and wires are new. I’ll check the plugs tomorrow before work. My exhaust leak is a crack in the collectors, so I’ll get the new headers and gaskets ordered now.

I’ve never done anything with timing before, I guess there’s a first for everything though.
 

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do yourself a favor and get calibration kit from edelbrock sounds like uneed to play with the metering rods and springs
 

HFolb23

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Plugs look good, headers showed up yesterday.

I actually found a package of Edelbrock 1464 step-up springs inside the truck today as well. The pack is opened but I located all ten springs. Makes me slightly concerned that someone may have bought the step-up springs, tried all of them and then realized that didn’t fix their issue.

Before I send this carb off to be tuned by someone who knows what they’re doing, is it even worthwhile? Being I already have a 750 cfm carb, is it too much carb for my application? Being it’s an air valve carb, should I use it or go with a vacuum secondary or the Q Jet?

Should I have the new headers installed before tuning the carb? Do I adjust timing first?
 

hirschdalechevy

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First off it would be huge if you knew the compression and cam spec's , but anyhow since your cap, rotor, plugs, wire's are new you should be good there but as far as carb. tuning goes you need to have the timing right first before you even think about the carb. A low compression 454 with stout cam will like a lot of initial timing and get it all in by 3,000 rpm. You can also play with vacuum cans on full vacuum to get more initial timing without hard starting. You will have to play with timing to give the motor what it wants without pinging.

Next is the carb. game, everybody has a different view on which carb. is the way to go, (like chevy, ford, dodge,) You have a eddie 750 right now and it will work just fine on your 454 if its tuned right. The only reason I would change carbs would be if you just wanted to try something new. As far as mileage goes, yes, the q-jet can get a bit better mileage because if the small primaries and they are great carbs. for mild motors but a holley or an eddie carb. tuned right on an afr gauge can get real close to the same as the q-jet. So a good carb. guy should get you there.

I have a 357hp, stout cammed, smallblock in my 77 k-5 right now and I have been playing with carbs. on it, (i have a bunch of carbs. laying around). I tried, one vacuum secondary 600 holley, one 600 quick fuel slayer, one 625 demon, and I got them to all run well, (using an afr gauge). I was tuning for good power but a nice leaner cruise ratio for mileage, but that is hard to do because you run into tip in problems, but anyway right now I have a quick fuel brawler 650 double pumper on there and it blows the others away. Better power everywhere and I have a nice 13.8 afr at hwy. cruise for good mileage and tuned out all tip in issue's. I get 14 to 15 mpg. on the hwy. all the time going out to the deep dessert.

So basically you can get any carb. to work well and get good mileage but alway's start with timing first and no carbs. out of the box will be perfect no matter which brand.

I forgot, to get good mileage you have to stay out of the secondary's while cruising as well with any carb.
 
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Matt69olds

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I’m not real familiar with edelbrock carbs. I’m sure with a little patience you can improve the performance.

Before doing anything, contact Edelbrock and find out exactly what metering rod, jets, power springs, etc are suppose to be in the carb. Get it back to factory specs before attempting to tune it. Since you found springs in the truck, who knows what has been changed. Get it back to stock so you have a good starting point.

The 1464 spring is for the power piston (think power valve on a Holley). You need to get the jetting right before messing with those springs.

You need to get the metering rod tuning kit for your carb.


Assuming you have the ignition timing set up and dialed in for your truck, pick a level road on a typical weather day. Get the truck fully warmed up, take it for a drive. Then install a set of metering rods a little leaner than what’s in the carb now, drive it again. Keep going leaner until you get a lean miss, it will kinda feel like a fish nibbling on the bait. Once you have found the lean limit, go a couple steps richer on the metering rods.

The metering rods are pushed up out of the jet by the power piston spring (the spring kit you found in the truck). The metering rods are pulled down into the jet by engine vacuum. The heavier the spring, the more vacuum is needed to hold the rods down.

Once you have the mixture right, you can play with the power springs. Get the truck back on your test road at highway speed. Gently accelerate while paying attention to how the truck takes the throttle. If the engine is lazy taking the throttle, you need to install a lighter spring. Ideally the truck will take light acceleration with zero hesitation. You may need to go lighter or heavier on the springs to find the happy place.
 

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