Throttle and Bog Down Issue

[Hewfil1]

Hey again everyone, got another issue, so here it goes again. My truck's been running funky since day one, and it's time I fix this issue for good. Idle is fine, at speed is fine, but acceleration from a dead stop, she chokes and sputters, before finally responding. I HAVE cleaned the carburetor along with my fuel lines, tank, new filters, everything. Timing has been set, plug wires checked, new plugs installed, carb tuned, yet there's still and issue.

My truck is a 1974 Chevy K10, listed GMC conversion under the title. My engine is a 5.7L 350ci SBC. On it, it has a Weiand dual plane and Carter AFB 725CFM carburetor. Inside I have a set of Keith Black Hypereutectic pistons. Exhaust is Hedman headers with a three inch exhaust system(Don't quote me on this, they may be 3.5). The exact cam lift I do not know, nor the heads.

Any advice on how to fix this would be greatly appreciated.
(Side Note: I have been told that a 725CFM is to large for my size motor and that throttle response will suffer. Is this also true?)

[geezer#99]

Nothing wrong with the carb size.
But your carb doesn't like more than 5 psi fuel pressure.
Put a regulator on it.
Also, what's your timing curve like?

[Hewfil1]

Originally Posted by geezer#99

Nothing wrong with the carb size.
But your carb doesn't like more than 5 psi fuel pressure.
Put a regulator on it.
Also, what's your timing curve like?

So why does everyone keep telling me, "725CFM is to much for a 350 SBC!"

[geezer#99]

Originally Posted by Hewfil1

So why does everyone keep telling me, "725CFM is to much for a 350 SBC!"

Because they're clueless!!
They think it's too big because it runs rich. It run's rich because it doesn't like more than 5 psi and floods a bit.
A common quadrajet is 750 cfm.
I've run 750 holley's on several motors.
Even ran a holley 800 double pumper on a stock cammed, low compression single exhaust 350 for awhile.

[Hewfil1]

Originally Posted by geezer#99

Because they're clueless!!
They think it's too big because it runs rich. It run's rich because it doesn't like more than 5 psi and floods a bit.
A common quadrajet is 750 cfm.
I've run 750 holley's on several motors.
Even ran a holley 800 double pumper on a stock cammed, low compression single exhaust 350 for awhile.

So basically I should just mess with tuning, add a reg, then check timing?

[Hewfil1]

Also,ed off my carb and my engine setup, which has like a 14:1 compression, could you recommend me a regulator?

[geezer#99]

Originally Posted by Hewfil1

So basically I should just mess with tuning, add a reg, then check timing?

That's the plan!!

Here's one I've used. Holley makes similar ones.
https://www.summitracing.com/oh/part...2518/overview/

[Hewfil1]

Originally Posted by geezer#99

That's the plan!!

Here's one I've used. Holley makes similar ones.
https://www.summitracing.com/oh/part...2518/overview/

I think I'll pick up a Holley to match the other stuff I'm getting. And where do these mount to? The firewall? I've never run a reg, so any info you have would be great!

[geezer#99]

I ran mine right in front of the carb behind the altenator. Use one of the intake bolts for the mount.

[Hewfil1]

Originally Posted by geezer#99

I ran mine right in front of the carb behind the altenator. Use one of the intake bolts for the mount.

Alright, thanks! I'll be sure to post an update thread on it.

[rich weyand]

Originally Posted by Hewfil1

Hey again everyone, got another issue, so here it goes again. My truck's been running funky since day one, and it's time I fix this issue for good. Idle is fine, at speed is fine, but acceleration from a dead stop, she chokes and sputters, before finally responding. I HAVE cleaned the carburetor along with my fuel lines, tank, new filters, everything. Timing has been set, plug wires checked, new plugs installed, carb tuned, yet there's still and issue.

Did you replace the accelerator pump in the carb. It needs to shoot gas in when the throttle opens, because the air rushes in faster than the gas flow can accelerate, runs lean, and, above 17:1 or so, the mixture is too lean to burn. Accelerator pump for that carb is like 15 bucks.

You can check by looking down the bores of the carb with the engine not running. As long as the float bowl has gas in it, when you twist the throttle cam by hand, you should see gas shoot into the bores.

[Hewfil1]

Originally Posted by rich weyand

Did you replace the accelerator pump in the carb. It needs to shoot gas in when the throttle opens, because the air rushes in faster than the gas flow can accelerate, runs lean, and, above 17:1 or so, the mixture is too lean to burn. Accelerator pump for that carb is like 15 bucks.

You can check by looking down the bores of the carb with the engine not running. As long as the float bowl has gas in it, when you twist the throttle cam by hand, you should see gas shoot into the bores.

That's the only thing I haven't done. Because of my carb model. I don't know which pump or gasket kit it needs. I''l direct you to my thread about it here. I have $25 left on my debit card so I really want to get these parts asap.
(P.S.: Turns out it was a 625, I forgot this and had someone recently tell me I was running a 725. Sorry about that guys.)

[rich weyand]

This *may* be it. Carter had different sized pistons for different carbs.

Best would be if you could get a model number off the Carter (probably 9625), and called Summit technical support and asked them. They're very helpful and usually knowledgeable

https://www.summitracing.com/parts/aed-5338/overview/

[rich weyand]

Have you tried looking down the bore of the carb and seeing what kind of squirt you get?

[Hewfil1]

Originally Posted by rich weyand

Have you tried looking down the bore of the carb and seeing what kind of squirt you get?

I will be able to later this week as I'm currently out of town.

[Hewfil1]

Originally Posted by rich weyand

This *may* be it. Carter had different sized pistons for different carbs.

Best would be if you could get a model number off the Carter (probably 9625), and called Summit technical support and asked them. They're very helpful and usually knowledgeable

https://www.summitracing.com/parts/aed-5338/overview/

My model number is 9635 which according to another member on my post about identifying it, is physically the same as the 9625, thanks.

[Gregski]

Originally Posted by Hewfil1

(Side Note: I have been told that a 725CFM is to large for my size motor and that throttle response will suffer. Is this also true?)

The number one mistake is over carbing yet guys keep repeating that mistake, yes you can make even a 750 CFM carb work on a 350 small block but you can also drink shots out of a 5 gallon bucket, but why would you

seriuosly, there is a formula to calculate carb size, it's not gospel or etched in stone but it goes something like this

CID x RPM / 3456 = CFM so...

350 x 5500 / 3465 = 557 CFM

so even if we round that up that says a 600 CFM carb will do you just fine, now I aint trying to be a jerk I am honestly trying to help so keep in mind that this 557 CFM that is at pedal to the metal engine spinning at MAXIMUM RPM of 5500, hub caps flying off and red lines melting, ok in other words it's where you will be 1% of the time of your Truck usage

the other 90% of the time you are around 2000 RPM cruising or at part throttle so you don't need to flow no 600 CFM, the other 9% you may be at WOT wide ope throttle but you aint Red Lining, you are just gunning it for a bit to git on the freeway or pass a FORD, ha ha

see what I am sayin, get a modest properly sized carburetor and live happily ever after

[Gregski]

Originally Posted by geezer#99

Because they're clueless!!
They think it's too big because it runs rich. It run's rich because it doesn't like more than 5 psi and floods a bit.
A common quadrajet is 750 cfm.
I've run 750 holley's on several motors.
Even ran a holley 800 double pumper on a stock cammed, low compression single exhaust 350 for awhile.

apples and bowling balls bro, the Quadrajet is a spreadbore carb it has two small holes for the primaries so it acts like a 550 CFM carb 80% of the time that's why it's such a good carb design, then when its go time the larger secondaries open up and the Truck comes to life but only for a fraction of the driving experience, not all the time especially at cruise or part throttle

[rich weyand]

Originally Posted by Hewfil1

My model number is 9635 which according to another member on my post about identifying it, is physically the same as the 9625, thanks.

The difference is that the 9635 has connections for EGR.

Ask Summit tech support if that one I posted above is the right pump for the 9625/9635. They should know. I also think that is the most common Carter AFB still in use, so it is probably the right one.

The seals on those little pump pistons age-harden, and they wear, and then you get the symptoms you describe. It *might* be something else, but you can check if that is what it is with some down-the-bore lookie-lookie when you get back home.

[Hewfil1]

Originally Posted by rich weyand

The difference is that the 9635 has connections for EGR.

Ask Summit tech support if that one I posted above is the right pump for the 9625/9635. They should know. I also think that is the most common Carter AFB still in use, so it is probably the right one.

The seals on those little pump pistons age-harden, and they wear, and then you get the symptoms you describe. It *might* be something else, but you can check if that is what it is with some down-the-bore lookie-lookie when you get back home.

I really appreciate it Rich, if this site had kudos, I'd definitely give you all mine.

[rich weyand]

Originally Posted by Hewfil1

I really appreciate it Rich, if this site had kudos, I'd definitely give you all mine.

De nada. I just like to see people have fun with their trucks.

[cadillac_al]

I'm just glad Rich is back. I was starting to worry. I always appreciate his tuning knowledge in these carb threads. I was planning to do some carb tuning this summer and if I got stumped I always knew I could come here and ask Rich. Stay healthy Bro.

[rich weyand]

Originally Posted by cadillac_al

I'm just glad Rich is back. I was starting to worry. I always appreciate his tuning knowledge in these carb threads. I was planning to do some carb tuning this summer and if I got stumped I always knew I could come here and ask Rich. Stay healthy Bro.

I had a minor heart attack at the beginning of the year. Couple tents put in.

I'm doing great now.