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#1 |
Registered User
Join Date: May 2016
Location: South Williamsport
Posts: 174
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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?) |
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#2 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,602
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Re: Throttle and Bog Down Issue
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? |
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#3 |
Registered User
Join Date: May 2016
Location: South Williamsport
Posts: 174
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Re: Throttle and Bog Down Issue
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#4 | |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,602
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Re: Throttle and Bog Down Issue
Quote:
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. |
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#5 | |
Registered User
Join Date: May 2016
Location: South Williamsport
Posts: 174
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Re: Throttle and Bog Down Issue
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#6 |
Registered User
Join Date: May 2016
Location: South Williamsport
Posts: 174
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Re: Throttle and Bog Down Issue
Also,ed off my carb and my engine setup, which has like a 14:1 compression, could you recommend me a regulator?
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#7 | |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,602
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Re: Throttle and Bog Down Issue
Quote:
Here's one I've used. Holley makes similar ones. https://www.summitracing.com/oh/part...2518/overview/ |
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#8 | |
Registered User
Join Date: May 2016
Location: South Williamsport
Posts: 174
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Re: Throttle and Bog Down Issue
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#9 |
Registered User
Join Date: Aug 2009
Location: Bowser
Posts: 13,602
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Re: Throttle and Bog Down Issue
I ran mine right in front of the carb behind the altenator. Use one of the intake bolts for the mount.
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#10 |
Registered User
Join Date: May 2016
Location: South Williamsport
Posts: 174
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Re: Throttle and Bog Down Issue
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#11 | |
Registered User
Join Date: Apr 2014
Location: Bloomington Indiana
Posts: 1,041
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Re: Throttle and Bog Down Issue
Quote:
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.
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Rich Weyand 1978 K10 RCSB DD. |
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#12 | |
Registered User
Join Date: May 2016
Location: South Williamsport
Posts: 174
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Re: Throttle and Bog Down Issue
Quote:
(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.) Last edited by Hewfil1; 08-10-2016 at 09:04 AM. |
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#13 |
Registered User
Join Date: Apr 2014
Location: Bloomington Indiana
Posts: 1,041
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Re: Throttle and Bog Down Issue
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/
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Rich Weyand 1978 K10 RCSB DD. |
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#14 |
Registered User
Join Date: Apr 2014
Location: Bloomington Indiana
Posts: 1,041
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Re: Throttle and Bog Down Issue
Have you tried looking down the bore of the carb and seeing what kind of squirt you get?
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Rich Weyand 1978 K10 RCSB DD. |
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#15 |
Registered User
Join Date: May 2016
Location: South Williamsport
Posts: 174
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Re: Throttle and Bog Down Issue
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#16 | |
Registered User
Join Date: May 2016
Location: South Williamsport
Posts: 174
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Re: Throttle and Bog Down Issue
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#17 | |
Post Whore
![]() Join Date: Aug 2014
Location: Sacramento, CA
Posts: 10,828
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Re: Throttle and Bog Down Issue
Quote:
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 Last edited by Gregski; 08-10-2016 at 09:22 PM. |
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#18 | |
Post Whore
![]() Join Date: Aug 2014
Location: Sacramento, CA
Posts: 10,828
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Re: Throttle and Bog Down Issue
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#19 | |
Registered User
Join Date: Apr 2014
Location: Bloomington Indiana
Posts: 1,041
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Re: Throttle and Bog Down Issue
Quote:
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.
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Rich Weyand 1978 K10 RCSB DD. |
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#20 | |
Registered User
Join Date: May 2016
Location: South Williamsport
Posts: 174
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Re: Throttle and Bog Down Issue
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#21 |
Registered User
Join Date: Apr 2014
Location: Bloomington Indiana
Posts: 1,041
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Re: Throttle and Bog Down Issue
De nada. I just like to see people have fun with their trucks.
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Rich Weyand 1978 K10 RCSB DD. |
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#22 |
Registered User
Join Date: Oct 2013
Location: Maine
Posts: 2,356
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Re: Throttle and Bog Down Issue
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.
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76 Chevy K20 76 GMC K15 77 Chevy C10 77 Chevy K10 |
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#23 | |
Registered User
Join Date: Apr 2014
Location: Bloomington Indiana
Posts: 1,041
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Re: Throttle and Bog Down Issue
Quote:
I'm doing great now.
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Rich Weyand 1978 K10 RCSB DD. |
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