Carburetor Tune Up Advice - The 1947 - Present Chevrolet & GMC Truck Message Board Network

RJBEATS95 · 01-28-2019, 02:02 AM

I own a 1970 Chevy C-20 Pick up with a Rochester Quadrajet carburetor and need some help with tuning. The truck idles just fine, but when I try to accelerate relatively quickly the truck bogs down before shooting off. I've heard that these carburetors have a reputation for having that issue but I haven't been able to find a solution, only tutorials on how to adjust it during idle. What can I do to solve this problem?

**WorkinLonghorn** · 01-28-2019, 03:34 AM

That sounds like the accelerator pump is malfunctioning or maladjusted.

When you open the throttle by hand while looking and listening down the carb (engine off) you should hear and/or see jets of gas spraying into the bores. If not, then you need to get a kit from the parts store that will contain a new little pump piston. You just take the top of the carb off and lift it out and replace it. The hard part might be getting the top back on with all the dangling needles and rods down into the correct locations but it can be easily learned how to get it right.

If you do hear/see spray, then maybe the setting is off. there are different holes that the connecting arm to this pump can be attached to. Very easy to set it to the inner hole for more spray.

Of course you should also make sure that you don't have a vacuum leak anywhere.

rpmerf · 01-28-2019, 07:34 AM

Quadrajet Problems:
http://www.florida4x4.com/tech/quadr...20Problems.doc

Quadrajet Tuning:
http://www.corvettefaq.com/c3/QJetTuningPaper.doc

Quadrajet Choke Setup:
http://www.vetteclub.org/warehouse/t...ke%20Setup.doc

Bunch of Quadrajet links:
https://www.florida4x4.com/tech/quadrajet/

I agree with WorkinLonghorn, sounds like accelerator pump. The accelerator pump gives a spray of fuel to help with starting and quick throttle changes. If the accel pump is not working, you will have a hard time starting, and it will lean out momentarily when you increase throttle.

demian5 · 01-28-2019, 12:35 PM

The secondaries may also be opening too soon and that can cause a bog and sometimes a pop as well before it scoots off. Easy adjustment if you know what you are doing.

I would pull carb and get a rebuild kit to make sure the accell pump is fresh as well as no emulsion tubes are laying in the float bowl....

webfoot · 01-28-2019, 01:42 PM

On a lot of older/never rebuilt carbs, the accel pump seal gets chewed up pretty good by the ethanol and don't work real well.

**MJN** · 01-28-2019, 02:32 PM

Sounds like a rebuild is in store as the others have stated. Here is some more tuning info on how to eliminate the quadra-bog when the secondaries open up.

https://www.dropbox.com/s/fhdxgp0wkn...s%202.pdf?dl=0

https://quadrajetpower.com/quadrajet-information/

Cap'n Hook · 01-28-2019, 10:34 PM

Just for motivation, as it’s been 10+ years since I last tuned a quadrajet, and can’t remember anything about them, once they’re tuned, they perform outstanding for their size. Plus they sound awesome!

Sparky dave · 01-29-2019, 07:13 PM

Something simple for you to check! My secondary’s were catching on the housing whilst in the closed position so check they move freely by pushing with your finger whilst the engine is off. Apparently the casings warp if 4 mounting bolts have been over tightened.

01-28-2019, 02:02 AM	#1
RJBEATS95 Registered User Join Date: Oct 2018 Location: Keizer Posts: 3	Carburetor Tune Up Advice I own a 1970 Chevy C-20 Pick up with a Rochester Quadrajet carburetor and need some help with tuning. The truck idles just fine, but when I try to accelerate relatively quickly the truck bogs down before shooting off. I've heard that these carburetors have a reputation for having that issue but I haven't been able to find a solution, only tutorials on how to adjust it during idle. What can I do to solve this problem?

01-28-2019, 03:34 AM	#2
WorkinLonghorn Senior Member Join Date: Dec 2006 Location: Studio City, Calif. Posts: 2,861	Re: Carburetor Tune Up Advice That sounds like the accelerator pump is malfunctioning or maladjusted. When you open the throttle by hand while looking and listening down the carb (engine off) you should hear and/or see jets of gas spraying into the bores. If not, then you need to get a kit from the parts store that will contain a new little pump piston. You just take the top of the carb off and lift it out and replace it. The hard part might be getting the top back on with all the dangling needles and rods down into the correct locations but it can be easily learned how to get it right. If you do hear/see spray, then maybe the setting is off. there are different holes that the connecting arm to this pump can be attached to. Very easy to set it to the inner hole for more spray. Of course you should also make sure that you don't have a vacuum leak anywhere. __________________ '69 GMC C2500 Custom Camper, 8 1/2' bed, New GM 350, NP 435 Close Ratio 4spd. Trans., 3.73 Dana-60 open.Camper and Trailer wiring, PS, PB, AC, tach , three gas tanks, 2nd owner, Work-Truck supreme. Best $300 I ever spent.

01-28-2019, 07:34 AM	#3
rpmerf Registered User Join Date: Feb 2011 Location: Parkville, MD Posts: 1,024	Re: Carburetor Tune Up Advice Quadrajet Problems: http://www.florida4x4.com/tech/quadr...20Problems.doc Quadrajet Tuning: http://www.corvettefaq.com/c3/QJetTuningPaper.doc Quadrajet Choke Setup: http://www.vetteclub.org/warehouse/t...ke%20Setup.doc Bunch of Quadrajet links: https://www.florida4x4.com/tech/quadrajet/ I agree with WorkinLonghorn, sounds like accelerator pump. The accelerator pump gives a spray of fuel to help with starting and quick throttle changes. If the accel pump is not working, you will have a hard time starting, and it will lean out momentarily when you increase throttle. __________________ 1970 C20 Custom Camper - 350, TH350 1997 GMC Suburban 1994 Acura Integra GSR 1987 Dodge Daytona Shelby Z

01-28-2019, 12:35 PM	#4
demian5 Registered User Join Date: Aug 2016 Location: Lake Forest, CA Posts: 2,177	Re: Carburetor Tune Up Advice The secondaries may also be opening too soon and that can cause a bog and sometimes a pop as well before it scoots off. Easy adjustment if you know what you are doing. I would pull carb and get a rebuild kit to make sure the accell pump is fresh as well as no emulsion tubes are laying in the float bowl.... __________________ "Work hard, use your vacation days." 1970 C15 GMC Long Bed 1986 C20 Scottsdale 1983 K2500 Sierra Classic Suburban 6.2 Instagram: C10sofOC

01-28-2019, 01:42 PM	#5
webfoot Registered User Join Date: Nov 2007 Location: Hermiston, OR Posts: 947	Re: Carburetor Tune Up Advice On a lot of older/never rebuilt carbs, the accel pump seal gets chewed up pretty good by the ethanol and don't work real well. __________________ *Running in place gets you nowhere fast.* -me 1968 GTO 4-spd covnvertible 1970 K-20 4-speed, factory AC.

01-28-2019, 02:32 PM	#6
MJN Registered User Join Date: Feb 2000 Location: Maquoketa, Ia/ Orr, Mn Posts: 2,998	Re: Carburetor Tune Up Advice Sounds like a rebuild is in store as the others have stated. Here is some more tuning info on how to eliminate the quadra-bog when the secondaries open up. https://www.dropbox.com/s/fhdxgp0wkn...s%202.pdf?dl=0 https://quadrajetpower.com/quadrajet-information/ __________________ '51 South Bend Model "9A" '56 Chevy 3600 NAPCO '74 Chevy Cheyenne Super K20-restored on 37's '72 GMC Sierra Custom K2500- in progress '08 GMC Envoy Denali '12 GMC Sierra 2500HD Denali '17 GMC Yukon XL VCCA member #58596 http://www.nohrco.com

01-28-2019, 10:34 PM	#7
Cap'n Hook Workin to live... Join Date: Jun 2003 Location: Texas Hill Country Posts: 261	Re: Carburetor Tune Up Advice Just for motivation, as it’s been 10+ years since I last tuned a quadrajet, and can’t remember anything about them, once they’re tuned, they perform outstanding for their size. Plus they sound awesome! __________________ Jason

01-29-2019, 07:13 PM	#8
Sparky dave Registered User Join Date: Nov 2017 Location: Leeds west yorkshire Posts: 307	Re: Carburetor Tune Up Advice Something simple for you to check! My secondary’s were catching on the housing whilst in the closed position so check they move freely by pushing with your finger whilst the engine is off. Apparently the casings warp if 4 mounting bolts have been over tightened.



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