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I just put in a new motor specs here....http://www.camaros.net/forums/showthread.php?t=162586

And everytime I step on the gas the motor stumbles and the catches after a few seconds. The carb is a holley street avenger 670 completly stock from the box. I was told its the accelerator pump cam. But im not really familiar with holleys yet. Anyone have any suggestions on what I should do?

Chris
 

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I had the same exact problem and it was the accelerator pump adjustment. Mines not perfect yet but not as annoying. Check out holleys site to get the adjustment specs. Youll need some of those flat shims to check it
 

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I have a 770 Street Avenger and am having the same problem. I will adjust the accelerator pump first, then try larger jets.
Thank You for the advice.
 

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The Holley web site should tell you which pump cam has what pump volume. If you get to the largest cam and you still have the stumble, you can swap out the acc. pump nozzle to a larger one. The nozzle size is stamped on the nozzle itself. Carefully remove it using a phillips head screw driver making sure the throttle blades are closed. It is trial and error, but what you are describing is an acc. pump shot issue 9/10 times.

Check here:

http://www.holley.com/types/Accelerator Pump Kits and Service Parts.asp

http://www.holley.com/types/Pump Discharge Nozzles.asp

alan
 

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Changed out the cam pump for the correct one and I runs "better". No hesitation but every once and awhile it will backfire through the Carb. I havent messed with the squirters yet, could that still be the problem? According to holley my carb has 0.031 pump discharge nozzle. What size do I need to go to?

Chris
 

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check out Holley's website. They have a video specifically regarding the accel pump, cam, nozzle, etc....I have the same problem and same carb. I will be picking up part #121-135 from Kragen. It's a 0.035" hole size.
 

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Carb adjustments can be tricky because any one setting affects the others. As a result, it's easy to find a knob (like accellerator pump cams) and tweak it until you're off in the weeds - fix one problem and introduce another. That 670 carb should be a good match for your 350 motor. If this is a new carb, it should be set to factory specs. If you've tweaked stuff, put it back to factory specs.

Timing is a critical factor affecting off idle performance. So don't over look timing as a factor. And the comments above related to vacuum advance are very important as well.

Vacuum leaks are another prime culpret. Remove the carb and carefully inspect the base gasket(s) to make sure they have a good witness mark indicating a good fit between the carb base and the intake manifold. I have an Edelbrock Performer RPM intake and the holley accelerator pump lever was contacting the number #1 runner making a huge vacuum leak. After installing a 3/8" phenolic spacer the vacuum leak went away. Another good place for vacuum leaks is at the intake manifold gaskets. Of all the methods used to find leaks (propane, starting fluid, WD-40), I like to use plane old water. Squirt water around the gasket area with the engine running while listening for sucking sounds.

Use a vacuum gauge to baseline your motor and measure improvements as you make your adjustments. The vacuum gauge will tell you alot about timing, vacuum leaks, and valve adjustments that will affect off-idle performance. Take things one step at a time so you can measure / see the effect of a single change. It's doubtful you'll need anything above an orange accelerator pump cam and I wouldn't be surprised if the carb didn't come with one already.

My money is on the vacuum leak / timing issue.
 
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Purchased the accelerator cam pump kit today.but does anyonehave any answer to my last question? Or is more like a trial and error thing?
Go ahead and change the jets back to the holly stock out of the factory for your modle carb. I have a 600 cfm and use 66 jets at just above sea level. If you are way up in altitude you may have to jet down due to the lack of oxygen (like 4000 or 5000 feet and higher). You may also want to increase the or decrease the accelerator pump knozzle. May not be getting enough gas on initial acceleration. Anyway, stock jets for your carb should be fine.
 

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I went up 2 sizes on the squirter size on my 670. I spoke with Holley & they advised me to do this first. It definately was more responsive, but it would backfire once every so often, but only after getting back in the thottle quicky after having coasted for a short time. I have since sold the car & now have a 67 w/LS1 :) I will never go back to a carb, I'm spoiled now. I wish I could have tinkered with it a little more. Like to have tried the vacuum advance tip.
Scott
 
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