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Old 06-30-2012, 04:42 AM
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Holley 670 avenger stumble

Hi guys,

I have a holley 670 avenger on my 351 Cleveland, which is a standard motor and auto. I have removed my 2 barrel manifold and motorcraft 2 barrel carby as well as the standard points distributor.

I have fitted an edelbrock rpm air gap manifold, the 670 avenger and an electronic distributor. Distributor has been graphed and set up to 12 degrees.

When I open the throttle at low rpm it stumbles sometimes to the point where it almost stalls. It feels like it accelerates fine (like the squirter is ok) but once it has squirted and reaches the throttle position it then stumbles.

I have read the Other thread "Holley hesitation" and I would say it is the exact same thing as what I am experiencing however, I have the 670 not 570 avenger.

should I be able to get this carby to run fine given it is effectively the same as a 600 just with the secondary metering block and is a standard motor?

Should I just be able to play around with the powervalve etc?

I will put a vacuum gauge on it tomorrow to see what's going on there...

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Last edited by Yz_rider125; 06-30-2012 at 04:52 AM.
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Old 06-30-2012, 05:50 AM
How fast is fast enough?
 
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Don't touch the power valve!

Adjust your idle feed screws to give you the maximum vacuum at idle. If you still have a problem after that you may need to trya different pump cam or squirter.

Only adjust the idle feed first!

A brand new carb should be set up fairly close on everything else for a stock engine.

You did set the timing with the vacuum advance disconnected right?
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Old 06-30-2012, 06:03 AM
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Yep timing set with vac disconnected and idle mixture screws already set.

Can spluttering and throwing fuel back out the top of the carby damage anything like the powervalve? When I put the new distributor back in it was a faulty unit and would not fire correctly, therefore the motor hardly ran and when it would stall it would splutter and throw fuel straight out the top of the carby quite aggressively...

Drives spot on apart from this just off idle hesitation
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Old 06-30-2012, 07:18 AM
How fast is fast enough?
 
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If it drives spot on otherwise the its not a power valve. try a pump can that comes in a little sooner. also try going a hair richer on your idle feed especially on the primary side.
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Old 06-30-2012, 07:59 AM
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Quote:
Originally Posted by Yz_rider125 View Post
Yep timing set with vac disconnected and idle mixture screws already set.

Can spluttering and throwing fuel back out the top of the carby damage anything like the powervalve? When I put the new distributor back in it was a faulty unit and would not fire correctly, therefore the motor hardly ran and when it would stall it would splutter and throw fuel straight out the top of the carby quite aggressively...

Drives spot on apart from this just off idle hesitation
The 670 is fine, it's not too big and will run great once it's tuned. Your carb has a blowout protection valve in the baseplate to prevent damage if it backfires.

It sounds like either not enough initial timing or the carb accelerator pump circuit. Possibly overexposed transition slot.

To start with, use manifold vacuum for the vacuum advance if you aren't already. If you weren't using manifold vacuum before, try driving it that way to see if the bog is gone.

If that didn't cure it, just as a test, crank in a healthy amount of initial timing to see if the response improves significantly. This will increase the total timing, so don't go out on the road hammering on it- this is just to see if increasing the initial timing helps or not. If it does, you can go about tailoring the timing curve of the distributor to match what the engine needs.

If that doesn't cure it, start on the accelerator pump circuit. One fast way to check if it's something with the accelerator pump is to set the cam to position #2. That will add pump shot later in the opening of the throttle. If that helps, the carb can be tuned to work w/the cam in that position, or another cam can be used.

I like to go as lean as I can, shooter-wise, then add pump duration and/or volume w/the cam if necessary. So if it still bods/hesitates the same way after using manifold vacuum for the vacuum advance, the shooter size can be decreased to get more duration of the accelerator pump shot. If this is too lean, the cam can be changed to get more duration w/the same shooter.

If that doesn't help, you need to determine how much of the transition slot is exposed at idle the way you have it now. If there's too much slot exposed (butterflies open too far) the off idle transition will be poor, and often the idle screws will be unresponsive. If your idle screws will stall the engine (or nearly so) when they'r screwed in, the slot is probably OK. But if the engine continues to run, the slots are overexposed and the primary throttle blades will need to be closed some.

See what the above does and get back to us.
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