Holley tuning question. - Chevelle Tech
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post #1 of 29 (permalink) Old Apr 16th, 19, 7:51 PM Thread Starter
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Holley tuning question.

I took my Holley 770 street avenger off of my 396 to look at the transfer slots. I had the motor idling at 900 rpm. When I looked at the transfer slots, none were visible. I adjusted the curb idle screw so that there was a perfect square at the transfer slot. I reinstalled the carb and now my idle is at 1200 rpm. How can I lower the idle without adjusting the curb idle screw? Can I lower the idle with the secondary adjustment screw? If I lower the rpm with the curb idle screw, the transfer slot will disappear.
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post #2 of 29 (permalink) Old Apr 16th, 19, 8:26 PM
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Re: Holley tuning question.

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I took my Holley 770 street avenger off of my 396 to look at the transfer slots. I had the motor idling at 900 rpm. When I looked at the transfer slots, none were visible. I adjusted the curb idle screw so that there was a perfect square at the transfer slot. I reinstalled the carb and now my idle is at 1200 rpm. How can I lower the idle without adjusting the curb idle screw? Can I lower the idle with the secondary adjustment screw? If I lower the rpm with the curb idle screw, the transfer slot will disappear.
Not much help but it is not that rare. I can not run my 1000HP square it will idle to high also. I backed off primaries and secondaries until I got it down to 900. BUT not sure about yours but mine has holes in all four butter flies have holes and that adds a lot of air.
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post #3 of 29 (permalink) Old Apr 16th, 19, 8:42 PM Thread Starter
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Re: Holley tuning question.

No holes in my butterflies. When I adjust the idle to 900 rpm, if I blip the accelerator quickly, the rpm tanks to 200 rpm and then recovers. I am thinking it does this because there is no transfer slot showing. When I adjust the transfer slot correctly, idle rpm is rock solid even when I blip the accelerator quickly.
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post #4 of 29 (permalink) Old Apr 16th, 19, 8:47 PM
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Re: Holley tuning question.

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No holes in my butterflies. When I adjust the idle to 900 rpm, if I blip the accelerator quickly, the rpm tanks to 200 rpm and then recovers. I am thinking it does this because there is no transfer slot showing. When I adjust the transfer slot correctly, idle rpm is rock solid even when I blip the accelerator quickly.
When you idle it down too 900 are you adjusting the primary and secondary at the same time or with just the primary?
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post #5 of 29 (permalink) Old Apr 16th, 19, 8:56 PM Thread Starter
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Re: Holley tuning question.

Just the primary. Can I adjust the idle lower with just the secondary adjustment screw?
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post #6 of 29 (permalink) Old Apr 16th, 19, 9:05 PM
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Re: Holley tuning question.

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Just the primary. Can I adjust the idle lower with just the secondary adjustment screw?
Okay I thought yours was a double pumper. So you had front and rear square and it idles to high. I would closing the back more so the front T slots are exposed to see what happens. Are you sure you don't have a vac leak right?
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post #7 of 29 (permalink) Old Apr 16th, 19, 9:08 PM Thread Starter
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Re: Holley tuning question.

No vacuum leak. My cam is a comp 280H. Vacuum is 12.75 steady.
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post #8 of 29 (permalink) Old Apr 16th, 19, 9:21 PM
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Re: Holley tuning question.

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No vacuum leak. My cam is a comp 280H. Vacuum is 12.75 steady.
If the secondarys are open at all try closing it and keeping the slots visible on the primaries
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post #9 of 29 (permalink) Old Apr 16th, 19, 9:34 PM
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Re: Holley tuning question.

One possibility - Check your accelerator pump adjustment.

When you have turned up the idle, you are engaging the pump more, and now when you blip the throttle you get a bigger pump shot.

When you turn the idle back down, and you blip the throttle, you might have backed it off enough that the accelerator pump is no longer giving you the shot you need which is why the rpm drops way off for a second.
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post #10 of 29 (permalink) Old Apr 16th, 19, 9:36 PM Thread Starter
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Re: Holley tuning question.

OK, tomorrow I will pull the carb an check the slots on the secondary. What will happen if I close the secondary butterflies and so slot is visible? Will this create other problems?
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post #11 of 29 (permalink) Old Apr 16th, 19, 9:57 PM
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Re: Holley tuning question.

Here is what I do on the secondary sides of Holley's. I take the screw that is adjustable from the bottom of the baseplate and unscrew it then put it in from the top so it's easier to adjust.
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post #12 of 29 (permalink) Old Apr 16th, 19, 10:04 PM
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Re: Holley tuning question.

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OK, tomorrow I will pull the carb an check the slots on the secondary. What will happen if I close the secondary butterflies and so slot is visible? Will this create other problems?
Does the rear have T slots? It does not have rear mixture screws. I would try closing the rear butter flies as they do not open when you blip the throttle anyway. Get the primaries set so that it idles right and responds well then get the vac secondaries set.
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post #13 of 29 (permalink) Old Apr 16th, 19, 11:39 PM
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Re: Holley tuning question.

How much initial timing are you running? Vacuum advance connected to a ported or manifold source? I agree with what has been said about trying to close the secondaries a bit if the idle stop has them open any further than what you need to avoid sticking plates in the bores.
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post #14 of 29 (permalink) Old Apr 17th, 19, 12:58 AM
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Leave the primary slot exposed as small squares, and close the secondaries more.
On my engine when I have the 770 street avenger on I have to close the secondary completely. I just have the idle screw touch the bracket, but not enough pressure to open it.
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post #15 of 29 (permalink) Old Apr 17th, 19, 5:19 AM
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Re: Holley tuning question.

Check your PCV valve, make sure it is closed at idle: put your finger on it at idle. If you can feel any movement, it is the pintle pulsing because the vacuum is not strong enough to pull the pintle valve against the seat at idle. Result is more air entering the engine than designed, needing throttle blades to be closed off more. If this is the problem, low vacuum PCV needed.
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