Change from Holley to Quadrajet, must change timing? - Team Camaro Tech
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post #1 of 11 (permalink) Old May 14th, 14, 03:31 PM Thread Starter
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Change from Holley to Quadrajet, must change timing?

On my basically stock BBC I changed my Holley 750 vac secondaries back to the original Quadrajet. With the Holley it ran great when punching the throttle. After putting the Quadrajet on it stumbles and weakens when hitting the throttle at low rpms. It doesn't conk out nor backfire but feels like it will then I back off the pedal and feather it into higher rpms and then it's okay.

I talked with the machine shop that does lots of BBCs and they said it's a whole different animal now so I have to adjust timing accordingly. Timing's now at 8 degrees initial so should I bump it up to like 12 or 14? Or....?

67 Plain Jane being assembled with 427, 4 spd Muncie, 12 bolt rear with Mosers, 4 wheel discs
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post #2 of 11 (permalink) Old May 14th, 14, 03:36 PM
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Re: Change from Holley to Quadrajet, must change timing?

I would get the carburetor working properly first. A Quadrajet has mechanically operated secondary butterflies and needs to have a properly operating secondary accelerator circuit and air valve.

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post #3 of 11 (permalink) Old May 14th, 14, 03:40 PM
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Re: Change from Holley to Quadrajet, must change timing?

I would also say get the carburetor squared away first. If it has been off the car for a while, buy a rebuild kit and freshen it up. Then use the right sandwich gasket with heat shield built in or layer fiber with hotslot if active, then tin heat then fiber without hotslot gash.

Then set your baseline tune using vacuum gauge for idle circuit. then do some passes at rpms to dial in your total advance and see how she responds.


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post #4 of 11 (permalink) Old May 14th, 14, 04:12 PM Thread Starter
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Re: Change from Holley to Quadrajet, must change timing?

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Originally Posted by 69-Pace View Post
I would also say get the carburetor squared away first. If it has been off the car for a while, buy a rebuild kit and freshen it up. Then use the right sandwich gasket with heat shield built in or layer fiber with hotslot if active, then tin heat then fiber without hotslot gash.

Then set your baseline tune using vacuum gauge for idle circuit. then do some passes at rpms to dial in your total advance and see how she responds.
Just had it rebuilt by the pro at the machine shop. Got the thick composite gasket in first, second the tin heat shield, and third the regular. So, the hotslot crossover in manifold is blocked off which I guess is the way it's supposed to be.

The Holley ran smooth as a baby's butt with the current timing at idle, mid-range, and WOT. Should I expect any different from the Quadrajet and adjust the timing?

67 Plain Jane being assembled with 427, 4 spd Muncie, 12 bolt rear with Mosers, 4 wheel discs
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post #5 of 11 (permalink) Old May 14th, 14, 04:27 PM
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Re: Change from Holley to Quadrajet, must change timing?

No the Quadrajet should be very responsive once the secondaries open if it is setup right. Did you set the idle mix and the idle circuit once it was installed?
Does the throttle open the secondaries only after the primaries are open?


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post #6 of 11 (permalink) Old May 14th, 14, 04:36 PM Thread Starter
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Re: Change from Holley to Quadrajet, must change timing?

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No the Quadrajet should be very responsive once the secondaries open if it is setup right. Did you set the idle mix and the idle circuit once it was installed?
Does the throttle open the secondaries only after the primaries are open?
Idle mix done by the needle screws up front, builder had them at 2 turns out then I turned them in to where it stumbles and then back out half-turn.
When you say "idle circuit" how would that be adjusted?

Throttle opens secondaries after primaries are open, did that manually under the hood. When going at higher rpms and punching it then the secondaries open pretty well.

It's when coming off a stop or red light when I start pushing the pedal it'll hit flat and stumble.

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post #7 of 11 (permalink) Old May 14th, 14, 05:15 PM
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Re: Change from Holley to Quadrajet, must change timing?

FWIW I would also check the accelerator pump - some stick in the bore and don't supply the added fuel for acceleration, etc. - just be sure it is working properly, and squirting additional fuel . Can you get it to bog when not under load ? If so, try giving it a little choke and try accelerating it again to see if things change, as it wouldn't be the first time I have seen a bad accelerator pump, or missing check ball, which leaves you virtually no additional fuel when accelerating.
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post #8 of 11 (permalink) Old May 14th, 14, 07:09 PM Thread Starter
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Re: Change from Holley to Quadrajet, must change timing?

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FWIW I would also check the accelerator pump - some stick in the bore and don't supply the added fuel for acceleration, etc. - just be sure it is working properly, and squirting additional fuel . Can you get it to bog when not under load ? If so, try giving it a little choke and try accelerating it again to see if things change, as it wouldn't be the first time I have seen a bad accelerator pump, or missing check ball, which leaves you virtually no additional fuel when accelerating.
I maxed out the accelerator pump by bending the arm, so looking down the bores it gives a good squirt.

Check ball is in there. Will advance the timing, to see. Here's hoping the guy at the machine is right. He also said the vacuum pulling on the distributor advance canister can be different between the Holley and the Rochester, so that might account for it?

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post #9 of 11 (permalink) Old May 14th, 14, 09:02 PM
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Re: Change from Holley to Quadrajet, must change timing?

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Originally Posted by gordr View Post
He also said the vacuum pulling on the distributor advance canister can be different between the Holley and the Rochester, so that might account for it?
Make sure you're running manifold vacuum on the Quadrajet if you were on the Holley.
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post #10 of 11 (permalink) Old May 15th, 14, 06:49 AM
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Re: Change from Holley to Quadrajet, must change timing?

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Originally Posted by gordr View Post
When you say "idle circuit" how would that be adjusted?
The idle circuit consists of the idle mixture screws - left and right, the curb idle setting on the throttle cam and the fast choke idle cam screw adjustment on the choke side.


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It's when coming off a stop or red light when I start pushing the pedal it'll hit flat and stumble.
That sounds like the accelerator pump as suggested above. Sorry I was under the impression this bog and stumble was a WOT situation, not from an idle.

Rebuilding a Rochester is an art and it is easy if you follow the prescribed procedure, but if someone just thinks they know what they are doing and rebuild it on a fly and miss one step the whole thing can be fubar and will need to be torn back down and rebuilt. The check ball is a critical component as well as the seal on the pump. As suggested start there.


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post #11 of 11 (permalink) Old May 15th, 14, 12:29 PM Thread Starter
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Re: Change from Holley to Quadrajet, must change timing?

Well, guys, it went from bad to worse. Adjusted timing back to 8 degrees and worked a bit better less flatspotting coming off idle. But after everythings gets warmed up and drive around town the idle fluctuates from 800 on down to below 450 and stalled a couple of times. It does not sound right. No fun! I'll be taking the requisite pictures with the Quadrajet in there with associated hardware as I'm selling the car, then I'll put the Holley back in for smooth riding.

Will take the Quad back to the builder at the machine shop and leave it with him for a few days. As I already asked him if it was doable and he indicated it was. I also told him if it was too much trouble I'd send it to a pro shop like the Carb Shop to have it redone. A little late now as I'm in hock to him for the work done so far.

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