The Ford Barn

19Fordy · 04-27-2023, 06:00 PM

I have a stock Holley 390 List 8007 on my 8BA. Engine is stock except for a Holley 390,List 8007 ,4 barrel carb,

When I rebuilt the carb, I removed the little set screw under the carb that adjust the secondary throttle plates. That Holley site this screw is preset at the factory and should not be adjusted unless you are using a non-stock cam.

In the photo below, you can see the tip of the set screw underneath the linkage arm that controls the amount of opening of the secondary throttle plate. To adjust the factory set screw you have to remove the carb.

Does anyone have a photo they could post showing the tip of THAT set screw as it appears when adjusted at the factory- when viewed from the top of the carb flange?

I have been tediously trying to set it correctly and am hoping a Fordbarner might have a Holley 390 correctly "set". Thanks, JIM

petehoovie · 04-27-2023, 06:34 PM

Quote:

Originally Posted by 19Fordy

I have a stock Holley 390 List 8007 on my 8BA. Engine is stock except for a Holley 390,List 8007 ,4 barrel carb,

When I rebuilt the carb, I removed the little set screw under the carb that adjust the secondary throttle plates. That Holley site this screw is preset at the factory and should not be adjusted unless you are using a non-stock cam.

In the photo below, you can see the tip of the set screw underneath the linkage arm that controls the amount of opening of the secondary throttle plate. To adjust the factory set screw you have to remove the carb.

Does anyone have a photo they could post showing the tip of THAT set screw as it appears when adjusted at the factory- when viewed from the top of the carb flange?

I have been tediously trying to set it correctly and am hoping a Fordbarner might have a Holley 390 correctly "set". Thanks, JIM

1948F-1Pickup · 04-27-2023, 07:45 PM

No photos. Kinda tricky to adjust……. obviously. Turn the screw so it has no impact on how closed the secondary plates are. Notice that the plates can get jammed in the bores of the carb. Adjust the screw the opposite direction until the plates no longer get jammed. Stop adjusting the screw when you get to this point.

scicala · 04-27-2023, 07:51 PM

Quote:

Originally Posted by 19Fordy

I have a stock Holley 390 List 8007 on my 8BA. Engine is stock except for a Holley 390,List 8007 ,4 barrel carb,

When I rebuilt the carb, I removed the little set screw under the carb that adjust the secondary throttle plates. That Holley site this screw is preset at the factory and should not be adjusted unless you are using a non-stock cam.

In the photo below, you can see the tip of the set screw underneath the linkage arm that controls the amount of opening of the secondary throttle plate. To adjust the factory set screw you have to remove the carb.

Does anyone have a photo they could post showing the tip of THAT set screw as it appears when adjusted at the factory- when viewed from the top of the carb flange?

I have been tediously trying to set it correctly and am hoping a Fordbarner might have a Holley 390 correctly "set". Thanks, JIM

You can't tell from the tang touching the set screw on how far the
secondary plates are opened. You need to take the carb off. With a stock cam once the set screw first touches the tang on the lever, with the secondary plates closed, turn the set screw in no more than about 1/4 turn. Just enough to keep the plates from sticking in the bores.
For the future, you can replace the set screw with one made for an allen wrench. Then you can turn it with the carb installed.

Sal

cjshaker · 04-27-2023, 07:52 PM

Like mentioned above, on a flathead, I'd just adjust it so it stops the throttle plates from contacting the bores. It's basically there just to stop the plates from sticking in the bore, but on bigger engines with more horsepower it can play a role in getting the idle right. You shouldn't have that issue with such a small engine and a mild cam.

1948F-1Pickup · 04-27-2023, 08:22 PM

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Quote:

Originally Posted by cjshaker

, but on bigger engines with more horsepower it can play a role in getting the idle right. You shouldn't have that issue with such a small engine and a mild cam.

Long duration camshafts/low vacuum......
That's when you play with that screw. Ask me how I know.

19Fordy · 04-28-2023, 12:33 PM

THANK YOU all for your valuable replies. What you suggest makes sense.

ALSO, THANK YOU Pete for enlarging the photos.

JayChicago · 04-28-2023, 02:17 PM

Quote:

Originally Posted by 1948F-1Pickup

Long duration camshafts/low vacuum......
That's when you play with that screw. Ask me how I know.

I don't get it. What can you play with? As said above, "It's basically there just to stop the plates from sticking in the bore". If you reduce that stop screw, plates will stick. If you increase that screw, won't it just let in more air and exacerbate the low vacuum issue?

cadillac512 · 04-28-2023, 05:02 PM

Quote:

Originally Posted by JayChicago

I don't get it. What can you play with? As said above, "It's basically there just to stop the plates from sticking in the bore". If you reduce that stop screw, plates will stick. If you increase that screw, won't it just let in more air and exacerbate the low vacuum issue?

Hi Jay-

The deal with adjusting the secondary stop screw with long duration cams is this. The engine needs more throttle to keep a decent idle speed when these kind of cams are used. If the primary idle screw is turned in too far, too much of the transfer slot (that narrow slot that the butterflies almost cover at idle) is uncovered and the off-idle fueling is affected, causing surging and a poor transition to the main metering circuit when the throttle is opened further.

By slightly opening the secondary butterflies, the transfer slot for those is slightly exposed and fuel is also delivered there to augment the idle speed the primaries provide. This way, the primary idle adjustment can be kept in the proper relationship with those transfer slots. Vacuum isn't lowered (any more than that which is due to the extra duration/overlap of the cam) because engine efficiency is maintained.

All this is necessary due to the idle being affected by the extra valve opening overlap, which causes exhaust to blow back into the not-quite-closed intake valve and dilutes the fresh intake charge.

Hot Rodders!

Terry

cjshaker · 04-28-2023, 05:34 PM

Quote:

Originally Posted by cadillac512

Hi Jay-

The deal with adjusting the secondary stop screw with long duration cams is this. The engine needs more throttle to keep a decent idle speed when these kind of cams are used. If the primary idle screw is turned in too far, too much of the transfer slot (that narrow slot that the butterflies almost cover at idle) is uncovered and the off-idle fueling is affected, causing surging and a poor transition to the main metering circuit when the throttle is opened further.

By slightly opening the secondary butterflies, the transfer slot for those is slightly exposed and fuel is also delivered there to augment the idle speed the primaries provide. This way, the primary idle adjustment can be kept in the proper relationship with those transfer slots. Vacuum isn't lowered (any more than that which is due to the extra duration/overlap of the cam) because engine efficiency is maintained.

All this is necessary due to the idle being affected by the extra valve opening overlap, which causes exhaust to blow back into the not-quite-closed intake valve and dilutes the fresh intake charge.

Hot Rodders!

Terry

Terry did a good job of explaining it. Not much to add to it. I run dual Holleys on my 427, with a big overlap cam. Trying to get the idle right strictly on the primaries makes a mess of the off-idle performance, not to mention the surging it can cause at idle. Trying to adjust 2 carbs with a big cam is twice the fun!

Hot rodders indeed

JayChicago · 04-28-2023, 07:11 PM

Terry
Excellent, well written explanation! Even I can understand that. You could author a book on carburetors. Thanks.

19Fordy · 04-29-2023, 01:36 PM

Terry, Thank you for the clear explanation of the relationship between primary and secondary throttle plate positions.

QUESTION: On a stock 8BA is it advantageous (with the 390 off the car) to first adjust the curb idle by first turning the curb idle screw so that the primary transition slots appear as a "square" and then install the carb on the engine and then fine tune the final curb idle RPM by adjusting only the secondary throttle plate (set) screw?
In order to do this, the OEM secondary idle set screw must be replaced with a short 12-24 hex head bolt that can be adjusted with a box end wrench with the carb on the car with the engine running.

My thinking is that doing the above would optimize the performance of the carb.
Thank you for your continued responses.

cadillac512 · 04-29-2023, 01:47 PM

Good question! I'd set the secondary as above...just short of sticking in the bores, and primary "square" as you say, then see how much more/less idle adjustment is needed. If it needs just a little, like 1/2 turn or so, I'd just use the primary screw. More than that and the secondary could sure be a viable option.

Most Holleys I've seen have the secondary slots covered or very nearly covered at the "just shy of sticking" setting, and work well there. I've not tried to use the secondary adjustment to set idle on a stock application so I can't say for sure how well it might work. I do know that it takes very little secondary opening to make the primary adjustment difficult to obtain a slow enough idle. A slightly sticking (open) secondary really messes with idle speed and mix on a stocker.

Terry

cjshaker · 04-29-2023, 02:15 PM

I would not attempt to use the secondaries as an adjustment on idle. Like Terry said, just set them to 'barely not touching the bores', then use the primary adjustments as they were designed. The secondary plates should only be fiddled with when a big cam is being used and a satisfactory idle can't be achieved with the primaries....and even then, only when all other issues have been corrected.

04-27-2023, 07:45 PM	#3
1948F-1Pickup Senior Member Join Date: Oct 2015 Location: Socal Posts: 795	Re: Holley 390 Secondary Throttle Setting Question. No photos. Kinda tricky to adjust……. obviously. Turn the screw so it has no impact on how closed the secondary plates are. Notice that the plates can get jammed in the bores of the carb. Adjust the screw the opposite direction until the plates no longer get jammed. Stop adjusting the screw when you get to this point.

04-27-2023, 07:52 PM	#5
cjshaker Senior Member Join Date: Jan 2023 Location: Ada OH Posts: 149	Re: Holley 390 Secondary Throttle Setting Question. Like mentioned above, on a flathead, I'd just adjust it so it stops the throttle plates from contacting the bores. It's basically there just to stop the plates from sticking in the bore, but on bigger engines with more horsepower it can play a role in getting the idle right. You shouldn't have that issue with such a small engine and a mild cam.

04-28-2023, 12:33 PM	#7
19Fordy Senior Member Join Date: May 2010 Location: Coral Springs FL Posts: 11,012	Re: Holley 390 Secondary Throttle Setting Question. THANK YOU all for your valuable replies. What you suggest makes sense. ALSO, THANK YOU Pete for enlarging the photos.

04-28-2023, 07:11 PM	#11
JayChicago Senior Member Join Date: Jan 2019 Location: Chicago Posts: 750	Re: Holley 390 Secondary Throttle Setting Question. Terry Excellent, well written explanation! Even I can understand that. You could author a book on carburetors. Thanks.

04-29-2023, 01:36 PM	#12
19Fordy Senior Member Join Date: May 2010 Location: Coral Springs FL Posts: 11,012	Re: Holley 390 Secondary Throttle Setting Question. Terry, Thank you for the clear explanation of the relationship between primary and secondary throttle plate positions. QUESTION: On a stock 8BA is it advantageous (with the 390 off the car) to first adjust the curb idle by first turning the curb idle screw so that the primary transition slots appear as a "square" and then install the carb on the engine and then fine tune the final curb idle RPM by adjusting only the secondary throttle plate (set) screw? In order to do this, the OEM secondary idle set screw must be replaced with a short 12-24 hex head bolt that can be adjusted with a box end wrench with the carb on the car with the engine running. My thinking is that doing the above would optimize the performance of the carb. Thank you for your continued responses. Last edited by 19Fordy; 04-29-2023 at 01:50 PM.

04-29-2023, 01:47 PM	#13
cadillac512 Senior Member Join Date: Apr 2019 Location: Kansas Posts: 983	Re: Holley 390 Secondary Throttle Setting Question. Good question! I'd set the secondary as above...just short of sticking in the bores, and primary "square" as you say, then see how much more/less idle adjustment is needed. If it needs just a little, like 1/2 turn or so, I'd just use the primary screw. More than that and the secondary could sure be a viable option. Most Holleys I've seen have the secondary slots covered or very nearly covered at the "just shy of sticking" setting, and work well there. I've not tried to use the secondary adjustment to set idle on a stock application so I can't say for sure how well it might work. I do know that it takes very little secondary opening to make the primary adjustment difficult to obtain a slow enough idle. A slightly sticking (open) secondary really messes with idle speed and mix on a stocker. Terry __________________ "It don't take but country smarts to solve the problem" (Smokey Yunick) '41 Merc Town Sedan / 260" 8CM engine '66 Fairlane four door / "warmed up" 302

04-29-2023, 02:15 PM	#14
cjshaker Senior Member Join Date: Jan 2023 Location: Ada OH Posts: 149	Re: Holley 390 Secondary Throttle Setting Question. I would not attempt to use the secondaries as an adjustment on idle. Like Terry said, just set them to 'barely not touching the bores', then use the primary adjustments as they were designed. The secondary plates should only be fiddled with when a big cam is being used and a satisfactory idle can't be achieved with the primaries....and even then, only when all other issues have been corrected.