Center carb vacuum question

Don Trombley

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So I have the famous high idle problem on my 440 six pack. I’ve been reading as many threads as I can find to trouble shoot, but so far no luck. My question is about the center carb vacuum ports. The top one is connected to the distributor advance and provides vacuum when the engine is rev’d. I connected it to my vacuum gauge and I believe it’s working properly. The lower one that’s connected to the outer carb diaphragms doesn’t seem to have any vacuum either at idle or at higher rpm’s. Is that right? The carb came from Holley and was listed as factory referb. The outer carbs are new from Holley. The motor is fresh from the shop and I’ve not driven the car yet

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davek

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Yes , are you using the solenoid on the 6 pack? My 2 setups I never used the solenoid.
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Don Trombley

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Yes , are you using the solenoid on the 6 pack? My 2 setups I never used the solenoid. View attachment 1368756 View attachment 1368757
Nope. No solenoid. I have found that if I push down fairly hard on the lever that’s used for a solenoid my idle drops down to near normal. The linkage is disconnected and the idle screw is all the way out. The throttle shaft has a minor amount of fuel leaking around the drivers side. Not sure if that’s normal.
 

493 Mike

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The port you are concerned with is a venturi port and to create vacuum it relies on air flow. It takes many CFM of flow to work but it does at high RPM under load. That is part of the reasoning for the large secondary carb diaphragms-the vacuum is low.
Mike
 

Don Trombley

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The port you are concerned with is a venturi port and to create vacuum it relies on air flow. It takes many CFM of flow to work but it does at high RPM under load. That is part of the reasoning for the large secondary carb diaphragms-the vacuum is low.
Mike
Thanks. Makes sense.
 

davek

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Nope. No solenoid. I have found that if I push down fairly hard on the lever that’s used for a solenoid my idle drops down to near normal. The linkage is disconnected and the idle screw is all the way out. The throttle shaft has a minor amount of fuel leaking around the drivers side. Not sure if that’s normal.
you should have no leaks. Are you using stock carb gaskets? The thick ones tend to catch the butterflys causing high idle.
 

Don Trombley

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you should have no leaks. Are you using stock carb gaskets? The thick ones tend to catch the butterflys causing high idle.
I think removing the carbs may be my next step. I just ordered some new gaskets. I’m assuming they are stock thickness. The ad Didn’t say otherwise. I’m still wondering about the small leak around the throttle shaft. Worn bushings?? Thanks.
 

Demonic

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From the factory there is no bushing at the shaft through the throttle plate. Excessively strong return springs can cause wear here, same as just regular age. Some Holley kits have spiral cut teflon sleeves for worn shafts. But the wear is never a true circle.
 

davek

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I think removing the carbs may be my next step. I just ordered some new gaskets. I’m assuming they are stock thickness. The ad Didn’t say otherwise. I’m still wondering about the small leak around the throttle shaft. Worn bushings?? Thanks.
Pull back your throttle by hand and feel if it is catching and see if it gos back to return.
 

OH71RRunner

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If there is leaking around the throttle shaft, that needs fixed. Mine was misshapen causing a vacuum leak that took forever to figure out. I bought a kit off ebay. It comes with the reamer and new bushings. There's several videos on YouTube about how to do it with a regular drill. No need for a drill press. I was worried about doing it myself, but it was easy. They're brass.
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Don Trombley

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If there is leaking around the throttle shaft, that needs fixed. Mine was misshapen causing a vacuum leak that took forever to figure out. I bought a kit off ebay. It comes with the reamer and new bushings. There's several videos on YouTube about how to do it with a regular drill. No need for a drill press. I was worried about doing it myself, but it was easy. They're brass.
View attachment 1368963
Super information! I bought the bushings already. Did you have high idle issues resulting from the vacuum leak?
 

OH71RRunner

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Super information! I bought the bushings already. Did you have high idle issues resulting from the vacuum leak?
I was having severe tuning issues, not just idle. My idle was all over the place. Sometimes would be high, then low. It was also creating adjustment issues. The throttle shaft hole was wollered out to where sometimes the shaft would sit just right to seal up.

It wasn't me that found it. I took it to a shop and they ended up finding it. Said to take the carb to a machine shop. After looking into it, I saw that I could do it myself for a few dollars.

Couple years later and it seems to be doing good.
 

493 Mike

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The operation of that venturi port can be verified by blowing compressed air past the opening in the casting-with carb off the manifold, wide open throttle. in correct direction of flow. Vacuum should be present at the hose connection tube if all is well.
Mike
 

Don Trombley

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The operation of that venturi port can be verified by blowing compressed air past the opening in the casting-with carb off the manifold, wide open throttle. in correct direction of flow. Vacuum should be present at the hose connection tube if all is well.
Mike
Thanks
 

Don Trombley

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I was having severe tuning issues, not just idle. My idle was all over the place. Sometimes would be high, then low. It was also creating adjustment issues. The throttle shaft hole was wollered out to where sometimes the shaft would sit just right to seal up.

It wasn't me that found it. I took it to a shop and they ended up finding it. Said to take the carb to a machine shop. After looking into it, I saw that I could do it myself for a few dollars.

Couple years later and it seems to be doing good.
Thanks
 
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