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Discussion Starter #1 (Edited)
Hey guys, I’ve got a vacuum leak coming from my carburetor spacer. I used new gaskets when I was installing it, but the engine increases in RPM when I spray carb cleaner on it. Car has a hissing sound when running and doesn’t idle smooth.

How can I ensure I install the new carburetor gaskets properly so they don’t leak?

Where do I plumb my vacuum gauge to check vacuum readings? What values should I be expecting or hoping for? I’ve heard 16-22 is good (at idle) — is that right?

Engine is a 289, no power brakes or anything. Just the vacuum line from the Autolite 4100 carb to the dizzy and the crankcase system.

Thanks in advance!


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No PCV valve or hoses?
 

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Did you over tighten the bolts? I forget the torque spec but it’s not very much, like barely over hand tight.
Did you replace anything? If a new spacer or carb it might not be flat.
 

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Discussion Starter #5 (Edited)
Did you over tighten the bolts? I forget the torque spec but it’s not very much, like barely over hand tight.
Did you replace anything? If a new spacer or carb it might not be flat.
I don’t recall the value either, but I think it’s something like 12 ft lbs. I do, however, remember checking when I installed, so I’m pretty sure they’re torqued right.

The spacer is new. The carb is original and has been rebuilt.

I seriously need to check if the spacer is flat? I guess that’s not ridiculous, considering the fit of some of the parts I’ve had recently. I’ll be sure to check when I’m doing the job.

Alex


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Holly says 60-80 in-lbs. (5-7 ft.-lbs). I'd check with your carburetor's manufacturer website.
Also, did you use the correct gaskets. There are a lot of variations. I'm no expert but I've seen some very different gaskets supposedly for the same carburetor and clearly at least one was wrong.
You can use any vacuum port on the manifold. Here's a chart that might help:

vacumm-reading-1.png
 
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Discussion Starter #7
Holly says 60-80 in-lbs. (5-7 ft.-lbs). I'd check with your carburetor's manufacturer website.
Also, did you use the correct gaskets. There are a lot of variations. I'm no expert but I've seen some very different gaskets supposedly for the same carburetor and clearly at least one was wrong.
You can use any vacuum port on the manifold. Here's a chart that might help:

View attachment 763273
I’m trying a different gasket this time. Fel-pro, says it’s for Ford v8s and for my application


Thanks for the chart, that has some useful information. I appreciate it!

Cheers, Alex


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Discussion Starter #8
Just in case this is useful: the engine has been completely rebuilt and has no road miles on it at ALL. Only use is from break-in and idling.


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The gaskets are a known problem. The Autolite 4100 requires a specific gasket, not a generic gasket. I'm not sure which gasket is the correct one but I know the wrong one creates vacuum leaks.
 

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Discussion Starter #10
The gaskets are a known problem. The Autolite 4100 requires a specific gasket, not a generic gasket. I'm not sure which gasket is the correct one but I know the wrong one creates vacuum leaks.
Dang, that’s not great news. I’ll give these a try then and if they aren’t the right ones I’ll make a dedicated search for the ones that work best.

Thanks again for the wisdom!
Alex


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I dont mess around!

Will make even the wrong gasket right and lots of times you can even reuse the gasket with another coat:)
 
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Discussion Starter #12
I dont mess around!

Will make even the wrong gasket right and lots of times you can even reuse the gasket with another coat:)
You apply on both sides of the gasket when installing?

Cheers, Alex


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You apply on both sides of the gasket when installing?

Cheers, Alex


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No, just the leaky side.:rolleyes:
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I’m trying a different gasket this time. Fel-pro, says it’s for Ford v8s and for my application


Thanks for the chart, that has some useful information. I appreciate it!

Cheers, Alex


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I had a vacuum leak on a 4100 and used these same gaskets. I did check the bottom of the carb and the top of the intake for flatness. They worked.
 

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Discussion Starter #15
I had a vacuum leak on a 4100 and used these same gaskets. I did check the bottom of the carb and the top of the intake for flatness. They worked.
Thanks for pitching in with your experience! I just got these installed. Use a dab of sealer on both sides of the gaskets just in case, and the engine sounds better already. I'll have to get the vacuum gauge hooked up so I can get a reading.
 

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Discussion Starter #17
IMG_2472.jpg

IMG_2471.jpg

Hey all,

Circling back around with my vacuum values. It’s clearly too low. I sprayed carb cleaner all around the carburetor spacer and carb, and have not gotten an increase in RPM like before. Same with the intake manifold; no effect when spraying.

Any ideas? What to check first? What could be wrong?

Thanks!
Alex


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Timing set right? Is this a stock cam? Aftermarket cam may have lower vacuum as there is more overlap. This is a full vacuum port not the ported/timed vacuum port?
 

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Discussion Starter #19
Timing set right? Is this a stock cam? Aftermarket cam may have lower vacuum as there is more overlap. This is a full vacuum port not the ported/timed vacuum port?
Timing is currently ~12° BTDC. Cam is not stock— would the cam card have the vacuum number on it?

I plumbed the vacuum gauge to the rear PCV valve hose going to the carb spacer port.

Thanks for pitching in!
Alex


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My old cam I would only get 12hg. I replaced it and I can only get 15-16hg now but it has less overlap on the new cam then the old cam. The cam card won't give your vacuum but you can plug in your numbers into overlap calculators to figure out your overlap see if its a lot or not. Also what is your idle set to? Also messing with he idle mixture screws can also change your vacuum as well, so that needs to be tuned as well.
 
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