I have a rebuilt 455 , that I have been chasing what I thought was a massive vacuum leak. Only have 10 inchs on the vacuum gauge. Checked and rechecked everything and I could never find a leak. I was checking the brake booster hose and forgot to hook it back up , started the car and now with a 11/32 hose unhooked I have over 18 inches on the vacuum gauge and the engine runs great, hook up the hose back to 10 or below. 66 quadrajet with a fresh rebuild. No gas smell, checked the carburetor and it doen't appear to be flooding the engine..
Never see this before and idea what I am missing.
Thanks Nick

 

Are you stating:

(1) With no 11/32 vacuum hose connected to the brake booster you have 18"Hg? and,
(2) With the 11/32 vacuum hose connected to the brake booster you have 10"Hg?

Just want to ensure I understand what you're saying.

What is the vacuum SOURCE of your brake booster vacuum hose? Direct to manifold?

 

A couple follow-up questions:

(3) Where is your vacuum gauge connected when taking the reading(s)?
(4) What vacuum do you read directly from the manifold where (I assume) your 11/32 brake booster hose is attached?

 

brake booster vacuum is from a manifold port by the distributor, I have the gauge hooked to a manifold port by the thermostat housing.

Yes with the hose connected to the booster, I get 10 inches of vacuum. with the hose connected to the manifold and hanging open by the booster I get 18 inches of vacuum on the gauge hooked to the front.

tried something different this afternoon, hooked up the booster hose, unhooked the hose at the pcv valve. Again with all hooked up 10 on the vacuum gauge, hose unhooked at the pcv valve 18 inches of vacuum. Pcv hose is attached at the front base of the carb.

I even opened up the carb to check the needle and seat, also the float level, both were good.

I didn't check the vacuum at the back manifold port, didn't really see the need as both my gauge and booster hose are connected to the manifold. I see if I have a fitting and check it.

 

If you have poor vacuum with the brake booster hooked up it may have a ruptured diaphragm. Dont hook up a vacuum line to your manifold and leave it open. That's invalid. Put a cap on the manifold vacuum port that goes to the brake booster and then take a manifold vacuum reading.

 

A ruff running engine that soothes out when air is added (vacuum leak in this case) is usually due to an overly rich air/fuel mixture. You state the carb is rebuilt and you rechecked the float system.
Is the choke fully open with a warm engine?
Have you attempted to adjust the idle mixture screws?
With the engine running at idle speed and everything connected is fuel dribbling down the primary venturies?
Are the power enrichment components properly assembled and closed at idle speed? (Power piston down, mixture needles seated in the jets closing off fuel flow)