Strange manifold vacuum
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Senior User
Joined: Feb 2006
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Strange manifold vacuum
Do I own the only 360 that has increasing manifold vacuum as The throttle plates open. I hooked the vacuum gauge up to a tee on the intake. one port goes to the brake booster and the other I hooked up the vacuum gauge. worked the throttle and watched the the vacuum increase. Every other motor I have had did the exact opposite. What gives? thanks, Joe
Posting Guru
Joined: May 2001
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From: Cupertino
Jodaddy, there is a port below the throttle plate that is unswitched vacuum and one above that is switched and controls the distributor advance. I think you have bitten into the top one.
Look back twenty or so threads to a post by Xstrange about how this vacuum port works. Maybe he can come in and fix you up with some advice.
Meanwhile, look for a lower port on the carb or one on the intake manifold if you need full-time vacuum.
Look back twenty or so threads to a post by Xstrange about how this vacuum port works. Maybe he can come in and fix you up with some advice.
Meanwhile, look for a lower port on the carb or one on the intake manifold if you need full-time vacuum.
Senior User
Joined: Sep 2009
Posts: 198
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From: Burbank, CA
Yep, it sounds like you're hooked into what's called "venturi vacuum" not "manifold vacuum". I'm guessing that the tee that you plugged into is in the vacuum line that runs down to the distributor advance, right?
The venturi vacuum comes from a port that's machined into the side wall of one of the carb bores, just above the edge of the throttle plate when it's closed. At that port, the vacuum is close to zero at idle, then jumps up when the throttle opens maybe ten degrees, then continues to go up in relation to the air speed through the carb. This provides the actuating power and the curve for the vacuum advance unit on the distributor. That's all it's used for on older trucks, although on newer engines it's used to power some emission control circuits (I think).
Maniold vacuum is what we normally refer to as "vacuum". It's the vacuum level in the intake manifold, under the throttles. It's pretty high at idle, then drops down as the throttle opens wider. It goes very high during deceleration.
The venturi vacuum comes from a port that's machined into the side wall of one of the carb bores, just above the edge of the throttle plate when it's closed. At that port, the vacuum is close to zero at idle, then jumps up when the throttle opens maybe ten degrees, then continues to go up in relation to the air speed through the carb. This provides the actuating power and the curve for the vacuum advance unit on the distributor. That's all it's used for on older trucks, although on newer engines it's used to power some emission control circuits (I think).
Maniold vacuum is what we normally refer to as "vacuum". It's the vacuum level in the intake manifold, under the throttles. It's pretty high at idle, then drops down as the throttle opens wider. It goes very high during deceleration.
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Joined: Jun 2003
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Adjusting mixture screws via the vacuum gauge method does not require manipulation of the throttle plates.
