vacuum advance
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Junior User
Joined: Jun 2006
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vacuum advance
on my 71 f-250, 360 v-8, i have a single vacuum distributor. the line is hooked to the advance, but when i followed it to the manifold vacuum fitting, it was unplugged, and the fitting on that side was capped off. when i hooked up, truck started running real rough, and died. got started again, and unplugged line, and it ran just fine. points are set at factory specs(new points), and engine is timed to specs. what would cause this, and since it runs fine without this, is it really necessary?
Fleet Mechanic
Joined: Nov 2004
Posts: 1,857
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From: Northern CA
It is not necessary, but it will help with fuel economy. I'd definitely reccommend hooking it up.
You need to hook the vacuum advance hose to the ported vacuum on your carb. Don't hook it directly to manifold vacuum.
What carb are you running? On a Holley it hooks on the side of the metering block near the idle mixture screw on the passenger side.
You need to hook the vacuum advance hose to the ported vacuum on your carb. Don't hook it directly to manifold vacuum.
What carb are you running? On a Holley it hooks on the side of the metering block near the idle mixture screw on the passenger side.
Last edited by bryyce62; Jul 21, 2006 at 03:32 PM.
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Junior User
Joined: Jun 2006
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running the fact motorcraft carb. noticed a vacuum port on side under auto choke body that doesnt have anything on it, but when engine is running, i dont notice any vacuum, and when i hook up advance hose, it dont make any difference. does motor have to be at certain rpm to create enough vacuum to work the advance? carb is newly rebuilt and has been vatted and cleaned, and new kit put in.
Postmaster
Joined: Apr 2002
Posts: 3,613
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From: Coal country
Good observation.
Try feeling for vacuum at that port when you lift the RPMs up a bit; it should come on almost as soon as you lift the throttle plates.
This is ported vacuum, or, vacuum that is used only when the engine is running above idle.
If a port has vacuum at any speed, including idle, it is called a manifold vacuum port, or, is said to have manifold vacuum. Manifold vacuum ports are used for transmission modulators, A/C controls, internal dash venting controls, and the like.
Ported vacuum is used for advancing distributor timing when the engine is running at more than idle.
Try feeling for vacuum at that port when you lift the RPMs up a bit; it should come on almost as soon as you lift the throttle plates.
This is ported vacuum, or, vacuum that is used only when the engine is running above idle.
If a port has vacuum at any speed, including idle, it is called a manifold vacuum port, or, is said to have manifold vacuum. Manifold vacuum ports are used for transmission modulators, A/C controls, internal dash venting controls, and the like.
Ported vacuum is used for advancing distributor timing when the engine is running at more than idle.
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Junior User
Joined: Jun 2006
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thanks so much for your replies. that easily explains the difference between ported and manifold vacuum. this port does have vacuum just above idle. this port being open may explain part of my rough idle. the advance has a factory crook on the end that almost points right to this port. i'll hook it here, and see what happens. i know my old gal is not new(one owner, 39000 actual miles), but maybe i can bring some more life to her. cant help but wonder how many 2006 fords will still be around in 2041. thanks again for a great site, and a bunch of knowledgeable people.
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All Vacuum ports directly related to the engine should be plugged.The complete vacuum system should be sealed.
Now, One thing to check to see is, if the vacuum advance is still working.
They tend to dry out and seize becoming inoperative.
If you don't have one you can borrow a vacuum pump and place the hose on the port to the vacuum advance and with the dizzy cap off,actuate the vacuum pump watching for the actuating arm inside the dizzy to be pulled toward the vacuum advance.
Now, One thing to check to see is, if the vacuum advance is still working.
They tend to dry out and seize becoming inoperative.
If you don't have one you can borrow a vacuum pump and place the hose on the port to the vacuum advance and with the dizzy cap off,actuate the vacuum pump watching for the actuating arm inside the dizzy to be pulled toward the vacuum advance.
Junior User
Joined: Nov 2004
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On my distributor there are 2 vacuum ports ('71 f100, 360, 2 barrel carb autolite 2100, 4 speed granny).
One (the one closer to the block) is connected to the ported vacuum at the carb and the other is left open. Why? Will this affect the running of the truck? Do I have to connect the other vacuum port to the manifold?
One (the one closer to the block) is connected to the ported vacuum at the carb and the other is left open. Why? Will this affect the running of the truck? Do I have to connect the other vacuum port to the manifold?
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New User
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IIRC, That is a retard/advance vacuum system.
Beginning in 1968, Ford went to a dual advance/retard unit with two vacuum hoses. This happened because of the need for reduced emissions. Under acceleration, it advances the spark. During deceleration, it retards the spark to reduce emissions. Despite Ford's best efforts, the dual advance/retard system (called IMCO for improved combustion) never worked very well.
Beginning in 1968, Ford went to a dual advance/retard unit with two vacuum hoses. This happened because of the need for reduced emissions. Under acceleration, it advances the spark. During deceleration, it retards the spark to reduce emissions. Despite Ford's best efforts, the dual advance/retard system (called IMCO for improved combustion) never worked very well.
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Post Fiend
Joined: Dec 2003
Posts: 6,658
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From: "Islander"
Mil1on; on 68 and 69 360 powered trucks here in California (have both) they still had single vacuum advance units from the factory. Also that is all I have seen the last 38 years looking under many truck hoods.
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