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How much should manifold vacuum advance timing at idle?


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So all of the carb adjustments I've done to date have been with all of the vacuum lines capped. I can get the car where I want it, idle at 850-900 in park, timing at 16*. All good and well. But if I hook up the dizzy to manifold vacuum, timing jumps up to close to 30, and the RMPS jump up to 1200. That's still in park at idle. I expected a slight increase in timing and rpms. but very slight. I don't even make that much vacuum. The dizzy is a Mallory Unilite, that I will be swapping out for the Duraspark soon anyway, but doesn't this sound like way too much advance at idle?? 

Edited by Big Red Mach 1

Chris - BIG RED MACH 1

Born in '73 - Drive a '73

Former U.S. Army 63B10-H8

1st Infantry Division - The Big Red One

 

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Hook the distributor to "ported" vacuum instead of manifold vacuum. The manifold tube is fed from below the throttle blades and ported is fed from just above the blades. In other words, ported is manifold vacuum it just comes into play when the throttle blades are opened bit off idle. Chuck

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You may have to drill a small hole in the front of each primary throttle plate  so that you will not have to turn the idle stop screw so far.  The sweet spot is around 1 turn after the screw contacts the lever.  If you have a Holley you can accomplish the same thing by adjusting the secondary throttle stop screw.

By having the stop screw turned so much the next circuit in the carburetor is cutting in and the ported vacuum is open to the vacuum advance, this effect will happen no matter which carburetor you are using.

You can google the process or get a Holley tuning book from Barnes & Noble.

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1 hour ago, c9zx said:

Hook the distributor to "ported" vacuum instead of manifold vacuum. The manifold tube is fed from below the throttle blades and ported is fed from just above the blades. In other words, ported is manifold vacuum it just comes into play when the throttle blades are opened bit off idle. Chuck

I'll give that a shot. Thanks Chuck. 

Chris - BIG RED MACH 1

Born in '73 - Drive a '73

Former U.S. Army 63B10-H8

1st Infantry Division - The Big Red One

 

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33 minutes ago, Kenny nunez said:

You may have to drill a small hole in the front of each primary throttle plate  so that you will not have to turn the idle stop screw so far.  The sweet spot is around 1 turn after the screw contacts the lever.  If you have a Holley you can accomplish the same thing by adjusting the secondary throttle stop screw.

By having the stop screw turned so much the next circuit in the carburetor is cutting in and the ported vacuum is open to the vacuum advance, this effect will happen no matter which carburetor you are using.

You can google the process or get a Holley tuning book from Barnes & Noble.

I've actually got holes in all 4 now. Was the only way to keep the transfer slot "boxed" and be able to idle properly. Prior to this, I had to use the main idle screw which opened up the transfer slot too much and rand so rich that there was no longer any adjustment at the metering block screws. This thread is really just about my timing jumping to 30 by connecting to manifold vacuum. 

Chris - BIG RED MACH 1

Born in '73 - Drive a '73

Former U.S. Army 63B10-H8

1st Infantry Division - The Big Red One

 

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Ported didn't advance me at all at idle, and I am totally fine with that. 

Chris - BIG RED MACH 1

Born in '73 - Drive a '73

Former U.S. Army 63B10-H8

1st Infantry Division - The Big Red One

 

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If the rpms jump with the manifold vacuum connected, some of the timing increase could be due to centrifugal advance. Are you using the DVCV to route vacuum lines between the dizzy advance module and ported/manifold?

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To answer your question directly, the timing will advance whatever the vacuum advance will move at that inch of mercury reading. If it'll move 16° at 16" mercury, then that's what it'll move if your idle vacuum is 16". As Steve noted, you may run into the start of the centrifugal curve if the idle increases enough. 

I'm a big proponent of using the DVCV valve as Ford intended, ported vacuum at normal temps, and switching to manifold to increase airflow if the engine gets hot. 

You may find that you'll need to tune the vacuum advance to your specific combination once you start driving it. Keep in mind that "most" adjustable vacuum advance units don't adjust the degrees of advance, but only the "rate" by changing tension on an internal spring. You can adjust the degrees of advance simply by adding tubing of various wall thickness over the 90° arm on the vacuum advance rod, or by physically adjusting the slot. 

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Posted (edited)

Is our DVCV valve the 2 port or 3 port? See a lot of both out there. 

Edited by Big Red Mach 1

Chris - BIG RED MACH 1

Born in '73 - Drive a '73

Former U.S. Army 63B10-H8

1st Infantry Division - The Big Red One

 

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If you had to drill holes then you have a big cam.  Vacuum advance will help run better FULL manifold vacuum, stick a allen wrench into can to adjust. Set up carb screws as previous posters, then play with idle speed with ca. Contrary to popular belief high timing at idle will NOT cause overheating, II run 12 degrees can advance + 22initial, 34 total= 46 total with can at speed ,runs great, hit the gas and vacuum goes away, ported vacuum was for emissions, not drivability, try it out, what you have to lose?

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Posted (edited)
1 hour ago, B229218 said:

If you had to drill holes then you have a big cam.  Vacuum advance will help run better FULL manifold vacuum, stick a allen wrench into can to adjust. Set up carb screws as previous posters, then play with idle speed with ca. Contrary to popular belief high timing at idle will NOT cause overheating, II run 12 degrees can advance + 22initial, 34 total= 46 total with can at speed ,runs great, hit the gas and vacuum goes away, ported vacuum was for emissions, not drivability, try it out, what you have to lose?

Yes the car does have a cam. Not one that people seem to care for very much. it's a Comp Cams 270H-10 https://www.compcams.com/magnum-224-224-hydraulic-flat-cam-for-ford-351c-351m-400m.html

Very interesting. I have to admit that I've never attempted to adjust my vacuum advance. 

Edited by Big Red Mach 1

Chris - BIG RED MACH 1

Born in '73 - Drive a '73

Former U.S. Army 63B10-H8

1st Infantry Division - The Big Red One

 

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All I can add is from my own "learning as I go", so not necessarily what others believe to be correct. Much of this has been gone over many times in the past. 

Some of this is going from memory, so I might be wrong. Big Red and I have (if I remember) the same Holley S/A 670 (4150) carb. Mine has a 3/32" hole in each primary plate and I do not have a big cam, just barely over stock for the 351C M code. These carbs a rich out of the box and mine was tuned by a professional tuner. The secondary plates are not cracked at all, set only so they don't stick. One difference is mine is a 4 speed trans, not auto, so idle speeds are a bit different. As for the DVCV 3 port switch, that only is for a dual diaphragm canister to retard timing when the engine is hot, like in traffic. I don't think it makes any difference when used with a single diaphragm canister and why mine is long gone and that port used for my idiot light sender. I do run the distributor advance on timed port as I have found it runs better. As for timing, it has 14 deg. initial, 20 deg crank + 4 deg on the vac canister with 17" Hg. at idle.

To go to Hemikillers post, I altered the distributor slot to equal a 10L (.410" width) or 20 deg. I do have a spare with a factory 13L slot and a piece of Nylon tubing over the post. This will give a very close approximation and cut the crank from 26* to around 20-21*, so a quick way to cut the crank degrees and increase the initial. I can't be sure about all years, but this 71 had only 6 deg. initial plus a slot size of 15L or 30 deg crank and we all know Cleveland's like 14 -16 initial. Below are a couple of pics to explain., one is the altered slot and the other is a piece of Nylon tubing over the pin, post or whatever you want to call it.

Just my opinion, take it or leave it.

IMG_0435.JPG

DSC00257_LI.jpg

Geoff.

 I learn something new every day!

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