vacuum advance

[citizendick]

hello fe guys: on my stock 390 i developed a rough idle, so i adjusted the carb, timed to 10 btdc i have a rock steady 20 in. of vacuum, runs and idles great, but when i hook vac back up to dizzy rough idle is back. my ? is how much vacuum should it take to pull the advance from start to full advance, mine starts at 10in.vac 30 in. for full advance. the diaphragm does not leak. thanks citizendick

[montana_hiboy]

Are you pulling vacuum from a ported source (above the throttle plates) or a manifold source (below the throttle plates)

[Thunderfoot]

Sounds like you are hooked to the manifold vacuum port on the carb, there should be almost no vacuum at the distributor at idle if hooked to port vacuum.
Also, be sure to check your total advance of the distributor (mechanical and vacuum) at about 3,000 rpm to see what you have for total advance. Should be about 32-35 deg total.
The only reason I point this out is that on mine the mechanical had 25 deg by itself and the vacuum was 20 deg by itself for a total of 45 deg, this with my initial of 12 put my total at almost 60 deg I just left my vacuum one unhooked and set my total at 32 and have about 14 initial. Just the FYI to check the total advance...

[citizendick]

ok this is an edelbrock 1405 i'm using the port on the front left side (the lowest port on the carb) at idle it has 20 in of vacuum, it did idle fine before, i'm guessing something is wrong here. dick

[Thunderfoot]

ok this is an edelbrock 1405 i'm using the port on the front left side (the lowest port on the carb) at idle it has 20 in of vacuum, it did idle fine before, i'm guessing something is wrong here. dick

That is the wrong port for the distributor, that one is manifold vacuum, it has the most vacuum when the carb has the butterflies shut for idle. Use the port on the right side that is a little higher, this is the port or venturi vacuum, it has more vacuum when the butterflies are open which is what the distributor is looking to do (advance with more throttle / air flow into the motor).

[convincor]

First read this- http://www.fordmuscle.com/forums/all-fo ... -port.html
My 72 which had a Edelbrock. It liked the timed port.
The Cobra's built 390 with a Holley Street HP likes full vacuum.
See what yours likes better.

Here's another- http://www.fordmuscle.com/forums/all-fo ... vance.html

[Thunderfoot]

First read this- http://www.fordmuscle.com/forums/all-fo ... -port.html
My 72 which had a Edelbrock. It liked the timed port.
The Cobra's built 390 with a Holley Street HP likes full vacuum.
See what yours likes better.

Here's another- http://www.fordmuscle.com/forums/all-fo ... vance.html

Good info. good read and sound theory I'm going to have to play with that on mine some and see how that works out my truck

[citizendick]

good link convincor , mine runs great w/vacuum disconnected, hooked up will not idle,the 20in of vacuum seems to be pulling dizzy to full advance, could a spring in dizzy be broken? thanks dick

[peanutman]

good link convincor , mine runs great w/vacuum disconnected, hooked up will not idle,the 20in of vacuum seems to be pulling dizzy to full advance, could a spring in dizzy be broken? thanks dick

I got this from an old retired ford mech. Ford autolite dizzys late 60s up until when they started adding electronic ign, the plates and springs was bad about wearing out and causing shorts and throwing the timing off. These worn parts was magnified thru the vac advance and sometimes just the manual advance (VAC ADVANCE NOT HOOKED UP) when engine speed is increased. Look for a worn out dizzy. You can replace it for around $70 and it comes with new points and condenser.

[mr_josh]

good link convincor , mine runs great w/vacuum disconnected, hooked up will not idle,the 20in of vacuum seems to be pulling dizzy to full advance, could a spring in dizzy be broken? thanks dick

How do you know that the diaphragm isn't leaking?

Have you put a timing light on the truck with the vacuum advance connected to make sure that it is actually pulling the timing advanced?

Have your tried moving the distributor to the ported vacuum source like Thunderfoot suggested?

[citizendick]

i checked the diaphragm w/20in.of vacuum and it held the pressure, today i timed it w/vac hose hooked up, now she runs and idles great, readjusted carb and getting 22in.of vacuum. i really don't get it but it runs strong through all rpms and the advance works. citizendick

[fordman]

i have to agree to check the springs and weights in the dizzy for being worn.