How can I reduce the amount of Vac advance?



I have too much vacume advacne on my engine.

I am running a DurasparkII dizzy with MSD6a hooked up with the Ford Magnetic pickup adapter. Matched to a CarterYF 1bl.

I have too much advance and need to drop it back some. Initial is fine, I cannnot retard that any more than I have it now. I just have too much advance off this carb.
Howdy Jimbo65:

Are you using a ported or full vacuum source? If you're using full vacuum try a ported source. The YF should have one.

'67 falcons suggestion also works. You disconnect the tubing from the cannister and insert the correct size Allen wrench into the opening and tighten to reduce and slow vacuum advance and loosen to increase and quicken response. Not all vacuum cannisters have this feature.

You can also drill and tap a small hole in the arm from the cannister to the distributor and insert as small screw that will interfer for the cannister body and limit advance. This is a trial and error system and should be a last choice.

Adios, David
If none of the above don't work, you can replace the vacuum advance mechanism with a aftermarket one for a V8. I bought a Crane adjustable one for mine...Just have'nt had a chance to install it yet... :D


I am not sure about the vac source on the carb.

I wish I had a picture, I will see about getting one tomorrow.

There are three ports on this carter one is at the base to the front of the engine (that is where I am currently hooked in) the other two are on the passenger side one around the throttle rod above the air mixture screw about halfway up the carb, and another at the top of the carb by the choke plate. Those are both capped off.

I will test the ports with a vac gauge and let you know, but I think it is ported, since at idle RPM's unplugging the line has no effect on the engine, and at high RPM's it pulls nicely. An intake source would have an effect even at idle correct?
Howdy back Jimbo66:

Right. Full vacuum is below the venturi and ported vacuum is somewhere above the venturi.

Did you check to see if you have an adjustible cannister yet?

what have you done to reduce or decarbonize your engine so far?

What is your initial advance set at?

Is your accelerator pump working as it should?

If it only pings when you floor it, it may be load sensitive rather than a vacuum advance problem. Vacuum advance should go away when you floor it. Hummmmm......

Keep us posted. The mystery continues.

Adios, David
Well I finally broke down and bought a Dial-back light from sears and a decent vacume gauge, here are my readings:

Cold Start: @1200 RPMs Vacume=17hg steady (Advance disconnected)
(Engine runs very rough and seems to be missing)

During Warm-up: (Advance disconnected)
RPM's drop from 1200 to 950 as chooke opens and vacume
stays constant at 17hg.
(Temp was warm here today so no trouble during warm-up that I
get when it's really cold)

At Temp: (200 degrees Advance disconnected)
@ 950 RPMs Timing 10 degrees initial. 17hg on the vacume.
Centrifugal Advance @ 10 degrees dialed back to 0 @2000 RPMs.

@ Temp: (2000 RPM's) Advance Hooked up. (Running smooth)
Mainfold vacume 17hg steady.
Vacume from carb to advance @ idle 0 / @ 2000 RPMs 17hg.
Dialed in Advance @2K is 35 degrees to move back to 10

On the old forum there was a great post about the amount of vacume and the added effect of centrifugal and advance to achieve the total engine vacume. I wish I still had that one.

Anyway, I cannot determine from the manual what my total advance and curve should be. It seems that 50 degrees above 0 @ 2000 RPM's is way too much. I shudder to think where 3000 RPM's is?

Can I get some advise on this?

Oh and 950 in neutral gives me about 600-650 is drive. I cannot get the engine down to under 600 without a drop in oil pressure.
@ 600 I get a drop in oil pressure at red lights from about 40 to 30.
I was just reading another post about advance problems and I saw a couple of things in there that made me think to add a few things here.

First I saw that cruise speeds should be around 2200?

I have a C4 auto and my highway cruise at 65 is at 3000 RPM's.
(Not sure if that helps or not.)

I am going to run the vac gauge into the car and run the car hooked up to the gauge tomorrow to see the results. I will post them back here once I get them.

Until then
Howdy Jimbo:

it sounds like you're having an adventure! Fun ain't it?

Load sensitive is when you get knock under a load. It could be anywhere from a low rpm acceleration, to passing speeds. It is frequently related to lugging an engine. It is usually less than full throttle. It is usually relate to too much vacuum advance still being called for.

WOT knock is usually related to initial and centrifugal advance, because vacuum is mostly gone under wide open throttle conditions.

Have you tried on of the other vacuum ports higher up on the carb yet?

Adios, david
I am in a holding pattern on this till Wednesday.

I am going to try the water trick to blow out some of that carbon.

And I will measure the other ports on the carb to see what vacumes I have coming out of there.

The one I am in right now has none at idle and full vacume at throttle.
I will see if there is another source that pulls less.

I will post back here tomrrow and let you know the results.