Trying to adjust vacuum advance, no change!

[340doc]

Just did an electronic ignition conversion on the Duster. Trying to take some of the vacuum advance out. When hooked up the vacuum adv is adding about 20 degrees. I tried turning the small allen head screw both ways with no change. Which way is more or less advance? How come I get no change?

 

[RustyRatRod]

That's not how it works. You cannot remove or add timing from the vacuum advance can, only WHEN it comes in. Early or late.

 

[Woods74]

Unplug it then set total timing, then plug it back in for an addition cruise timing, 48-52 degrees total INCLUDING vacuum advance.

 

[TrailBeast]

That's not how it works. You cannot remove or add timing from the vacuum advance can, only WHEN it comes in. Early or late.

I second that motion.
Adjusting it only changes when, not how much.

How much has to do with initial plus mechanical plus vacuum, so you want to set your initial then possibly limit your mechanical for a WOT total of around 34 degrees.
THEN you connect the vac advance and adjust how soon or later in RPM/throttle opening that extra 20 comes in so as not to cause detonation at low and mid throttle.
All three together should total 50-52 degrees or so.
This all changes a little from engine to engine depending on cams, compression ratio, fuel, and other details.

 

[340doc]

That's not how it works. You cannot remove or add timing from the vacuum advance can, only WHEN it comes in. Early or late.

Oh ok, well no wonder. Thanks. That's about where I'm at Poison Dart. Thanks.

 

[340doc]

Thanks Trailbeast.

 

[TrailBeast]

Thanks Trailbeast.

Just hope it helps.

 

[jos51700]

IIRC, the total amount of advance is stamped on the arm of the vac advance (that reaches into the distributor). This requires changing canisters.

The rate of application is controlled by the allen screw.

 

[TrailBeast]

IIRC, the total amount of advance is stamped on the arm of the vac advance (that reaches into the distributor). This requires changing canisters.

The rate of application is controlled by the allen screw.

He said he measured 20 degrees added by the vac advance, so if he ends up with 54 total and brings in the vac advance a little late/high vac he will probably be fine.

If not he can change the can or bring the initial back a couple of degrees, either one.

 

[AJ/FormS]

My slanty likes advance, and lots of it. Every slanty I ever had liked it.
What's your slanty telling you?

 

[340doc]

Id like to run the total mechanical at around 33 degrees but when I add the vacuum it bumps up to 55. I then get a slight high rpm miss like too much timing. When I unplug the vacuum can while holding at that rpm the slight miss goes away. So I backed the initial down a little. That's why I want to limit the vacuum a little. I will have to check the stamping on the arm. Thanks to all!!

 

[TrailBeast]

Id like to run the total mechanical at around 33 degrees but when I add the vacuum it bumps up to 55. I then get a slight high rpm miss like too much timing. When I unplug the vacuum can while holding at that rpm the slight miss goes away. So I backed the initial down a little. That's why I want to limit the vacuum a little. I will have to check the stamping on the arm. Thanks to all!!

This is because you are using probably using manifold vacuum instead of ported off the carb.
With ported vacuum it doesn't add any advance until the RPM's are up a little and under a light throttle.
That's when you want the vacuum to come in, and then under zero and heavy throttle that 20 degrees from the can goes away.
Under zero throttle conditions (idle) your vac and mechanical advance should both be doing nothing, leaving you with your initial only.
As the RPM's come up under light throttle your mechanical and vacuum both start coming in, and a WOT the vacuum advance goes away leaving your initial and mechanical total of 34 or there abouts.

 

[mguner]

Ok guys yes and no! The strength of the vacuum signal will vary with throttle position right?! When you back out the screw you increase the pressure on the adjustment spring. IF, IF your signal and throttle position are at a point weaker than the increased spring pressure you will indeed limit total vacuum advance. To tell for sure what is happening you may want to run a vacuum gauge off a T and see what vacuum you are getting at part throttle cruise with the engine under load. Then check how much advance you get with that same amount of vacuum applied. Ported vacuum is usually better suited on a stock to mild engine. You still have the "potential" for more advance than you want but it is not likely. I backed out the adjustment screw on my 76 slant until it skipped and was able to fix an overheating issue at cruise.

 

[TrailBeast]

Ok guys yes and no! The strength of the vacuum signal will vary with throttle position right?! When you back out the screw you increase the pressure on the adjustment spring. IF, IF your signal and throttle position are at a point weaker than the increased spring pressure you will indeed limit total vacuum advance. To tell for sure what is happening you may want to run a vacuum gauge off a T and see what vacuum you are getting at part throttle cruise with the engine under load. Then check how much advance you get with that same amount of vacuum applied. Ported vacuum is usually better suited on a stock to mild engine. You still have the "potential" for more advance than you want but it is not likely. I backed out the adjustment screw on my 76 slant until it skipped and was able to fix an overheating issue at cruise.

Details, details

 

[AJ/FormS]

If you tune this can right, it has a lot of potential

 

[emsvitil]

The spring shifts when the advance starts and ends.

The range stays the same.


If the can has 0-20 degrees of advance @ 10-20 inches Hg, you can move that 0-20 to say 15-25 or 5-15 or whatever. (with 30 being the limit. in which case the max advance will also finally decrease (25-30 would be 0-10 degrees advance))

 

[RedFish]

If you are enjoying this you can look forward to doing it again someday ( when the rubber breaks ). You'll first have to determine the vacuum leak is not somewhere else.
There is a alternative. Disconnect the hose, plug it, forget it. So I've opened the same door for the same naysayers again. Want to read all about welding the slots in the centrifugal advance plates to limit that ? Go for it. Want to get out from under the hood now and never go back for a vacuum advance fault ? Try it.