Vacuum Advance question...

[rbond]

In reading several well written posts on this topic... I understand how the spark advance system works... or is supposed to work... but in my case, I'm scratching my head...

F350 7.5L with CA emissions shenanigans making life more complicated...

I've read that after setting timing/idle (8˚@ 800 for me) you should be able to re-attach the vac hose to the advance diaphragm and the idle shouldn't change... but mine does... increases to about 1100 from 800 rpm and I'm suddenly at almost 30˚ of advance

I have the usual 15" of vac (a bit higher actually) at idle at the vac advance diaphragm... which, according to my calibration decal, is plumbed as straight manifold vac once the engine has reached OT. Before OT, the vac advance is getting vac source through a restrictor.



So, I popped the dist cap off and put a manual vac pump directly on the advance diaphragm just to see how much advance is applied at 15" and the plate basically rotates to full advance. When I apply a timing light at idle with the vac hose attached I'm at 29˚ advance and my idle jumps up by 300rpm. I would think that this much advance at lowish rpm would cause the motor to come to a complaining halt.

This can't be right... right?  Shouldn't I only be getting about 5" of vac at the advance diaphragm at idle? At this extreme amount of advance, the system has nowhere to go at higher rpm.

The Vac Advance unit is new... and the spring tension was set to mimic the replaced unit
The Spark Coil is new... and rotates freely

... so what am I missing?

[Rembrant]

rbond wrote

This can't be right... right?  Shouldn't I only be getting about 5" of vac at the advance diaphragm at idle? At this extreme amount of advance, the system has nowhere to go at higher rpm.

The Vac Advance unit is new... and the spring tension was set to mimic the replaced unit
The Spark Coil is new... and rotates freely

... so what am I missing?

Firs things first, the vacuum advance and centrifugal (RPM) advance are activated separately, so even if the vacuum advance is maxed out, the centrifugal advance will still work.

I don't know how this is supposed to work on a CA emissions vehicle, but I know that with normal ported vacuum, you won't have any vacuum at idle.

You can adjust the vacuum advance spring until it is not activated at idle. If you tighten up the spring, it will still advance the full 20 deg, it just won't begin until later.

I can't say much more than that...maybe one of the other guys on here is familiar with the CA calibration...

[PetesPonies]

I would bag the stock mess and run it the way it should be done. Who is going to know the difference? The hourly people at inspections? yeah right.

[rbond]

PetesPonies wrote

I would bag the stock mess and run it the way it should be done. Who is going to know the difference? The hourly people at inspections? yeah right.

wish I could... but I have to pass smog first

So... what would be the "way it should be done"?

[rbond]

Rembrant wrote

Firs things first, the vacuum advance and centrifugal (RPM) advance are activated separately, so even if the vacuum advance is maxed out, the centrifugal advance will still work.

... right... so if the vac adv is maxed out all the time... why even have it? You could simply adjust for it in timing setting

I don't know how this is supposed to work on a CA emissions vehicle, but I know that with normal ported vacuum, you won't have any vacuum at idle.

that was my understanding as well... hence my confusion...

You can adjust the vacuum advance spring until it is not activated at idle. If you tighten up the spring, it will still advance the full 20 deg, it just won't begin until later.

True... and I may try that. But still baffled by the full vac draw at idle...

I can't say much more than that...maybe one of the other guys on here is familiar with the CA calibration...

[Rembrant]

rbond wrote

... right... so if the vac adv is maxed out all the time... why even have it? You could simply adjust for it in timing setting

Well, you could adjust it to account for the maxed out vacuum advance, but then it would sort of work in a backwards fashion. Remember, the reason that vacuum is there is because the throttle plates are closed at idle. Once you step on the gas and open the throttle plates, vacuum decreases. That would cause your timing to retard terribly IF you had it set to account for max vac advance. So...don't do that. The vacuum advance is beneficial, you want it to work correctly.

I think what Pete was suggesting was to run the vacuum advance as it was normally run (outside of CA, I guess) with a ported supply off the carb. This would ensure zero vacuum at idle.

Or, in my case, using the ported vacuum on my Holley carb, I was still seeing about 10" vacuum at idle, so I simply adjusted the advance so that it doesn't activate until it pulls 10" vacuum. The vacuum advance adjustment doesn't change the amount of vacuum, it just changes when the vacuum is applied. There's about 20 degrees available there, and it takes about 10" vacuum to go from 0 to 20. This is why at 15", you're already pegged at full advance. Adjusting the spring tension simply moves the 10" of travel. Mine is activating now (approximately) between 10" and 20" of vacuum, instead of zero to 10".

Mine may not be correct where it is, but the truck is working really well, so I left it alone as it is. I'm going to do more ignition curve tuning later on, so I consider my current set-up a temporary one.

Or...last option, is to leave it all as it is, and try to find out what the California calibration is exactly. I assume it is in the service manual...what year is this thing again?

[ArdWrknTrk]

rbond wrote

In reading several well written posts on this topic... I understand how the spark advance system works... or is supposed to work... but in my case, I'm scratching my head...

F350 7.5L with CA emissions shenanigans making life more complicated...

I've read that after setting timing/idle (8˚@ 800 for me) you should be able to re-attach the vac hose to the advance diaphragm and the idle shouldn't change... but mine does... increases to about 1100 from 800 rpm and I'm suddenly at almost 30˚ of advance

I have the usual 15" of vac (a bit higher actually) at idle at the vac advance diaphragm... which, according to my calibration decal, is plumbed as straight manifold vac once the engine has reached OT. Before OT, the vac advance is getting vac source through a restrictor.

 When I apply a timing light at idle with the vac hose attached I'm at 29˚ advance and my idle jumps up by 300rpm. I would think that this much advance at lowish rpm would cause the motor to come to a complaining halt.

This can't be right... right?  Shouldn't I only be getting about 5" of vac at the advance diaphragm at idle? At this extreme amount of advance, the system has nowhere to go at higher rpm.

... so what am I missing?

You always need to reset the throttle stop after reattaching the vacuum line.

With this system you will see quite a bit of advance at idle.
This is an emissions engine and with more advance there is more time for fuel to burn before the exhaust valve opens.

 The VCV should apply restricted vacuum to the advance can unless the truck starts overheating.
At that point full manifold vacuum is applied, which increases idle speed -making the water pump and fan turn faster- to help cool the engine.

[baddog8it]

Wow! A smog test on an '86!  Are you in California?  Can you clue us in to what that entails?

Here in Missouri, years ago, we actually had to put our vehicles on a dyno with a sniffer up the tailpipe.  What a pain.  Now it's just a matter of checking the OBDII for codes.  If there are no codes, it passes.  Of course, only vehicles with OBDII are required to do this.  This is part of the reason that I insisted on my (then) next vehicle (The Beast) be pre-1995.  The emissions test is only required in some counties in/around the St. Louis area, and some counties in the Kansas City area.  There are also some exemptions for SOME diesels and vehicles over a certain GVWR.

[Steve83]

rbond wrote

...the plate basically rotates to full advance.
...

... so what am I missing?

The centrifugal advance (as Rembrant noted), and the fact that the restriction SLOWS the vacuum advance's action after the engine is started, and the detailed instructions printed on the REST of the VECI label.

[rbond]

ok... thx for all the feedback.

I always think of vacuum advance as working "in conjunction" with centrifugal... that it would "add to" the effect of the centrifugal... but in this case, it's working to compensate for "load"... bringing the point of ignition closer to TDC as the throttle opens more.

Rembrant... your reply was very helpful to clarify my question... thx. Your solution makes sense. As it is right now, my advance unit is set to start pulling a 5"... seems a bit low. I'll tighten up the spring on the advance unit to 10" and see how it responds... adjust for the faster idle.

Thx, Gents.

[rbond]

baddog8it wrote

Wow! A smog test on an '86!  Are you in California?  Can you clue us in to what that entails?

Here in Missouri, years ago, we actually had to put our vehicles on a dyno with a sniffer up the tailpipe.  What a pain.  Now it's just a matter of checking the OBDII for codes.  If there are no codes, it passes.  Of course, only vehicles with OBDII are required to do this.  This is part of the reason that I insisted on my (then) next vehicle (The Beast) be pre-1995.  The emissions test is only required in some counties in/around the St. Louis area, and some counties in the Kansas City area.  There are also some exemptions for SOME diesels and vehicles over a certain GVWR.

In CA for vehicles pre OBDII, we get the tailpipe treatment... and the DMV has really tightened up on the testing process... used to be a more "manual" process where you could get away with iffy engines depending on the mood of the tester... now, all the data is sent directly to Sacramento... so... yeah.

[rbond]

ArdWrknTrk wrote

You always need to reset the throttle stop after reattaching the vacuum line.

With this system you will see quite a bit of advance at idle.
This is an emissions engine and with more advance there is more time for fuel to burn before the exhaust valve opens.

 The VCV should apply restricted vacuum to the advance can unless the truck starts overheating.
At that point full manifold vacuum is applied, which increases idle speed -making the water pump and fan turn faster- to help cool the engine.

... thx for confirming this... I had read in several posts that vacuum at idle should be pretty much zero... or not enough to affect the advance... but clearly, that is only on ported vacuum systems.

As for the VCV... I'm getting a slight variation... I get restricted vacuum until OT... then I'm at full manifold. I tested when the valve opens up... and it's actually opening a little before the temp stabilizes. Not sure that there's any variance for "overheating" unless the valve opens up further. But at OT I'm already at full manifold vac through the VCV.

[ArdWrknTrk]

There are very good instructions available on the Jegs website.

If you search there for the Comp Cams adjustable advance unit for Ford/Mercury you'll see a PDF link that goes through all the steps in correct order.

Also, Bill (85LeBaron2t) owned and ran a carburetor/tune-up shop for years.
He is very detailed in his replies.

[rbond]

ArdWrknTrk wrote

There are very good instructions available on the Jegs website.

If you search there for the Comp Cams adjustable advance unit for Ford/Mercury you'll see a PDF link that goes through all the steps in correct order.

Also, Bill (85LeBaron2t) owned and ran a carburetor/tune-up shop for years.
He is very detailed in his replies.

... found it... you're right... very informative doc!

thx

[ArdWrknTrk]

rbond wrote

As for the VCV... I'm getting a slight variation... I get restricted vacuum until OT... then I'm at full manifold. I tested when the valve opens up... and it's actually opening a little before the temp stabilizes. Not sure that there's any variance for "overheating" unless the valve opens up further. But at OT I'm already at full manifold vac through the VCV.

I'm really interested in this, and wonder if the wax pellet inside a VCV can go bad or leak.

I do understand that the one controlling the EGR would shift ports at OT but was under the impression that the one for the distributor didn't move until ~250-F.

Sorry I can't offer more specific advice for your 1986 California model.
I do hope you can get this into compliance with the emissions inspection there.  

[rbond]

ArdWrknTrk wrote

I'm really interested in this, and wonder if the wax pellet inside a VCV can go bad or leak.

I do understand that the one controlling the EGR would shift ports at OT but was under the impression that the one for the distributor didn't move until ~250-F.

Sorry I can't offer more specific advice for your 1986 California model.
I do hope you can get this into compliance with the emissions inspection there.  

I agree, that would make sense... and I'm fascinated by it too as I'm on a steep learning curve here. In my case the VCV is brand spankin new... (I replaced most all the vacuum components I could based availability) so I would expect it to be operating correctly.

I set up my vacuum system to match the sticker for my vehicle verbatim... (it had been modified by the previous owner as I found out) and I tested the new VCV which controls the Vac Adv diaphragm... mostly to see how it functioned. It's a 3 port unit with the center port routed to the Vac Adv unit... and when cold, the
 vacuum source is fed from the top connection via a blue "restrictor"... then when the thermostat opens up... the vac source is shifted to the lower port which is straight manifold vac.

Since this vac system is basically controlling advance at "load" (which I'm now understanding much better thx to this and other threads ;) I would surmise that the purpose of the restrictor is to slow the vac advance timing to the diaphragm if the motor is under "load" pre OT... otherwise... it still builds up full manifold Vac eventually. The restrictor doesn't limit the amount of vacuum change... only the amount of time it takes to achieve complete vacuum change.

I'm still amazed that at 30˚ of advance at idle... the thing still runs... but hey...  That would never fly on an Alfa motor... even at 4 cyl.

[Rembrant]

rbond wrote

I'm still amazed that at 30˚ of advance at idle... the thing still runs... but hey...  That would never fly on an Alfa motor... even at 4 cyl.

My '84 302 had 29 degrees initial timing when I bought it. It would ping like a mofo with even the slightest bit of load on the engine...lol. Once the centrifugal and vac advance was included, I have no idea where it was...60 degrees?. The vac advance was partially seized and only moved about 10 degrees, but the centrifugal advance was still working.

[85lebaront2]

Your California 7.5L is just a little different from what mine used to be and Jim's (ArdWrknTrk) has. There are some minor changes in some of the plumbing, if I remember correctly from another person I helped unravel the maze it was the bowl vents for the carb, being electric rather than vacuum operated.

The purpose of (a) the vacuum restrictor is to delay the rate of vacuum advance until the engine is at the temperature where it needs a rapid vacuum response, and (b) the bypass to direct manifold vacuum is done when the engine is hot enough to need it.

Ford used a ported spark advance for years and early emission engines actually retarded the timing at idle to allow a very lean mixture to ignite with the breaker point ignition systems. These only put out about 25,000 volts for the spark and would not fire a super lean mixture. Static timing on a lot of these was around 6° BTDC, some engines by other manufacturers ran timing at ATDC settings and many had both vacuum advance and retard units on the distributors.

With the introduction of electronic ignition systems starting around 1974 engineers discovered what hot rodders had know for years, a hotter (higher voltage) spark would jump a wider gap and fire a lean mixture with no problems. The systems evolved to even hotter spark (Duraspark II on Fords) and required a larger span between distributor towers and larger plug wires to prevent the spark from jumping to the wrong cylinder tower or arcing to ground.

The use of full idle vacuum through a restrictor allows the engine to run cleaner with everything functioning than the older ported spark system.

If you think 30° is a lot, the computer controlled systems on the EFI engines would blow your mind, my 7.5L idles at somewhere between 28-32° and full advance at cruise is in the neighborhood of 60° BTDC.

Good luck on passing smog, once everything is right, a fresh oil change will help also.

[grumpin]

Interesting info. I had my OBDII tester hooked up to the Suburban the other day.

I was looking at live data, the spark advance at idle was 25°.

[1986F150Six]

An unloaded engine, whether @ idle or at cruise conditions can tolerate quite a bit of advance.