Vacuum Advance question...

[rbond]

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]

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]

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.

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]

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]

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]

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]

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]

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.

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]

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.

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]

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°.

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