bouis

Okay, here's what I've learned about vacuum advance.

The vacuum advance absolutely will not work if you follow the rats nest removal tutorial. There's just no vacuum from that nipple, ever.

Ideally, the vacuum advance should advance the leading timing a total of 4.5 degrees leading and 15 degrees trailing. Additional manifold vacuum will not advance the timing any more. This should happen only on throttle, but should fall off before 4000 RPM. Mechanical advance should be 25 degrees total.

I can't think of any solution to get this working properly. Hooking up the vacuum advance to the switched vacuum on the manifold nipple that controls the purge valve causes the vacuum advance to work on throttle as it should, but it never cuts off before [or after] 4k rpm, either. This amounts to 29.5 degrees of total advance. Hooking it up this way could cause detonation (though I never heard any or saw evidence of detonation).

Presumably, the vacuum advance operation is controlled by some of those soleniods we removed. I'm going home soon, and when I get there I'll figure out exactly how much of the rats nest is required for proper vacuum advance functioning.

j9fd3s

iTrader: (3)

i put a vacuum guage on my 83, the advance works like this.

0 throttle: solenoid off, not advancing

above 0 throttle, solenoid on, advance from engine vacuum. vacuum is greatest at cruise, and least at wot.
solenoid stays on after 4k (at least on the 83)

bouis

So it does stay on after 4k? This goes against what some people have said. I wish I had a car with the full rats nest to experiment with.

Rx7carl

Update: The vacuum advance pots go the 2nd nipple counting front to back. The solenoids only cut vacuum advance under certain conditions using the emissions control unit and switches on the transmission. This is for emissions control only and does not have any affect on driveability. The vacuum advance, once activated will continue to see vacuum but becomes effectively IN-effective once the mechanical advance takes over, as the vacuum advance only goes to a certain degree and stops. HTH and I apologize for any mistakes in the tutorial.

aGoGo

Ummm....... Should I follow the tutorial then ? Thanks

RotaryRyan

yes. I think so

bouis

I'm confused. Carl did you put a vacuum gauge on it? 'Cause I've never seen any vacuum from that nipple.

Jeff20B

iTrader: (13)

Yikes! I guess I'll keep drinkng 7UP. Or no wait, make that Mountain Dew.

bouis

I can also say from experience that, given a constant vacuum source, the vacuum advance will *not* be overridden by the mechanical advance; it'll be stacked on.

Jeff20B

iTrader: (13)

Hmm, if I remember right, 12A mechanical advance is 20º. The 5º of vacuum advance *adds to* the total advance whive gives us 25º.

Rx7carl

No, I traced the hoses on the emissions diagram.

bouis

Well, my cent advance is 25 (verified with a timing light) and it's a 12a.

Here's what the shop manual says:


I don't know what to make of this.

bouis

Well Carl, I put a vacuum gauge on it and it never sees a single cm of vacuum. So something is not right here.

bouis

Really, according to that diagram, the mechanical advance should be only 10 degrees from idle.

Jeff20B

iTrader: (13)

Heh, now I'm confused.

bouis

Yeah. I've verified with a timing light that that the vacuum advance works just like that (+4.5 leading, +15 trailing) but I'm at a loss for the mechanical advance. Mine is definately 25 degrees from 750 rpm to 4k rpm.

Jeff20B

iTrader: (13)

I can tell you my experience with GSL-SE dizzies. with no vacuum advance, they go from 0TDC at idle to 24-25ATDC at 4k RPM. So maybe I am remembering it correctly? Maybe 12A dizzies only advance by 20º with mechanical only?

j9fd3s

iTrader: (3)

the distributor advances 10-12 DISTRIBUTOR degrees witch is 20 on the CRANK.

you guys are also misunderstanding how it works. yes the advance stacks on but vacuum at wide open is almost 0, so at full load you get 0 vacuum advance. revving it in neutral with the advances hooked up will give you full mechanical advance and full vacuum advance.

again my 83 with everything hooked up stock and working only cut vacuum to the leading @0 throttle, its very possible other years work differently.

bouis

Ah, okay, that does make sense. I guess. Well, not really.

The vacuum-switched purge valve runs off the same vacuum... are you saying that it's not working at all under WOT? I wish I had some way to verify this. Maybe I'll buy 6 feet of vacuum hose and put the damn gauge in the cockpit and take it for a ride.

Sigh.

cdrad51

more like 7, if we are talking about a 12A

Junia

So, please correct me if I'm wrong. If I don't have anything hooked up to my dizzy for the vacuum advance, I'm not really missing out on anything. Is this correct?

RotaryNotary

Educate me, someone...What is WOT?

Rx7carl

Junia, as long as the ports on the dizzy stay uncapped, its not a big deal, except your milage will suffer a bit.

WOT= Wide Open Throttle

RotaryNotary

Shoulda had that one! Oh well, that's what the board is about - 1 embarassing piece of info found after another!