Vacuum Advance question

[lesingepsycho]

I have the Pertronix Flame Thrower II complete distributor with vacuum advance.

I had the vacuum advance hooked up to manifold vacuum even though I see now that it says it is supposed to be ported vacuum. Is there a possibility I blew-out the diaphram? It will not hold vacuum. When vacuum is applied, it will pull the breaker plate forward but then it quickly bleeds off.

Secondly, if it supposed to go to ported vacuum and my SUs don't have a ported vacuum outlet (AUG-549), then where/how should I be hooking it up? (Assuming I have a good diaphram)

JACK

 

[Billm]

Since noone else is talking I'll just put in my 2 cents worth-
Hooking your vacuum advance up to the manifold is about as useless as not hooking it up at-all. It MAY help to start a really balky engine but once it is running then there is no benefit.
Once the engine is running the manifold will have somewhere between 10 and 29 inches of vacuum, both of which will pull the vac-advance plate in the distributor to its full travel and will virtually never allow it to go anywhere else. The only way to see ANY movement would be to go at full throttle, in 4th up the side of a cliff and just before the engine died the vacuum plate will move some.
The port on the carb (if it has one, some sets don't) will regulate the vacuum correctly, the manifold won't.
About the only way to put a port on a carb is to drill out the boss area that should be there but I wouldn't try it until I had studied it until I knew exactly where and how deep to go and then I probably wouldn't.
If I were you I would procure another carb body that has the port and go from there.
Sorry
BillM

 

[jvandyke]

I was foggy on this so I googled and got this;
FWIW

I don't know anything specific about that distributor though.
manifold vs. ported

 

[lesingepsycho]

WOW. Great tech article! Thanks, that was very informative!

JACK

 

[Bayless]

I think I finally understand.

 

[jvandyke]

That was a good one. If I'm understanding it correctly, you really do want manifold vacuum as it changes with engine speed (throttle position); idle/cruise lots of vacuum so the timing is altered a little, WOT or accelerating: little vacuum therefore the vac advance is not effecting timing, it's just static (where you set it at idle) and mechanical (the weights in the dizzy).
Ported vacuum (the air filter side of the throttle plate) will have the opposite, almost no vacuum at idle and very little at cruise (plate closed or nearly) and therefore no vac advance when it's needed most, more vacuum at WOT (throttle plate wide open) but still not nearly as much as the manifold side would see at idle/cruise.

I think Bill, that as soon as you open the throttle, the vacuum falls off and the vac advance diminishes immediately, not only at WOT up a hill in 4th. I had a vacuum gauge hooked up for a while, sometimes they are/were "economy" meters. If manifold vacuum is high, you're at cruise/idle your throttle is closed mostly and you're not using much fuel. If you're accelerating hard, throttle open, manifold vacuum low gauge reads low or "poor" fuel economy.

 

[Pythias]

I too have seen vacuum gauges used as "ecomony" gauges, but when used as such they are totally misleading. Studies performed by BMW in the mid 1980's and again in the late 1990's were run. Nine total "conditions" were used for acceleration.. in combination, light, half and full throttle, and shifting at 2000, mid rpm and redline. the tests were done with multiple cars IIRC. The surprising result was that it's more economical to accelerate full throttle to redline, than it is to ease on the gas and shift at 2000 rpm! The BEST is full throttle to 2000 rpm. The worst is to ease the throttle all the way up to redline. As a result of those studies BMW released the 325 ETA. The engine was tuned to deliver maximum low end torque (for a BMW anyway).

It has since always amused me to see driving tips for economy warning against "jack rabbit starts" as being wasteful. Quite the contrary, they're fuel savers!

 

[jvandyke]

That's the best news I've heard all day!

 

[Roger]

Be careful with this advice. What works for a GM product ain't necessarily so for a BMC. Distributor advances are different, and so for that matter are carburetter characteristics.
Vacuum advance has the primary effect of retarding the ignition for low speed, high throttle opening conditons - like setting off from rest, or accelerating up a hill, particularly with fixed-choke carburetters without accelerator pumps, which is where the idea started pre-WWii.

Which is "better"? Start with the one your equipment was designed for, then try the other!

 

[Guest]

My 1500 has a vacuum RETARD, not adavance.

I don't know what you have, but better make sure it's gonna go in the right direction.