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Manifold vs carb vacuum - how to tell?

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drooartz

drooartz

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Okay, I know we've discussed this before, but I was a bit confused then. I'm a bit confused now as well. :smile:

Due to some bad notes on my part (and the fact that it was over a year ago), I'm not sure which way my dizzy was set up when I had Jeff at Advance rebuild it. My carbs and manifold both have the spots for pulling vacuum. I *think* it was set up to use manifold vacuum, but I'm not sure.

Is there a good way to test and see which way it is is set up? I tried to call Jeff at Advance this morning, but they're closed until Monday. I'd like to do some tuneup work this weekend, and need to make sure the timing is correct.

The engine *seems* to be running properly with the vacuum from the manifold, but I don't have enough experience to know if it is *really* running correctly. Essentially, I don't really know what "correct" will be like.
 
I will throw out an answer just for fun. Hopefully someone who actually knows what he's talkin' about will come along soon.

If you set your initial timing with the vacuum hose disconnected from the dizzy and plugged and your idle at 1000 rpm or less, whether you use ported (carb) or manifold vacuum should only affect the idle. If your dizzy is plugged to manifold vacuum, you will get full vacuum advance at idle, if it is plugged into ported vacuum, you will get no vacuum advance at idle. If you have a strong smooth idle, you should be good to go.

I *believe* ported vacuum came along with the invention of pollution controls although I can't recall the totally suspect reasoning for it.
 
Here's my take take on it:

Put a short piece of hose onto the dizzie vacuum unit and suck on it (preferably with a MityVac).

Note which way the contact breaker plate moves. You can then determine if the vacuum unit is a retard or an advance. (eg. if the plate moves counter clockwise it will retard the ignition).

Vacuum advance is ported before the carb butterfly and retard is ported after (eg from the intake manifold)

Cheers
 
It is a vacuum advance. He knows that. He needs to know is it is designed to run from the ported vacuum of the carb or the full manifold vacuum.

I'd run it from the ported one, and worse case scenario the fuel efficiency isn't that great.

Timing is set with the advance disconnected anyway.
 
vacuum advance is designed to run with ported vacuum... No advance at idle(which is where vacuum is high) then as the throttle is opened and the vacuum moves up from the manifold to the carb ports and the engine rpm increases is when the vacuum is applied for timing advance assist(in addition to mechanical weights) and when you stomp on it, vacuum goes away from the advance so you don't detonate...

that's the design and conception per basic auto engineering and smog schooling.
 
Most vacuum advance dizzys (on Spridgets) use the carb for the advance.
Vacuum retard dizzys used the manifold.
On a Sprite, well all Sprites were vacuum advance from the carb except 1967, it was centrifical only.
These dizzys have the vacuum tip on the center of the cone.
Later Midgets, 73 & 74, maybe 72s also, used a vacuum retard system to meet the pollution laws of the times. This dizzy has the vacuum tip offset on the side of the vacuum cone. And this one needs manifold vacuum.
 
Thanks, guys. I'll be digging into this on Saturday and I think I understand a bit better now. I'll still be calling Jeff at Advance on Monday to make sure I have it hooked up the correct way.
 
Well, the real truth here is that I'm now thoroughly confused as to what I have and what I need. I'm going to wait to meddle with the car until Monday, when I can call Hap and Jeff and sort out my confusion.
 
Drew, carb to dizzy.
 
That depends, Jack, on exactly how Jeff set this dizzy up (keep in mind nothing in this engine bay is stock Bugeye). These carbs and motor are originally from a '74, so I'm not even really sure if the carbs have a vacuum port on them.
 
I would like to make a small wager Drew.
 
You may very well be correct, Jack, but if so then I probably have a problem with a mismatch of dizzy and carbs. Either way, I should have it sorted out once I can make a few phone calls.
 
A Smithwick for the winer.
 
If it was me I'd call Advanced Distributors and ask them.
 
davester said:
If it was me I'd call Advanced Distributors and ask them.
Click to expand...
That's my plan. I tried earlier in the week, but they were closed for the week. We'll get this sorted out in a few days.
 
And the winner is...

Manifold!

I talked to Jeff at Advance today, and the carbs I have (from a '74 Midget) have *no* vacuum port -- the only one available is on the manifold. So I do have it the right way, and all the parts should match up properly.

Something I should have double checked before, but at least it's all as it should be. I was figuring it was right, as the engine seemed to be running as it should be. Just nice to know it is correct.

So Jack, you owe me a Smithwick.
grin.gif
 
Well I'll be, yep, owe ya one Drew.
 
I certainly learned something.

I'll make it down there one of these days, Jack.
 
I've been gone to the vintage races at Road Atlanta, so I missed out on most of this, but did have some PMs with Drew. Actually ported vacuum, is a feature most commonly seen on the earlier carbs, later they went to manifold vacuum. The port vaccum hole is right at the butterfly in the carbs where when the butterfly is near closed to keep vacumm at minimum at lower rpms like idle, but in reality, it always drawing a bit of vacuum, because your butterfly is never completely closed. Ported vacuum is considered to be a more efficent way of pulling vacuum, as it is directly in the path of the vaccum source, where manifold vacuum has to travel a bit more to get there. Somewhere along the line with the later cars the vacuum got moved to the manifold, I guessing for something to do with emissions, but I really don't know why they did it. The old wise tale is to never miss and match carbs and distributors vacuum sources, meaning if the distributor was designed for manifold port vacuum, leave it that way.
 
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