which vacuum?

[alaskajosh]

I got a new carb (Weber 38/38) for my Robert Davis engine. This carb has a couple of vacuum ports to choose from to supply the vacuum advance canister on the distributor. One reads about 0 at idle and runs up to around 18 at full throttle. The other port reads 18 at all times, regardless of throttle. Which should go to the advance canister? I've tried to Google the question but it seems like different guys choose different vacuum for different driving?? I wound up further confused.
Thank you, Josh

[JimCT]

What would be the purpose of using a port that had the same value all the time? I think you answered your own question. And that is a great carb, we are running it on our '68 ambulance.

[alaskajosh]

I know it seems intuitive but then, if you make the mistake of looking into it, you find conflicting theories.

Here's a page I have up right now, for example.
http://www.gofastforless.com/ignition/advance.htm
Granted this is a hot-rod guy but you don't have to look around for long to find some guys saying to hook up to the constant (MV?) source.
Or maybe I just REALLY don't understand what I'm reading.. wouldn't be the first time.

Thanks, Josh

[alaskajosh]

This is simple enough even for me (skip down to "How vacuum is used") :
http://www.lbfun.com/warehouse/tech_info/timing%20&%20vacuum%20advance/vacuum_explained.pdf

I think the port that shuts vacuum off at idle is the "ported vacuum" and it's purpose is for emissions control.. allowing timing to retard at idle and advance normally as the throttle plates open.

If one is not concerned with emissions regulations then he may be better off using manifold vacuum.
Frankly, it's still over my head, most of it.

Thanks, Josh

[lstrvr]

I do see your concerns and you have done ANYTHING but answer your own question with the first post! There is no constant vacuum value in an engine which you may have confused yourself with by measuring vacuum at WOT with no load vs. the engine under load. Both ports can be used for vacuum advance, it just depends on how the engine has been tuned. It is a matter of tuning the engine with regards to the centrifugal timing, vacuum advance unit, and the vacuum port, whether it be ported or manifold. There are a few ways to determine which port should be best used for your application.

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