Cleaning up my wiring. What does this plug go to?

It might be. Chevy used to, and might still, use one wiring harness for all of the same model. I've had trucks with no A/C and radio delete and if you unravel the wiring harness the wiring and connectors are there, just not used. A PO might have unraveled it and not tied it back up.

MisterB said:

it might be just barely long enough to reach the front of the head, but not to where my current alternator is.

Here's a Pic from yesterday when I was changing out the valve cover gaskets

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A small side note here, from this picture your vacuum advanced line is going into the passenger side of the edelbrock carb which is full time vacuum and is usually for the transmission. While the passenger side is timed vacuum and for vacuum advanced.

Dutch Rutter said:

A small side note here, from this picture your vacuum advanced line is going into the passenger side of the edelbrock carb which is full time vacuum and is usually for the transmission. While the passenger side is timed vacuum and for vacuum advanced.

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That's interesting. Should I try switching it then? What all would I gain by doing that?

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Its always worth a shot. The timed port would advance your timing based on your RPM. So my truck currently is set at 12 initial at idle then when raised to 3000rpm the timing increases to around 34 or so. If running on that constant vacuum your timing will always be in that "advanced position" and not change based on your RPM. at least that is what they say.

Did some more digging on this. It depends some on the type of dizzy you have. This is from edelbrock.com

Before Removing Old Carburetor
Determine if the distributor vacuum advance it timed (no vacuum at idle) or full (vacuum present at idle).
With the engine fully warmed up and idling, pull the vacuum hose off of the carb and "feel" for vacuum by putting your finger on the vacuum port. If your distributor has timed vacuum advance, hook the distributor vacuum hose up to the passenger side vacuum port on the carburetor. If it has full vacuum advance, hook it up to the driver's side vacuum port.

All the ones I've had were timed. but yours could be full. Basically the driver's side port always has vacuum pulling while the passenger side I believe does not pull vacuum at idle but does as the RPM increase. It should only affect your idle so not too big of a deal, try both and go with what works better.

Dutch Rutter said:

A small side note here, from this picture your vacuum advanced line is going into the passenger side of the edelbrock carb which is full time vacuum and is usually for the transmission. While the passenger side is timed vacuum and for vacuum advanced.

Click to expand...

it depends on your vac canister. they generally add 12+ degrees of timing at high vacuum. so if you have a canister that adds 12* at idle (high vacuum) and your base timing is set at 10* you'll have 22* if advanced by full manifold vacuum source. then you have your mechanical advance which depending on what type of weights you have may add about 20* once "all in" at higher rpm. so at WOT you'd be at 30ish in this example by your "all in" rpm. many shoot for all in near 3000rpm. at WOT you are near atmospheric pressure(no vacuum), so the vacuum advance is doing nothing. to further complicate things, when cruising at high rpm you'll get a combination of mechanical advance and vacuum advance. for instance, your rolling down the freeway at 2800 rpm, but only barely into the throttle with low load--this would have you at near full mechanical advance and near full vacuum advance. your timing would be advanced up into the 40-50* range depending on vac canister and weights installed.

the debate over ported vacuum and full vacuum for distributor is back and forth with internet folks, but I think performance oriented guys go full manifold and smog era stuff is ported.

Dutch Rutter said:

Did some more digging on this. It depends some on the type of dizzy you have. This is from edelbrock.com

Before Removing Old Carburetor
Determine if the distributor vacuum advance it timed (no vacuum at idle) or full (vacuum present at idle).
With the engine fully warmed up and idling, pull the vacuum hose off of the carb and "feel" for vacuum by putting your finger on the vacuum port. If your distributor has timed vacuum advance, hook the distributor vacuum hose up to the passenger side vacuum port on the carburetor. If it has full vacuum advance, hook it up to the driver's side vacuum port.

All the ones I've had were timed. but yours could be full. Basically the driver's side port always has vacuum pulling while the passenger side I believe does not pull vacuum at idle but does as the RPM increase. It should only affect your idle so not too big of a deal, try both and go with what works better.

Click to expand...

Ported vs manifold is an in depth subject. The short version is generally for performance manifold is prefered and was how it was originally done back before the mid 70's. In the mid 70's auto manufactures were trying everything the could to reduce emissions. running retarded timing at idle was one way they attempted to accomplish this, they used ported vacuum so you would not have any vacuum advance at idle. If you have a distributor with a SMOG curve it may not like manifold vacuum.

Vacuum advance has nothing to do with RPM's it is strictly designed to advance timing based on engine load/vacuum. Mechanical advance is based on RPM's

Thanks for clearing that up @C10MixMaster and @Frankenchevy