Vaccum Advance Question

[jlmiller5]

I have been going through this truck I bought, looking for problem areas and doing general maintenance.  It is a 1979 GMC Heavy Half with the 454 big block in it.  Im trying to check all the devices to make sure they are working properly.

I was setting the timing, and I am curious about what this thing is in the line between the carb and the vac advance on the distributor. Do I need it, and how can I check that its working?  It appears to be in the right place according to the vac diagram on the radiator support, but it doesn't call out what it is.

[mcintyrederek]

That's a check valve.  You can "suck" vacuum through it one way but air won't be able to go back through it the other way.  There are probably others elsewhere around the engine.  The red side indicates the side that connects to a device.

Mack.

[enaberif]

I would think a check valve would defeat the purpose for a vacuum advance. The vacuum advance needs to be able to adjust based off the amount of vacuum and the check valve is only allowing vacuum in not out.

[jlmiller5]

The 2 lines on the side, that come from outside the picture, go to one of those thermal vacuum switches that look like it is in the path of the coolant.  Here is the vacuum diagram from my radiator bracket. 

Also Im pretty sure the advance source is ported vacuum, I've seen a lot of debate here on that subject also.  Does anyone think it would be a good test to run directly from carb vac source (either ported or manifold) to the vac advance without this check valve?  I am just wondering about the integrity of a 30 year old device like that, and I don't know how to test it.  Most of the other bits (EGR, EFE, Vac Canister, etc.) have been plugged, gummed up, or stuck when I have looked into them.

Thanks for the help!

[enaberif]

The 2 lines on the side, that come from outside the picture, go to one of those thermal vacuum switches that look like it is in the path of the coolant.  Here is the vacuum diagram from my radiator bracket. 

Also Im pretty sure the advance source is ported vacuum, I've seen a lot of debate here on that subject also.  Does anyone think it would be a good test to run directly from carb vac source (either ported or manifold) to the vac advance without this check valve?  I am just wondering about the integrity of a 30 year old device like that, and I don't know how to test it.  Most of the other bits (EGR, EFE, Vac Canister, etc.) have been plugged, gummed up, or stuck when I have looked into them.

Thanks for the help!

If you move that vac line to manifold your going to have to tweak a bunch of stuff.

[blazer74]

Going by your diagram you have ported from the front of the carb and full manifold from the rear of the carb going to that subject valve with the red ring. Appears to be a dual function piece. To be sure if you have full manifold to the vac advance pull the hose off the dist at idle and see if it has vac at idle.

If not its ported vac if so its full manifold vac,


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[enaberif]

Going by your diagram you have ported from the front of the carb and full manifold from the rear of the carb going to that subject valve with the red ring. Appears to be a dual function piece. To be sure if you have full manifold to the vac advance pull the hose off the dist at idle and see if it has vac at idle.

If not its ported vac if so its full manifold vac,


Sent from my iPhone using Tapatalk

Ah that is what that other line is doing! Interesting function for the distributor.

[mcintyrederek]

Yes you are correct on the vacuum advance and the check valve.  What was I thinking.  The brake booster is the only thing that uses a check valve in my C10.  My check valve on the brake booster looks similar but is a lot larger and has a red band like in the photo.

Mack.