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So I have a very stumping question/situation. Let me lay down what I know and correct me if I'm wrong. Turning the distributor on a sbc 350 counterclockwise advances the timing with clockwise retards it. When I advance it with everything plugged it seems to increase the rpm. Shouldn't it be doing the opposite? Also the more you turn it from 12 degrees and lower towards tdc on the damper it seems as if it won't run when it's at tdc. Is it supposed to do that?

Yes, counter-clockwise advances the timing and clockwise retards the timing.
Assuming that you have correctly stabbed the distributor and the mechanical weights and springs are working smoothly, the distributor Batt+ and tach connector leads generally point straight out to the side.
The timing should be set "WITHOUT" the vacuum advance connected. Once the timing is set, connecting vacuum will make the timing mark advance to the point where you cant see it with a timing light. I am assuming you are using full manifold vacuum to the distributor.

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Previous setup: 4.1 Buick V-6/TH 350
Current setup: 400 Small Block Chevy V-8/TH 350
9" ford rear end/short bed/blue
What do Water, Electricity and Humans have in common... They all travel the path of least resistance.

I'm using the vacuum advance provided off of the summit 750 carburetor. It seems to run better when it get's advanced running at about 20+ degrees. All ports are plugged except for the pcv valve, which is hooked up. Just put in a new valve too. I'm just wondering if it's normal to have the motor struggle as it moves from 20+ degrees downward to 12.

If you're certian that the distributor is propery stabbed and the mechanical advance is working smoothly, and the firing order is correct (1,8,4,3,6,5,7,2) set your timing at 12 degrees and check your idle speed. If it's too low, the motor seems to struggle to stay running. Also, If that's a brand new carb, the basic mixture screw adjustments may not suit your motor. Turn the mixture screws in till they lightly bottom out and back them out 1 and a half turns. This will get you in the ballpark.
It's also a good idea to spray carb cleaner at all the gaskets along the heads and carb base. If the motor is sucking air, all the timing adjustments wont fix it.

_________________
Previous setup: 4.1 Buick V-6/TH 350
Current setup: 400 Small Block Chevy V-8/TH 350
9" ford rear end/short bed/blue
What do Water, Electricity and Humans have in common... They all travel the path of least resistance.

The motor runs great but I know for a fact that my vacuum advance isn't working due to the distributor being left out in the rain for half a year. Replaced the top portion of the distributor but that's not what I'm really getting at. All I'm asking is if it's normal for the motor to want to die down as it decreases from 20+ degrees down to 0?

The motor isn't running as efficiently as it should with the timing closer to TDC. So yes, the motor is much smoother at 20 degrees than at 5 degrees.
OK, now concider this when you get your vacuum advance fixed. You can connect the vacuum line to the fitting on the carb that Summit points out (ported vacuum). This vacuum connection will basicly not provide any vacuum pull at idle. It's a federal emissions requirement to have this port for ignition. Any carb before made before federal emissions standards did not have this port. Vacuum to the distributor was from a manifold vacuum source. Some find that they like ported vacuum because they can feel the hit of an aftermarket cam. Others like manifold vacuum because having the vacuum fully advancing the timing at idle makes for a much smoother running engine.
Only at idle will you have any difference with either ported or manifold vacuum. When you're cruising at a steady speed, they react the same. When you floor it, neither vacuum source is working because the throttle plates are wide open and there's no vacuum pull thru the motor.
Experiment with both ported and manifold vacuum to the distributor and see what suits you better. I'm thinking you'll want manifold vacuum because of the added timing. Your baseline timing of 12 degrees or so with another 10 or so degrees of vacuum advance will give you around 20 or so degrees of timing at idle.

_________________
Previous setup: 4.1 Buick V-6/TH 350
Current setup: 400 Small Block Chevy V-8/TH 350
9" ford rear end/short bed/blue
What do Water, Electricity and Humans have in common... They all travel the path of least resistance.

YES, it is normal for the engine rpms to go down as the timing is retarded. At idle your distributor vacumn advance should be connected to a non vacumn scource or connection on the carburetor. Sometimes referred to as a " ported"connection, on a Holley carburetor there are two connections on the front of it, the one you want is the one on the passenger side, this one will not have any vacumn coming out of it at idle. But when you are setting your timing it is still a good idea to remove and plug the vacumn line going to the distributor, this because sometimes with a big cam the idle screw might be turned in enough to partially un cover this port. Good luck!
BTW Blue Meanie, carburetors have had this ported vacumn ever since the Holley 1850 came out in 1957, long before any federal regulations for carburetors existed.
You will find this vacumn connection located on the side of the metering block, it is a 1/8" pipe threaded opening.

Yes, the 1850 was manufactured with a ported vacuum source. But there wasn't many vehicles leaving the assembly line with a 4 bbl bolted to the motor. The majority of autos built had sedate, smooth running, 2 bbl carbed motors and having a V-8 under the hood was the selling point.
Here's a redirect that basicly covers what i have learned over the years...

http://www.corvetteactioncenter.com/for ... vance.html

Another GM engineer (Lars Grimsrud) has written numerous articles on the subject. Google him and you'll find detailed info on tuning.
What it comes down to is that there are 2 sources of vacuum for ignition. It's finding out which one works best for each application.

_________________
Previous setup: 4.1 Buick V-6/TH 350
Current setup: 400 Small Block Chevy V-8/TH 350
9" ford rear end/short bed/blue
What do Water, Electricity and Humans have in common... They all travel the path of least resistance.

Thanks this is an excellent article, I enjoyed reading it. Makes a lot of sense too, and could save a lot of gas!

I myself, am a big fan of using vac advance on a street engine. But there are limits. Highly modified, fire breathing engines that get stuffed into a vehicle that still has licence plates wont benefit from it because of the erratic vacuum signal.
On a street engine, there can be some subtle changes required to the timing and carb when going back and forth between ported and manifold vacuum. It just comes down to what works best to a particular engine configuration and what the driver wants.
As for aftermarket manufacturers, to sell parts that are "Legal for use on pollution controlled vehicles", they will provide the non-law breaking instructions. So with distributors, ported vacuum is the correct method.
There is no real power advantage having vac advance but having a faulty vacuum canister is a problem. Odds are if a canister isn't functioning, it has a hole in the diaphram. Air leaks make it tough to correctly tune.

I myself used to rely on the HEI as bullet-proof (which it basicly is), but things do wear out over time. The articles written by Lars Grimsrud had me looking deeper into my ignition system. Now i am very much concidering getting a MSD HEI. These things have such a broad range of adjustment.

10310... I hope you have a handle on your timing issue. I kinda hijacked this thread with stuff you didn't ask for. But i hope you find this info usefull.

_________________
Previous setup: 4.1 Buick V-6/TH 350
Current setup: 400 Small Block Chevy V-8/TH 350
9" ford rear end/short bed/blue
What do Water, Electricity and Humans have in common... They all travel the path of least resistance.