2 vacuum advance hose connections on a 289?

[garystocker]

I have the base timing at 6BTC with the vacuum advance line disconnected and plugged. When reconnected the timing advances 20 degrees at idle! I am told this is due to a vacuum advance connection with a constant vacuum source as opposed to ported, which will give me no vacuum at idle.

The edelbrock carb has 2 vacuum connections on the front. The one on the left is ported. Both lines are connected to the distributor.

The right side (constant vacuum, even at idle) is connected to the outside diaphrame connection. The left side (ported) on the carb is also connected to the diaphram unit, only more toward the distributor.

Do I need to switch those lines? Do stock 289 distributors have 2 vacuum lines connected to the carb? If so, how are they connected? Which side is ported? UGH!!

Here is my theory. The constant vacuum should be connected to the inner port on the distributor diaphram unit which keeps a suction on the diaphram toward the distributor, preventing vacuum advance. The other side of the diaphram has a ported connection which does not pull until hihs RPMs are achieved. Then the diaphram is sucked away from the distributor, pulling the breaker plate and advancing the timing.

What do you all think about this?

I need a push in the right direction...

 

[GoinBroke]

The manifold vacume should be to the inboard port on the dizzy and the ported vacume should be on the outside port.

 

[jcode68]

I had a dual diaphram advance on my original 302 for my 68 Mustang. If I recall properly, the connections were made to a ported vaccum source on one side at the carb(don't recall which) and the other was connected to a thermostatically controlled port off the manifold. I guess when the temp reached a certain point, it would open or close the port for vacuum. I know my response probably only adds more confusion and not a solution. I don't think the dual diaphram units added any real benefit, so if your not hung up on being original, I would replace the advance with a single unit available from most parts stores. Cost should be $20 or less and will simplify the hookup and probably be more reliable, especially if yours is the original unit.

 

[Rapid]

My Edelbrock has the ported vent on the passenger side of he carb. I am running that side on my 289. I understand that base timeing, about 10-12 at idle no vac.Vacume advance is for idle and total timing which should be around 34 at 3500 rpm. The vac advance takes over after idle and gets the engine going till the centrifical weights take over. I have mine set about 12 at idle, 19 total advance with vacume and weights that advance the rest of the way by 3500 rpm. It runs well with no pinging and great throttle response.