Originally Posted by damanx
So, I left the base timing at 12 degrees and I tubed in one of those distributor vacuum valves that has the manifold port and carb port that feeds vacuum to the distributor. The manifold supply is actually going to a thermal vacuum valve then to the valve.
I noticed that when the engine was warm, the vacuum from the manifold applied the advance at idle bringing the timing to 22 degrees.
Upon acceleration where manifold vacuum dropped, the timing drops suddenly then comes back up until it seems like the mechanical kicks in.
So I think I am on the right track as it seems like the pull is much smoother than on the ported vacuum.
The other thing I'd like to try is placing a delay valve before the advance can to slow down the loss of vacuum a little.
One issue I need to deal with is the secondary boosters leaking also.
Will update once I install the valve.
Using a delay to the vacuum advance can work, but you need to realize that when you floor it and the engine's under a load (pulling up a hill for example), the delay in dropping the vacuum advance could cause it to detonate. There can be a transient ping at the hit of the throttle even w/o a delay.
The rest of the dynamics involved also come into play, so whether this will be a problem or not remains to be seen. But it's something I wanted you to be aware of (and you prollably already were) so you'd be watching for detonation under a load (pinging).
In general, you're seeing one of the downsides to using a lot of vacuum advance. If the amount was 10-12 degrees from the vac. adv, the drop wouldn't be as noticeable, and that would have allowed you to add that timing (all or part) to the initial advance.
If the engine is running on a lot of vacuum advance, it needs to be matched up very close to optimum as far as torque converter stall, gear ratios, cam, etc. goes. Because if there's a mismatch like a too-tight torque converter as an example, that alone can cause the engine to drop vacuum (and it'll take longer to recover) at the hit of the throttle.