Vacuum advance

[19Walt93]

In my experience, relying on the vacuum advance for power never works well. It's there when the throttle is at a constant position, but as soon as you step on the throttle to accelerate, the advance it was giving goes away. You get a much crisper and responsive engine if most of the timing advance is there all the time with the weights + static.

I just advance the static as much as I can because it's easy. Modifying the springs and weights is the proper more permanent way to do it.

I don't rely on vacuum advance for power, I use it to add advance at lower rpms, especially when tipping into the throttle. Setting up the distributor is easy to do and costs nothing unless you choose to buy different springs. If you recurve your distributor and drive it you'll regret not doing it sooner. During the 70's when the car makers really didn't know how to meet the emissions standards they would sometimes run manifold vacuum to the vacuum advance. It caused surging at low, steady speeds and low power accelerating which was aggravated by the late centrifugal advance curves. I can't count the cars and trucks we altered(illegally,no doubt) to make customers happier with their new vehicles. My dealer seemed to specialize in boring, Grampa cars, lots of 6 cylinders, 302's and 351's when 400's and 460's were available. A late 70's LTD with a 302 or 351 would usually idle at around 600-650 in drive but run at 1000-1100 in neutral because it was so lean and retarded that it would lose that much speed when put in gear. And get 12-14 mpg with Grampa driving. Tiny calibration changes can make amazing results.

 

[bilbo]

I just changed mine to a Holley 5200 carb and it seems to work much much better with the vacuum advance on the manifold. I would get a lot of surging and unstable idle when using ported vacuum for the distributor. I've still got some tuning and tweaking to do. But at least I think I finally figured out my tip-in stumble. I believe it's a faulty PCV valve. It opens up too early and leans out the mixture really bad right at transition. That lean spot goes away completely when I remove the PCV valve from the system. I ordered a Motorcraft one and will see how that goes.