[TJ's GMC]

I tried my engine(305E with a 600 cfm 4 barrel and modded magnum manifold) on Manifold vacuum advance and it would hesitate off idle after idling for so long. Thing is, weather on Ported or Manifold....both are useless under load. Under load your relying on the weights in the dizzy for advance. The vacuum timing only comes in while cruising at a constant set speed to aid in power and mileage. Therefore, not Enough or to Much can have an affect on power and mileage. Which is why I backed mine down to 8 initial and left it on Ported timing. I did do 12 initial once, but it felt like to much and didn't really make a difference, so I left it at 8-10. 4-6 is is factory for stock, but with the extra fuel I have found advancing a little helps. Also the addition of an HEI makes a difference in itself. That and hot wires and plugs will aid in power. I still need to open the plugs in mine a tad more, but I haven't as it runs like a top and I drive it Everyday...so if it ain't broke I don't fix it. lol If I had a dyno that would be something I would play with and get some paper results.