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I know this topic probably has been beaten to death but, I'm a little confused due to other posting I have read. I thought on a GM 350 you need to connect the dizzy to a vacuum source that had vacuum at idle and it drops off as the RPM come up then mechanical takes over.

I have read in the Holley manuals that the dizzy should be connected to intake vacuum not ported (vacuum goes up with RPM). Which is totally opposite of what I have always thought. :confused:
 

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I run holley carb's and after experimenting with both ports I also run to the full vacuum port. Car idles and runs all around better for me. IMO
 

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I run holley carb's and after experimenting with both ports I also run to the full vacuum port. Car idles and runs all around better for me. IMO
I agree after reading the posts mentioned here I hooked it up to full manifold vacuum did not do much drove for a few months still was not right- I figured out the Vacuum canister was not working (diaphragm shot) replaced early this week- It was the best $20 I spent on my car. Hooked up to full vacuum it idles smoother, I was able to turn down the idle screws now there is no more dieseling at the throttle plates are more closed- it idles cooler and accelerates great. I hope that helps.
 

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Originally the vac advance was setup to full vacuum. It was doring the 70's when compression ratios dropped and fuel economy was starting that it switched to ported vacuum.

There is really very little difference between the two. Full vacuum is high at idle and ported is low at idle. That's basically it. When the throttle is wide open, both operate virtually identically.

Good reading on teh subject:
http://www.lbfun.com/warehouse/tech_info/timing & vacuum advance/Timing101Article.pdf

For the record, I use full vacuum.
 
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