Vacuum advance issues

[Wrenchbender Ret]

You can run without vacuum advance. It is for economy basicaly but does help driveability in some cases. Weather it is hooked to full vacuum or ported should not effect the pinging problem. The advance should drop back under load as vacuum drops. You might need an adjustable vacuum chamber so you can set it to come on at a higher vacuum & limit the amount. How about the centrifical advance? Often they are stiuck on an HEI. You may need stronger springs. You will need to experament with the tining & advance as every engine has its variations.

[cppursell]

After disconnecting vacuum, I set my initial timing at 8 adv. 3000rpm takes me to 27 adv total. I drove it and it pings at WOT and under heavy acceleration.

I hooked up manifold vacuum, and have an idle timing of 32, and 52 at 3000rpm. It seems to idle better but driving it pings easily.

I read that manifold vacuum is necessary)(http://rockridgefarm.com/vettdoc/Timing_101.pdf) "...you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively."

But I've heard from others including that mechanic that you need ported. Also, here (http://www.73-87.com/7387garage/drivetrain/hei.htm) gives tips on setting timing with an adjustable vacuum can.

I'm so confused!! What is right?!?

[Coley]

The easiest and perhaps the cheapest way to beat this is to get another 2bbl carb with ported vacuum on it and swap it out to try it.
You can probably pick up a reasonable one for a good price off of Ebay or equivalent.....lots of Rochester stuff on there.
Good Luck
Coley

[cppursell]

Originally Posted by Coley

The easiest and perhaps the cheapest way to beat this is to get another 2bbl carb with ported vacuum on it and swap it out to try it.
You can probably pick up a reasonable one for a good price off of Ebay or equivalent.....lots of Rochester stuff on there.
Good Luck
Coley

I have considered that and was going to do it until I saw the article saying you need manifold vacuum. It said that ported came around in the smog era so there would be less advance, less efficiency, hotter engine, just for hotter exhaust to reduce hydrocarbons. Man, that is some backwards thinking right there...

[mud.man.rj]

I would pickup a carb with ported vacumn and run both this will give you your best ecomomy, but if not wanting to change carb then eliminate vac advance and just run a bit lighter spring on mech advance to get between the best of both. The vacumn doesn't work or induce advance until you are under part throttle operation and helps a bit to stop initial bog or lag off of bottom end only then mechanical advance takes over. I run a mechanical adv only with big cams and change springs till I get it where I want it.