New Carb
Old School Hot Rodder
Joined: Feb 2006
Posts: 6,472
Likes: 11
From: Exmore, VA
One of the issues I used to see was some engines liked full vacuum on the advance at idle, others didn't. Chevrolet engines would backfire under sudden throttle opening with ported advance, they always used direct vacuum until 1968. Chrysler engines would misfire horribly with direct vacuum at idle, most of them ran 12.5 degrees static timing. Ford used ported spark for years.
One item will be how much manifold vacuum your cam will give you at idle, if it isn't enough to fully advance the vacuum advance them the idle will be unstable. At least with a manual trans it isn't as big a problem as with an automatic.
One item will be how much manifold vacuum your cam will give you at idle, if it isn't enough to fully advance the vacuum advance them the idle will be unstable. At least with a manual trans it isn't as big a problem as with an automatic.
FTE Legend
Joined: Jul 2010
Posts: 32,875
Likes: 48
From: Northeast, OK
But, where I've had problems are when the vacuum at idle isn't sufficiently strong to fully advance the timing regardless of what else happens. In those cases I get an unstable idle. For instance, I've had engines set so they idle properly in gear with an auto, but take them out of gear and they run too fast. Or, the A/C compressor comes on and the R's drop which causes the vacuum to drop which causes the R's to drop....... and the thing falls on its face.
So, to say that all street engines should run manifold vacuum to the advance tells me the author of those words doesn't live in the real world - or at least the world in which I live. There are just waaaaay too many variables to make such a statement.
Old School Hot Rodder
Joined: Feb 2006
Posts: 6,472
Likes: 11
From: Exmore, VA
Gary, you hit the nail on the head, I wish I had a nickel for every kid with a 283 or 327 he put a hot cam in, curve kit in the distributor and couldn't figure out why his car ran at 1500 rpm in neutral, and would stall when he put the Powerglide in gear.
I used to keep a set of real heavy advance springs in my toolbox so I could essentially lock the advance and set a static timing, tweak the idle mixture and speed with no vacuum to get a baseline idle. Then I could go to my distributor machine and set initial centrifugal advance a little above that. If I had enough vacuum at idle in gear to work the advance I would hook it direct, if not I would use a ported vacuum source.
I used to keep a set of real heavy advance springs in my toolbox so I could essentially lock the advance and set a static timing, tweak the idle mixture and speed with no vacuum to get a baseline idle. Then I could go to my distributor machine and set initial centrifugal advance a little above that. If I had enough vacuum at idle in gear to work the advance I would hook it direct, if not I would use a ported vacuum source.
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