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By Duck - 16 Years Ago
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Fellas- I was trying to get my vacuum lines/ sources figured out earlier, when I realized the re-maned Holley I have is missing a vacuum source. There should be a port just ahead of the choke assembly, but all I have is a tapped blind hole. Can I utilize manifold vacuum as an advance source, or will that cause me problems? Thank, /Duck
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By Pete 55Tbird - 16 Years Ago
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Actually, that is the whole idea. Cruise, equals high vacuum MAX advance equals BEST MPG whereas lots of throttle equal low manifold vacuum NO advance and the engine will not detonate (ping). Pete
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By GREENBIRD56 - 16 Years Ago
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Sorry to disagree - but full vacuum advance at idle works just fine. Lowers the engine temperature and leans it out while at no load. When you open the throttle and the vacuum advance backs off - the mechanical keeps raising with the rev's - but should stay low enough not to knock. When you reach cruise - the vacuum kicks back in and leans the mixture again. There are a few zillion detroit vehicles that came with this arrangement, so it must work..
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By crenwelge - 16 Years Ago
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In the little over 50 years I have been tinkering with point ignition, I find vacuum advance to be one of the hardest things to explain. For some reason, people seem to have preassumed that advance meaning it advances as rpm goes up. This of course is done by the centrifugal advance and the vacuum advance actually retards it under full power demands and allows it to advance under low power demands. A statement like the vacuum advance allows it to retard when you floorboard it doesn't seem to make sense.
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By charliemccraney - 16 Years Ago
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57 Ford Kustom (9/12/2009)
Maybe saying that vacuum advance is dependent on amount of vacuum. the more the throttle plates are open the less vacuum you have. T  M
That's not what I've observed. What I have observed is that the vacuum advance stops increasing at some point regardless of an increase in vacuum and the manifold vacuum increases with a gradual increase in the throttle position. It drops with abrupt increases in the throttle position.
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By DANIEL TINDER - 16 Years Ago
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Interesting note re: vacuum advance:
Talked to a kid at a car show last week with an otherwise bone-stock/orig. '55 292 Mercury (recently inherited) with a HUMONGOUS Edelbrock carb. on it. When I noticed the Loadomatic distributor and asked "how's that working for you?", he said "fine".
I tried to explain the problems inherent, but I don't think he took me seriously.
We all know why it shouldn't work, but long ago I once ran around for over a year with a leaking/split vacuum diaphragm on mine, and (aside from the vacuum leak) never noticed a problem with it. I had set the initial timing by ear (until starting was difficult), and essentially had NO advance of any kind! It still ran OK.
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By GREENBIRD56 - 16 Years Ago
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I think that for me anyway - it works to just consider the dizzy vacuum pot a "lo-load mileage device". Remember the "mileage meters" they used sell - maybe still do...? The face was marked with ECONOMY (green) on one side and swept over to POWER (red) - a vacuum gauge with no numbers. My Dad had one on his Merc and used it faithfully. His kid (who would that be?) usually jerked the economy doo-dads off his own stuff and went straight for dual points, light springs and full centrifugal ASAP. When the load is low (high vacuum) - it adds advance and boosts economy. When your foot asks for power - it gets out of the way. Have you guys ever seen this site? Aussie propaganda ...... the little needles jumping is a crack-up. But it helped me explain things to my kid 1500 miles away. http://www.users.bigpond.com/ergoff/vac1.htm
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By Duck - 16 Years Ago
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Thanks fellas- I've just finished up drilling & tapping a new port in my manifold. I really appreciated the opinions... /Duck
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By oldcarmark - 16 Years Ago
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Hello Steve!I had a look at that "Aussie propaganda" .As much as its funny do you agree with the problems that are represented by each diagram?Reason is the ongoing problem i am having with my rough idle.I am using a vacuum gauge to try and figure out whats causing it and nothing I am seeing on my gauge indicates a problem.Steady high reading of 19 with only a slight movement of 1/2 inch of vacuum change.Tried most of the suggestions offered and still haven't figured out my problem.
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By GREENBIRD56 - 16 Years Ago
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I'm thinking there is one cylinder that just doesn't match the rest - that one has got stuck rings or something on that order. Fires - but doesn't make the same sort of kick as the others.... makes your vacuum gauge "tick" a bit. If you remove the front vent/ valley cover breather and hold your hand over the pipe - is there any sort of crankcase pressure pulse at all? I'm headed off to Idaho for a weeklong job - maybe the thin air will clear out my thinking.....
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